Bulletin of the American Physical Society
2005 47th Annual Meeting of the Division of Plasma Physics
Monday–Friday, October 24–28, 2005; Denver, Colorado
Session QP1: Poster Session VII |
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Room: Adam's Mark Hotel Grand Ballroom I & II 9:30am |
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QP1.00001: DIII-D II, HEATING AND CURRENT DRIVE, DIAGNOSTICS |
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QP1.00002: ECH at DIII-D J. Lohr, I.A. Gorelov, D. Ponce, J.F. Tooker, R.W. Callis, K. Kajiwara, R. Ellis The gyrotron installation on the \hbox{DIII-D} tokamak is being upgraded with the acquisition of three 110~GHz, 1.0~MW, 10~s gyrotrons having cryogen-free magnets. In addition to these gyrotrons, which will have diode geometry, a single stage depressed collector prototype gyrotron, also operating at 110~GHz, but capable of generating in excess of 1.0~MW at increased efficiency is being tested to maximum parameters. A number of specialized system components are being developed to accommodate the increased output power of this tube. The ultimate system will generate up to 5.5~MW for experiments, with injected power in excess of 4.0~MW. Fast scanning of the injection angles, improved control of the high voltage power supplies, more flexible fault processing capability, better sweeping of the electron beam over the gyrotron collectors and enhanced control of the output power are being implemented. [Preview Abstract] |
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QP1.00003: Benchmarking of ECH Codes for ITER R. Prater Many computer codes have been developed for wave propagation, absorption, and current drive using electron cyclotron waves. These codes include ray tracing codes, like BANDIT-3D, GENRAY, TORAY-GA, and TORAY-FOM, and Gaussian beam codes like ECWGB (now GRAY), OGRAY, and TORBEAM. For absorption, codes may use analytic models or Fokker- Planck calculations, as in BANDIT-3D, CQL3D, and OGRAY. Detailed comparisions of the codes has been made (with the active participation of their authors) for a projected ITER discharge (Scenario~2). To better test the propagation part of the codes, a discharge with higher density and greater refraction was also used. Several code problems were fixed due to these studies, and the resulting profiles of power density and current density lie within a narrow range. Some issues needing further work will be discussed. [Preview Abstract] |
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QP1.00004: Study of the Temperature Time Evolution of the ECH Launcher Mirror in DIII-D K. Kajiwara, C.B. Baxi, J. Lohr, I.A. Gorelov, M.T. Green, D. Ponce, R.W. Callis The ECH launchers in the \hbox{DIII-D} tokamak have poloidal and toroidal scan capability with steering provided using movable mirrors. Minimizing eddy current-induced forces on the mirrors while maintaining low loss rf performance presents conflicting design requirements. To solve this problem, a mirror design using a sandwich structure of Glidcop and stainless steel was developed. In order to prevent melting of the surface of the mirror, the mirror temperature is monitored at the back surface. An appropriate thermal model can estimate the actual peak temperature at the front surface by using the monitored temperature. A previous study showed the simulation of the base temperature increase is in agreement with the experiments. However, there has been a discrepancy for the time evolution, which must be resolved to give confidence in the model for the peak surface temperature. The comparison of the time evolution of simulations and experiments will be presented. [Preview Abstract] |
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QP1.00005: Performance of Diamond Gyrotron Windows at \hbox{DIII-D} I.A. Gorelov, J. Lohr, R.W. Callis, D. Ponce, K. Kajiwara Artificially grown diamond disks prepared by chemical vapor deposition (CVD) are widely used for gyrotron windows and for tokamak rf port windows due to their ability to transmit high frequency (greater than 100~GHz) and high power (greater than 1~MW) rf beams with minimum losses and excellent thermal performance. A large database has been accumulated during the last few years with results of simulations and experiments on thermal and mechanical properties of CVD diamond under different conditions of rf power transmission, cooling and neutron irradiation. We present experimental measurements and computer simulations of the performance at 110~GHz and 1~MW of three CVD diamond gyrotron output windows on production gyrotrons operating for electron cyclotron heating and current drive experiments on the \hbox{DIII-D} tokamak. We also report model calculations for a 110~GHz depressed collector gyrotron with output power in excess of 1~MW. The model calculations are compared with experimental temperature data from infrared monitoring of the windows during 5~s pulses with 1~MW transmitted rf power. [Preview Abstract] |
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QP1.00006: DIII-D Experiments on Cyclotron Harmonic Absorption of Fast Waves R.I. Pinsker, C.C. Petty, R. Prater, M. Choi, W.W. Heidbrink, Y. Luo, F.W. Baity, M. \hbox{Murakami}, M. Porkolab In the presence of a sufficiently dense population of fast ions from either neutral beam heating or from fusion products in a burning plasma, ion cyclotron damping of fast waves (FW) is important even at moderate to high ion cyclotron harmonics. DIII-D experiments have coupled up to 3 MW of FW power at 60 MHz and at 116 MHz to investigate damping on fast ions at harmonics from the 2nd harmonic of hydrogen to the 8th harmonic of deuterium. In agreement with linear theory, strong absorption was observed on fast ions from an injected 81 keV deuterium beam at the 4th and 5th harmonics for 60 MHz FW. Comparison of 4th and 8th deuterium harmonic absorption at a fixed magnetic field have shown that the latter is weak at low density, also as expected. However, at high density the 8th harmonic absorption continued to be much weaker than at the 4th, in apparent contradiction to predictions of strong absorption at both harmonics under those conditions. Possible explanations of the discrepancy are discussed. [Preview Abstract] |
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QP1.00007: Dust Measurements with the DIII-D Thomson System B.D. Bray, W.P. West, J. Burkart The DIII-D Thomson scattering system, consisting of eight ND:YAG lasers and 44 polychromator detection boxes, has recently been used to observe the existence of dust in the scrape-off layer and divertor regions during normal plasma operations. In order to maintain absolute sensitivity calibrations of the Thomson system, each polychromator box contains a detector channel with a filter at the laser wavelength that is sensitive to Rayleigh/Mie scattered light. Dust particles residing in the region of the laser beam viewed by a detector produce a very large scattered light signal in this Rayleigh channel compared to the usual Thomson scattering from the plasma electrons. Background from stray light scattered in the machine and neutrons can be estimated and average density profiles for various plasma configurations are determined. [Preview Abstract] |
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QP1.00008: An Improved Detector Electronics and Data Acquisition System Design for Thomson Scattering Diagnostic on DIII-D C. Liu, C.L. Hsieh, B.D. Bray, D. Sellers The detector electronics and data acquisition system for the Thomson scattering diagnostic on \hbox{DIII-D} is being upgraded to replace the present CAMAC-based system. Besides more modern electronics, the proposed design contains a number of improved features. For instance, to reduce the gain drift with temperature in the avalanche photodiode, the diode will be mounted on a thermally insulated copper block and maintained at an elevated temperature using feedback control. Since the plasma background light plays a dominant role in the measurement noise, a model is used to analyze the noise contribution in regard to the time widths of the electronic output pulse and the signal integration gate. The building blocks of the detector electronics are GHz OpAmps and the ns analog switches. The method of differential gating [1] is used to cancel the charge injection induced by the high speed operation in the analog switch.\par \vspace{0.5em} \noindent [1]~X.~Wang, AIP Conf.\ Proceeding 333, Beam Instrumentation Workshop, Vancouver, Canada (1994). [Preview Abstract] |
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QP1.00009: An Upgrade of the DIII-D MSE Diagnostic that Utilizes the Counter-Injected Neutral Beam for Improved Current Profile Reconstruction C.T. Holcomb, S.L. Allen, R.F. Ellis, R. Geer, R.J. Jayakumar, M.A. Makowski, J.M. Moller, K.L. Morris, L.G. Seppala The motional Stark effect (MSE) diagnostic is being upgraded this year to take advantage of the new \hbox{DIII-D} neutral beam arrangement. Our three existing MSE views will be maintained as they are while a fourth view will be added to image the counter-injected neutral beam. The fourth view will use two separate imaging systems for coverage of the plasma from the magnetic axis to the low-field edge. These systems each contain a single polarization preserving fold mirror and a lens relay system to transfer light to a polarimeter. The details of this design are discussed. The addition of the new views will eliminate a gap in the profile where the existing MSE views cannot accurately discriminate $B_z$ and $E_r$. Uncertainty in $E_r(R)$ will be reduced by a factor of 2 in the core and 5-6 in the edge. Uncertainty in $B_z(R)$ will be reduced by 20-30\%, and the radial resolution will improve by about a factor of 3 in the core. [Preview Abstract] |
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QP1.00010: Density Fluctuation Measurements Using Fixed-Frequency Quadrature Reflectometers on DIII-D G. Wang, W.A. Peebles, T.L. Rhodes, E.J. Doyle, N.A. Crocker, X. Nguyen, L. Zeng, S. Kubota, G.R. McKee, G.J. Kramer, R. Nazikian, E.J. Valco, W.W. Heidbrink, M.A. Van Zeeland Two fixed-frequency reflectometers utilizing quadrature phase detection are employed on \hbox{DIII-D} to study both coherent and turbulent density fluctuations with high spatial/temporal resolution. Antennas can be orientated to launch and receive either O- or \hbox{X-mode} waves to probe different plasma regions. For coherent fluctuations, a 1-D phase screen model is applied to infer fluctuation levels. The result for an m=3/n=2 tearing mode agrees well with beam emission spectroscopy (BES). Reflectometry measurements of compressional Alfven modes at two cutoff locations will be compared with predictions of the eigenmode radial structure. For turbulent measurements, a 2-D full wave code is employed to deduce density fluctuation levels which are in reasonable agreement with BES. These results together with data from the \hbox{H-mode} pedestal will be presented. [Preview Abstract] |
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QP1.00011: Spatially Localized mm-Wave Backscattering Measure- ments of High-k Turbulence in the DIII-D Tokamak M.A. \hbox{Van Zeeland}, T.L. Rhodes, W.A. Peebles, X. Nguyen, J.C. DeBoo, R. Prater, M.A. Gilmore, W.M. Solomon A collective Thomson backscattering system has been installed on the DIII-D tokamak with the goal of measuring high-k turbulence such as expected from ETG modes. The diagnostic is based on microwave backscattering in X-mode polarization at 96 GHz to probe wavenumbers in the range of $\sim$30-40~cm$^{-1}$. This system utilizes a DIII-D steerable ECH antenna for launching and complements a similar system located at the tokamak midplane as well as an array of other turbulence diagnostics. Fluctuation localization, beyond that typically achievable through collective scattering, is accomplished by taking advantage of wavenumber matching criteria. The radial distribution of $\sim$35~cm$^{-1}$ turbulence in an Ohmic L-mode plasma is obtained and preliminary results presented. Application of a generic algorithm to de-convolve the spatial distribution of fluctuations from scattered power measurements will also be discussed. [Preview Abstract] |
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QP1.00012: Characteristics of H-mode Pedestals in Improved Confinement Regimes in DIII-D R.J. Groebner, A.W. Leonard, T.C. Luce, C.M. Greenfield, G.L. Jackson, T.H. Osborne, D.M. Thomas, M.R. Wade, M.E. Fenstermacher The characteristics of H-mode pedestals in improved confinement regimes are studied and compared to conventional ELMing H-mode discharges in DIII-D. These improved regimes include VH-mode, hybrid H-mode and Advanced Tokamak (AT) discharges. Initial results of this study show that across all regimes, 1)~confinement improves as the pedestal electron beta-poloidal [beta-pol$_{e(ped)}$] increases; 2)~the global beta-poloidal of the plasma is linearly related to beta-pol$_{e(ped)}$; and 3)~the scale length for the electron pedestal pressure profile is of similar magnitude. Thus, the initial results of this study show that there is a continuum of pedestal parameters with various confinement regimes falling within this continuum. In other words, the improved confinement in these regimes does not result from a dramatic change in pedestal characteristics. [Preview Abstract] |
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QP1.00013: Pedestal Optimization Through Shape Variation in DIII-D A.W. Leonard, T.H. Osborne, P.B. Snyder, R.J. \hbox{Groebner}, P. Gohil Pedestal pressure dependence upon the shape parameters triangularity and squareness are measured experimentally in DIII-D and compared to the calculated stability limit. The pedestal pressure is an important parameter in optimizing the performance of a tokamak, affecting global confinement and stability limits as well as ELM heat flux into the divertor. The pedestal pressure limit is found to vary significantly with squareness as well as the previously described triangularity dependence. Control of the pedestal pressure through squareness has the advantage of not affecting the divertor operation, fueling and pumping. High spatial resolution edge measurements from Thomson Scattering and CER determine the pedestal pressure profile just before an ELM. Numerous magnetic equilibria are constructed about this operational point by scaling the edge pressure and current about the measured values. The edge stability operational space in each shape configuration is then mapped out by an ELITE calculation of the ELM instability growth rate in each constructed magnetic equilibrium. [Preview Abstract] |
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QP1.00014: Coupled 1-D Plasma-Neutral Transport Simulations of Pedestal Behavior in DIII-D H-Mode Plasmas L.W. Owen, R.J. Groebner The relative roles of plasma and neutral transport in the structure of the H-mode density pedestal are investigated. 1-D (radial) equations for the plasma density and temperatures are solved iteratively with Monte Carlo calculations of the particle and energy source terms. With the pedestal height prescribed as a boundary condition, various forms are assumed for the particle diffusivity D in \hbox{H-mode}, including finite width barriers and functions with a single finite scale length step-up inside the separatrix. A general trend in \hbox{DIII-D} experiments is for the density pedestal width to vary inversely with the height, but this is not always true. As the density pedestal rises after the L-H transition the width often increases. It is shown that the observed temperature increase cannot explain the magnitude of the width increase. The simulations suggest that the transport coefficients change as the plasma evolves toward the first ELM. If simultaneously D decreases and the scale length of the step-up increases in the rise phase, the pedestal width increases with height. [Preview Abstract] |
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QP1.00015: Turbulence Dynamics Across the L-H Transition in DIII-D D.J. Schlossberg, R.J. Fonck, G.R. McKee, D.K. Gupta, M.W. Shafer, G.R. Tynan, C. Holland Turbulence characteristics and the effects of sheared poloidal flows on turbulence in the pedestal region of \hbox{DIII-D} plasmas preceding and across the L-H transition are examined using high time resolution time delay estimation (TDE) as well as nonlinear analysis techniques. Density fluctuation measurements are obtained with the recently upgraded, high-sensitivity beam emission spectroscopy system (BES). Poloidal flow shear rates in \hbox{L-mode}, measured with BES and CER, are found to be roughly one quarter the measured decorrelation rates, while in \hbox{H-mode}, they are roughly four times the decorrelation rate in the pedestal region, consistent with the E$\times$B shear suppression of turbulence model for the LH transition. Furthermore, \hbox{H-mode} eddy structures have nearly double the poloidal elongation as \hbox{L-mode} turbulence structures. The evolution of this structure asymmetry as well as counter-propagating modes prior to and during the L-H transition will be investigated using biorthogonal decomposition methods. [Preview Abstract] |
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QP1.00016: Investigation of Edge Pedestal Profiles in DIII-D W.M. Stacey, R.J. Groebner We have previously found [1,2] that, when experimental profiles of $E_r$ and $V_\theta$ and average values of momentum and heat transfer coefficients inferred from experiment were used as input, the radial profiles of $n,T_i$ and $T_e$ in the edge pedestal could be calculated directly from particle, momentum and energy balance requirements. The pedestal structure (localized steep gradient region) was found to result from a combination of an edge peaking in the ion outward $V_r$ caused by the ionization of recycling neutrals and a strong edge peaking in the inward ion $V_{pinch}$ caused by strong edge peaking in $E_r$ and $V_\theta$. There is theoretical evidence that the peaking in $E_r$ and $V_\theta$ may be caused by the peaking in $V_r$, suggesting the ionization of recycling neutrals as the ultimate cause of the edge pedestal structure. We are carrying out further calculations and examinations of the data to investigate this possibility.\par \vspace{0.5em} \noindent [1] W. M. Stacey, Phys. Plasmas, $\bf {11}$, 5487 (2004).\par \noindent [2] W. M. Stacey and R. J. Groebner, Phys. Plasmas, $\bf {12}$, 042504 (2005). [Preview Abstract] |
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QP1.00017: A Neoclassical Calculation of Toroidal Rotation Profiles in DIII-D R.W. Johnson, W.M. Stacey, J. Mandrekas A calculation model based on neoclassical viscosity and on momentum and particle balance was found to predict global momentum confinement times in DIII-D [1,2] and other tokamaks [3]. An extended model [4,5] has now been applied to calculate the radial distribution of toroidal rotation in several DIII-D shots in a variety of energy confinement regimes (L and H mode, ITB, QDB). Calculated toroidal rotation velocities were generally found to over-predict central experimental values by a factor of 1.5 to 5, reducing to a factor of 1.2 to 2 at $\rho\approx\,$0.8-0.9. Calculations are in progress to determine what part of this over-prediction is due to outward momentum convection and what part should be attributed to anomalous transport. The observed improvement in confinement of L-mode and ITB discharges with neon injection was predicted.\par \vspace{0.5em} \noindent [1]~W.M.\ Stacey, and M.~Murakami, Phys.\ Plasmas, $\bf {8}$, 4450 (2001).\par \noindent [2]~W.M.\ Stacey, and J.~Mandrekas, Phys.\ Plasmas $\bf {9}$, 1622 (2002).\par \noindent [3]~W.M.\ Stacey, and D.R.\ Jackson, Phys.\ Fluids $\bf {B, 5}$, 1828 (1993).\par \noindent [4]~W.M.\ Stacey, Phys.\ Plasmas $\bf {11}$, 3096 (2004).\par \noindent [5]~W.M.\ Stacey, Phys.\ Plasmas $\bf {9}$, 3874 (2002). [Preview Abstract] |
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QP1.00018: Comparison of the Improved GTNEUT with Monte Carlo for DIII-D Neutrals Experiments D. Zhang, J. \hbox{Mandrekas}, W.M. Stacey The Transmission-Escape-Probability method [1] has been developed and implemented in the GTNEUT code [2] as an accurate and efficient calculation of 2D neutral particle transport in complex plasma edge geometry. Three extensions of TEP methodology-anisotropic angular fluxes to calculate transmission coefficients [3], intra-region diffusion theory directional refinement of escape probabilities [4], and calculation of the local neutral energy distribution [4]-have been developed which significantly improve the range of validity. Detail comparisons of the improved GTNEUT with Monte Carlo methods for neutral measurements in DIII-D L- and H-mode shots have been made. Agreement of GTNEUT with Monte Carlo and the data has improved significantly.\par \vspace{0.5em} \noindent [1] W.M. Stacey and J. Mandrekas, Nucl. Fusion $\bf {34}$, 1385 (1994).\par \noindent [2] J. Mandrekas, Comput. Physics Comm, $\bf {161}$, 36 (2004).\par \noindent [3] D. Zhang, J. Mandrekas and W.M. Stacey, Contributions to Plasma Phys. $\bf {44}$, 45 (2004).\par \noindent [4] D. Zhang, ``Neutral Particle Transport in Plasma Edge Using Transmission/Escape Probability (TEP) Method," PhD Thesis, Georgia Institute of Technology (2005). [Preview Abstract] |
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QP1.00019: Comparison of DIII-D Phase Contrast Imaging Measurements in the Edge and Core Regions C.J. Rost, M. Porkolab, J.R. Dorris, K.H. Burrell During the last \hbox{DIII-D} run period, the phase contrast imaging (PCI) turbulence diagnostic acquired data in an upgraded configuration. The improvements include 10~MHz digitizers with a data record covering the entire discharge and a wavenumber range increased to 30/cm. The beampath was previously tangent to the LCFS but now passes through the LCFS and reaches r/a=0.8. The PCI was previously sensitive only to radial modes, but it is now sensitive to modes with finite poloidal wavenumber. Measurements of turbulence near the ITG range, particularly the S(k,f) spectra, now show a Doppler shift that was never observed previously. Analysis of this new data includes the variation of the magnetic field along the beam path. Comparisons with previous PCI measurements in the old beam geometry give us a more complete picture of the edge turbulence. The increased wavenumber range also allows us to examine how experimental frequency spectra of plasma turbulence depend on the wavenumber range of the diagnostic. [Preview Abstract] |
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QP1.00020: Characterization of Geodesic Acoustic Modes and a Lower-Frequency Zonal Flow Feature in DIII-D with BES D.K. Gupta, R.J. Fonck, G.R. McKee, D.J. Schlossberg, M.W. Shafer The spatiotemporal characteristics of geodesic acoustic mode
(GAM) and lower-frequency zonal flows are examined and compared
with theoretical predications as a function of safety factor,
$q$ and elongation. High time resolution poloidal turbulence
flow velocities are determined from multi-point beam emission
spectroscopy (BES) data using time-delay-estimation analysis.
Localized 2D measurements of density fluctuation are obtained in
L-mode plasmas over 0.6$ |
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QP1.00021: Core Turbulence Structures and $\rho_*$ Scaling Properties in H-Mode Plasmas G.R. McKee, R.J. Fonck, D.K. Gupta, D.J. Schlossberg, M.W. Shafer, J. Candy, C.C. Petty, M.R. Wade, R.E. Waltz The characteristics of long-wavelength density fluctuations ($k_\perp\rho_i < 1$) are examined in the core region (0.5 $<$ r/a $<$ 0.9) of H-mode discharges and compared to turbulence in L-mode discharges. Measurements are obtained with the upgraded 16-channel (4-radial $\times$ 4-poloidal), high-sensitivity beam emission spectroscopy system at DIII-D. The $\rho^*$ scaling of turbulence structures in hybrid scenario H-mode plasmas demonstrates that the radial correlation lengths scale closely with the local ion gyroradius, as predicted theoretically and observed in L-mode plasmas. Eddy spatial structures, in contrast, differ dramatically between L and H-mode plasmas, with H-mode turbulence exhibiting a highly tilted structure in the radial-poloidal plane, as measured via 2D spatiotemporal correlations. Whether this difference results from flow-shear, radial propagation, or inherent turbulence dynamics will be investigated via comparison to measured flow shear, as well as with comparisons of GYRO simulations. [Preview Abstract] |
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QP1.00022: Local Core Turbulence Dynamics via BES During q-Triggered Internal Transport Barriers M.W. Shafer, R.J. Fonck, G.R. McKee, D.K. Gupta, D.J. Schlossberg, M.E. Austin, R.J. Groebner Low order rational q surfaces have been associated with the triggering of ITBs in negative central shear \hbox{L-mode} discharges on \hbox{DIII-D}. Using the high sensitivity beam emission spectroscopy (BES) diagnostic, core fluctuation measurements were obtained for $r/a \sim 0.3-0.7$. Fluctuation levels are found to drop transiently during the appearance of low-order rational q surfaces, where the largest drop is found to be near the rational q surface. Local poloidal turbulence flow is simultaneously found to increase by $\sim$50\% during the event and is compared to CER E$\times$B flow. A spatial transition from broadband turbulence at larger radii to coherent modes in the core is observed. The coherent modes are present before and after the onset of the ITB. The time evolving spatial structure of these coherent modes is analyzed. The phase coupling of the coherent modes to broadband turbulence is examined using wavelet-based cross bicoherency for the non-stationary data. [Preview Abstract] |
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QP1.00023: Non-Driven Bulk Ion Velocity in DIII-D ECH \hbox{H-Mode}Discharges J.S. deGrassie, L.R. Baylor, W.M. Solomon H-modes generated with ECH have nonzero toroidal and poloidal ion fluid velocity, although the auxiliary torque input is negligible [1,2]. We have measured such toroidal and poloidal velocity profiles for both the bulk ion, helium, and the dominant impurity ion, carbon. We will show comparisons of these data with neoclassical predictions. The poloidal Mach number is computed from the experimental data and the magnitude tested for relevance in establishing the H-mode pedestal [3].\par \vspace{0.5em} \noindent [1]~J.S.\ deGrassie et al., Phys.\ Plasmas ${\bf 11}$, 4323 (2004).\par \noindent [2]~J.S.\ deGrassie et al., EX/6-4Rb, Proc.\ 20th IAEA Fusion Energy Conf., Vilamoura (2004).\par \noindent [3]~N.~Kasuya and K.~Itoh, Phys.\ Rev.\ Lett.\ {\textbf 94}, 195002-1 (2005). [Preview Abstract] |
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QP1.00024: A Recalculation of the Angular Momentum Flux in the Pfirsch-Schluter Regime S.K. Wong, V.S. Chan The toroidal angular momentum flux in the small rotation version of neoclassical transport theory in the Pfirsch-Schluter regime was calculated by Hazeltine [Phys.\ Fluids ${\bf 17}$, 961 (1974)]. The validity of the drift-kinetic-equation-based approach employed in that reference has recently been questioned by Catto and Simakov, [Phys.\ Plasma ${\bf 12}$, 012501 (2005)], who have also evaluated the flux from a fluid approach. We have found that Hazeltine's approach can be justified with the additional assumptions of a small poloidal field compared with the toroidal field and updown symmetric poloidal flux functions, besides the usual approximation of small gyroradius and large collisionality. We have revisited his calculations using an adjoint equation approach followed by expansions in Sonine polynomials, and have found substantial differences. We shall also compare the drift-kinetic and the fluid approaches. [Preview Abstract] |
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QP1.00025: Sustained High Beta Operation With Internal Transport Barriers on DIII-D E.J. Doyle Sustained high beta, high confinement operation with $\beta_N\sim\,$4, $\beta_T\sim\,$3-8\% and $H_{89}\sim\,$2.5 for $\sim$2~s has been achieved in discharges with internal transport barriers (ITBs) and negative central magnetic shear (NCS) on DIII-D. The minimum safety factor was maintained at $\sim$2, transiently leading to high bootstrap current fraction operation, $f_{BS}\sim\,$60\%. This combination of high beta and high confinement operation with moderate safety factor, $q_{95}\sim\,$5.5-3.5, also leads to high normalized fusion performance, with fusion gain factor spanning the anticipated ITER performance range. Transport barriers are observed in the ion temperature and rotation profiles, but not in the electron temperature profile, which is broad. These results address a critical issue for ITB operation in AT plasmas, obtaining sustained ITB profiles compatible with high beta limits. Previously, typical ITB operation has been limited to $\beta_N <\,$3 by pressure peaking. Pressure peaking in these discharges remains low due to a broad electron temperature profile. [Preview Abstract] |
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QP1.00026: Observation of Ion Transport Barriers Based on ${\bf T_e}$ Perturbations in QH-Mode Discharges in DIII-D J.C. DeBoo, T.C. Luce, C.C. Petty, K.H. Burrell The quiescent H-mode (QH-mode) in DIII-D is an ELM-free, high-confinement mode of operation that contains an internal transport barrier, ITB, in a stationary state that yields high performance plasmas. The stationary nature of the ITB and lack of ELM perturbations allows the barrier characteristics to be studied with the application of heat pulses initiated outside the barrier by monitoring the amplitude and phase of the heat pulses as they propagate toward and through the barrier. $T_e$ perturbations of 3-5\%, localized at $\rho=0.7$, were produced with ECH from 110~GHz gyrotrons. The $T_e$ heat pulses produced $T_i$ perturbations of 2-3\% and thus both electron and ion transport behavior can be studied. An ion ITB was much more prominent than was an electron ITB. Localized reduction in the amplitude of the $T_i$ perturbations was observed and can be used to infer an ion barrier that is a spatially localized region of significantly reduced thermal diffusivity centered at $\rho~0.5$ with a width of up to 15~cm. Perturbations in toroidal rotation velocity will also be discussed. [Preview Abstract] |
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QP1.00027: Electron Cyclotron Modification of Profiles in Quiescent Double Barrier (QDB) Discharges on DIII-D T.A. Casper, C.J. Lasnier, J.M. Moller, K.H. Burrell, P. Gohil, A.W. Leonard, P.B. Snyder, W.P. West, E.J. Doyle, J. Weiland High performance QDB conditions form when an internal transport barrier is created with intense neutral-beam injection into quiescent H-mode plasmas. We are exploring a variety of techniques to improve QDB parameters. In experiments using ECH/ECCD we observed a strong dependence on the q-profile when EC-power is used inside the core transport barrier. While strong electron heating is observed, we also observe a drop in the other core parameters; $T_i$, rotation, $n_e$ and impurities. These dynamically changing conditions provide a scan of core temperature profile ratio with $0.3<(T_e/T_i)_{axis}<0.8$ observed. We are exploring the correlation and effects of observed density profile changes with respect to these time-dependent variations in the temperature ratio. Thermal and particle diffusivity calculations indicate a consistency between the rise in temperature ratio and an increase in transport corresponding to the changes in density. [Preview Abstract] |
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QP1.00028: The Connection Between Upwind Dissipation, Entropy Production, Velocity-Space Resolution and Steady-States of Turbulence in GYRO Simulations J. Candy The connection between dissipation and steady states of turbulence in gyrokinetic simulations has been discussed by Krommes [1] who argued that nondissipative simulations cannot achieve a true turbulent steady state. The issue was revisited in the context of Eulerian simulations by Watanabe [2], providing a clear and precise confirmation of Krommes' analysis. In this presentation we show how the upwind advection schemes used in GYRO [3] provide the dissipation required for the achievement of steady states of turbulence. These steady states are grid-converged not only with respect to transport coefficients but also with respect to entropy. We put to rest the commonplace but ill-founded notions that Eulerian simulations (a) require velocity-space dissipation and (b) miss important velocity-space structure.\par \vspace{0.5em} \noindent [1]~J.A.\ Krommes and G.~Hu, Phys.\ Plasmas ${\bf 1}$, 3211 (1994).\par \noindent [2]~T.-H.\ Watanabe and H.~Sugama, Proc.\ of 20th IAEA Fusion Energy Conf., Vilamoura, 2004, Paper TH/8-3Rb.\par \noindent [3]~J.~Candy and R.E.\ Waltz, J.~Comput.\ Phys.\ ${\bf 186}$, 545 (2003). [Preview Abstract] |
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QP1.00029: Profile Similarity in GYRO Simulations of Bohm and GyroBohm Scaled DIII-D L- and H-mode Pairs R.E. Waltz, J. Candy, C.C. Petty Because the largest $\rho_*$ variation is only 1.6-fold, the experimental difference between Bohm and gyroBohm scaling in DIII-D dimensionally similar pairs corresponds to bottom and top of error bars in the temperature profile. Here we provide an error analysis from profile dissimilarity in full physics GYRO simulations of the best DIII-D L- and H-mode pairs. Dissimilarity can arise from imperfect experimental data or from the simulations failing to keep profiles fixed. The GYRO simulations track the experimental Bohm scaling of DIII-D L-mode pair (good similarity) and the super-gyroBohm scaling of the H-mode pair (poor similarity). We test the scalings with perfect similarity pairs obtained by “projecting” the experimental data from one discharge to another via the similarity rules, e.g. $T_2(r) = T_1(r)( \rho_{*1}/\rho_{*2})$, etc. The L-mode (H-mode) GYRO simulations of perfectly similar profiles have Bohm (exactly gyroBohm) scaling maintaining temperature gradients fixed. We also track the breakdown of these scalings by projecting beyond the 1.6-fold experimental variation in $\rho_*$. [Preview Abstract] |
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QP1.00030: Quantitative Studies of Nonlinear Interactions Between Shear Flows and Turbulence in Experiment and Simulation C. Holland, G.R. Tynan, R.J. Fonck, G.R. McKee, R.E. Waltz, J. Candy Using newly developed analysis algorithms, we present the first direct experimental measurements of the nonlinear shearing of drift-wave turbulence by a geodesic acoustic mode (GAM, a finite-frequency zonal flow). Density fluctuation measurements obtained via beam emission spectroscopy are combined with a velocity inference algorithm to provide a direct measurement of the shearing action. The measurements indicate that the shearing leads to a transfer of internal energy to smaller scales, in agreement with expectations from theory and simulation. To provide better context for these results, simulations from the GYRO gyrokinetic code are used to quantify the relative importance of zonal flow shearing vs. ``self-shearing'' of drift-waves, and differences in turbulence shearing by ``Rosenbluth-Hinton'' zonal flows with zero mean frequency and finite-frequency GAMs. The implications of strong GAM shearing in the tokamak edge are also discussed. [Preview Abstract] |
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QP1.00031: A Comprehensive Study of the Parametric Dependencies of Transport Using Gyrokinetic Simulations J.E. Kinsey, R.E. Waltz, J. Candy Nonlinear gyrokinetic simulations are used to systematically study the effects of E$\times$B shear, magnetic shear, safety factor q, $T_i/T_e$, collisionality, plasma beta, and elongation on turbulent energy, particle, and momentum transport due to ion temperature gradient (ITG) and trapped electron modes (TEM) in toroidal geometry using the GYRO code [1]. Previous work has tended to focus on studying ITG modes with adiabatic electrons for a single reference case. Here, we report on over 150 nonlinear kinetic electron simulations to be used for benchmarking and transport model development. Including kinetic electrons, we have verified that the effect of E$\times$B shear on both ITG and TEM transport is well modeled by a simple quench rule. In simulations varying q, the ion and electron energy transport exhibit a linear q-scaling while the particle diffusivity is insensitive to q. The nonlinear results are compared against quasilinear (QL) diffusivity ratios to assess the accuracy of QL theory on a per-mode basis.\par \vspace{0.5em} \noindent [1]~J.~Candy and R.E.\ Waltz, Phys.\ Rev.\ Lett.\ ${\bf 91}$, 045001 (2003). [Preview Abstract] |
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QP1.00032: Complete Physics for the Trapped Gyro-Landau Fluid Equations G.M. Staebler, R.E. Waltz, J.E. Kinsey A new system of equations for the fluid moments of the gyrokinetic equation has been recently developed which unifies trapped and passing particles. This new TGLF system adds trapped particle physics to the previously developed gyro-Landau fluid (GLF) equations. In the TGLF system a single set of moment equations is valid over the full range of drift-waves from low frequency trapped ion modes all the way up to electron temperature gradient modes. The linear growth rates of the TGLF system have been shown to be quite accurate (11\% average deviation) compared to gyrokinetic linear growth rates over a wide range of parameters. This benchmarking was done with limited physics however (shifted circle geometry, electrostatic, collisionless). The physics has now been extended to general geometry. Collisions and electromagnetic terms have also been added. Comparison of the linear stability results for the TGLF system with exact gyrokinetic results with the new physics will be reported. [Preview Abstract] |
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QP1.00033: Gyrokinetic Turbulent Heating F.L. Hinton, R.E. Waltz Expressions for turbulent energy transport and heating are derived, which are consistent with the drift-kinetic or gyrokinetic approximations for the turbulent fluctuations. The mean distribution function is the solution of a drift-kinetic equation that is the Hazeltine (1973) equation with additional terms containing the turbulent fluctuations. In the variables $u,\mu$ (where $u$ is parallel velocity and $\mu$ is magnetic moment), Liouville's theorem is satisfied, so the equation can be written exactly in conservative form. The energy flux and heating rate are identified from the energy moment of the mean drift-kinetic equation. The heating includes the energy moment of the parallel nonlinearity, which appears in the drift-kinetic equation for the fluctuations (which is appropriate for electrons). We also obtain new turbulent heating terms that depend on finite gyroradius. These are negligible for electrons, but could be significant for ions. Preliminary evaluations with GYRO DIII-D simulations suggest that the new turbulent heating terms, when radially integrated, represent no more than 5-10\% of the usual gyrokinetic power flows. [Preview Abstract] |
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QP1.00034: Methodology and Application of GCNM to Tokamak Transport H.E. St. John, L.L. Lao, M. Murakami The Predictive Transp project was recently approved and funded. The General Atomics contribution to this project is the development of a set of core solution methods that will be used to solve the typically very stiff diffusion equations that are encountered in recent theoretical models of particle, momentum and energy confinement. Here we describe the progress in this work, the special methods required to overcome numerical difficulties such as adaptive grids, changing support sets and dynamic dependent variable selection methods. A combination of steepest descent, Newton and trust region strategies and evaluation of Jacobians using algorithmic differentiation in conjunction with OpenMp and MPI parallel methods will also be incorporated. Applications to ITER time dependent and steady state AT discharges with fast wave, neutral beam and ECH current drive are presented. [Preview Abstract] |
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QP1.00035: Energetic Impurity Transport in Multispecies Gyrokinetic Plasmas C. Estrada-Mila, J. Candy, R.E. Waltz We summarize a systematic study of the behavior of energetic impurities using the GYRO code [1]. Historically, the focus in this area has been on wave-particle interaction of energetic ions (beam ions or alpha particles) with global MHD modes (Alfven eigenmodes or fishbones), with little or no discussion of effects arising from bulk plasma turbulence. Experimental results [2] and theoretical analyses tend to support a view where the interaction between energetic ions and a turbulent background is relatively unimportant. This is expected since at high energies particles have a large orbit width that, crudely speaking, averages over the turbulent transport coefficients. To understand this transition, we study the nonlinear gyrokinetic dynamics of (hot) helium ash as a function of the temperature ratio $T_{He}/T_D$ in the range 1 to 100. Preliminary results indicate that helium confinement improves as $T_{He}/T_D$ increases.\par \vspace{0.5em} \noindent [1]~J.~Candy and R.E.\ Waltz, J.~Comput.\ Phys.\ $\bf{186}$, 545 (2003).\par \noindent [2]~S.J.\ Zweben, et al., Nucl.\ Fusion $\bf{40}$, 91 (2000). [Preview Abstract] |
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QP1.00036: ECH Density Pumpout and Small Scale Turbulence in DIII-D K.-L. Wong, R.V. Budny, R. Nazikian, W.A. Peebles, T.L. Rhodes, R. Prater, C.C. Petty, R.J. Jayakumar Core density pumpout has been observed on DIII-D during ECCD experiments with 3~MW of injected ECH power absorbed at $\rho <\,$0.3 and 4~MW of counter neutral beam injection. The central plasma density dropped by almost 30\% in 150~ms after ECH was turned on. FIR scattering data showed a strong increase of turbulence at $k_\theta\sim\,$7~cm$^{-1}$, approximately synchronous with this density pumpout. The fluctuation scale is intermediate between $\rho_e$ and $\rho_i$ ($k\rho_i\sim\,$4). Mode identification (ITG, TEM [1] or other modes) is pending detailed data analysis. These results will be presented and compared with previously published data from TFTR [2] where a correlation between similar scale of turbulence and local electron energy transport was observed.\par \vspace{0.5em} \noindent [1]~C.~Angioni, et al., Nucl.\ Fusion $\bf {44}$, 827 (2004).\par \noindent [2]~K.L.\ Wong, et al., Phys.\ Lett.\ A $\bf {236}$, 339 (1997) and $\bf {276}$, 281 (2000). [Preview Abstract] |
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QP1.00037: Full Wave Modeling of Lower Hybrid current Drive with non-Maxwellian Distributions C.K. Phillips, H. Okuda, H. Qin, J.R. Wilson, P.T. Bonoli, J.C. Wright, M. Brambilla In lower hybrid current drive (LHCD) experiments, it is well known that the resulting current drive is determined by absorption on an electron distribution that itself has been significantly distorted by resonant interactions with the lower hybrid waves. Furthermore, previous theoretical studies with combined ray tracing and Fokker-Planck codes have indicated that energetic alpha particles in burning plasma devices such as ITER may parasitically absorb lower hybrid waves. The effects of these distributions must be included to accurately simulate LHCD scenarios in devices such as C-Mod and ITER. Recently, the full wave lower hybrid code, TORLH, has been used to simulate LHCD scenarios in plasma with cold ions and hot, thermal electrons. Modifications to TORLH to include non-Maxwellian electrons or ions will be described in this poster. Initial simulations that compare LHCD scenarios in plasmas with thermal and non-thermal electrons will be presented. [Preview Abstract] |
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QP1.00038: Self-Consistent Studies of Lower Hybrid Current Drive in ITER Relevant Regimes P.T. Bonoli, J. Liptac, R. Parker, A.E. Schmidt, J.C. Wright, R.W. Harvey, A.P. Smirnov Lower hybrid current drive (LHCD) is currently under consideration for off-axis current profile control in the ITER device. Upcoming LHCD experiments in the Alcator C-Mod tokamak will be performed in a wave physics regime close to that of ITER where B $\cong $ 5T, n$_{e }\cong $ 1 $\times $ 10$^{20 }$m$^{-3}$, and ($\omega _{pe }$/ $\omega _{ce})^{2 }\cong $ 1 and are therefore ideally suited to test physics issues related to off-axis current generation in a reactor regime. We have implemented the combined Fokker Planck -- ray tracing code (CQL3D-GENRAY) [1] at MIT and have used this code to carry out detailed studies of LH current drive for candidate target discharges in both ITER and C-Mod. The role of 2D velocity space effects in determining the driven current and radial power deposition profiles will be discussed as well as the efficacy of using ``compound'' LH grill spectra for controlling the location of power deposition. [1] R.W. Harvey and M.G. McCoy, Proc. of IAEA TCM on Advances in Simulation and Modeling of Thermonuclear Plasmas, Montreal, IAEA, Vienna (1993) p. 492. [Preview Abstract] |
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QP1.00039: Self-consistent full-wave simulation of wave heating and current drive in tokamak plasmas Atsushi Fukuyama, Akitsugu Sonoda The time evolution of a velocity distribution function affects the wave propagation and absorption in plasmas. Self-consistent analysis of wave heating and current drive requires to describe the time evolution of wave structures, velocity distribution functions and background plasmas. Using the integrated tokamak simulation code, TASK, we have carried out full wave simulations of ICRF minority-ion heating in a large tokamak and ECRF heating in a small tokamak. The formation of energetic ions enhances the absorption of ICRF waves and modify the radial profile of power deposition. The tunneling of the EC waves through the cutoff layer and the absorption near the upper hybrid resonance were described for the first time. In order to extend the applicability of the analysis, formulation of the full wave analysis and the Fokker-Planck analysis including the finite gyroradius effects will be also presented. [Preview Abstract] |
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QP1.00040: Analysis of Lower Hybrid Current Drive Scenario on Alcator C-Mod Analysis of Lower Hybrid Current Drive Scenario on Alcator C-Mod with a Full Wave Code J.C. Wright, P.T. Bonoli, M.A. Brambilla, R.W. Harvey, C.K. Phillips, H. Okuda The newly installed lower hybrid system on Alcator C-Mod are to be used to drive current and shape the current profile in the plasma. The deposition location is sensitive to the launched parallel refractive index, $n_\|$. The experiment can vary $n_\|$ between 2 and 3. We use the full wave lower hybrid code, TORLH (TORIC code modified for LH simulations), with non-Maxwellian electrons and the Ehst-Karney parameterized adjoint model for current drive, to predict the variation with $n_\|$ of the current drive deposition location in the plasma for a reference target plasma. We will discuss the importance of including full-wave effects such as focusing and diffraction in the calculation of the LH power deposition. In addition, we will discuss the role of 2D velocity space effects in the nonthermal electron distribution function on the LH power deposition. [Preview Abstract] |
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QP1.00041: Electron heating in the lower hybrid current drive problem Valentin Shevchenko, Vitaly Galinsky Due to resonant interaction with lower hybrid waves in the LHCD problem, part of electrons are accelerated and, as a result of this, the tail of energetic electrons is formed on the electron distribution function. This distribution can be unstable with respect to excitation of oblique Langmuir waves - so called fan instability [1, 2] that is an effective mechanism of pitch angle diffusion of the tail electrons. In this case the tail electrons interact simultaneously with both (i) LH waves that accelerate them (Landau resonance) and (ii) waves excited in the process of the fan instability (anomalous Doppler resonance) that lead to their pitch angle diffusion. Because velocity diffusion lines of electrons formed due to heir interaction with each type of waves intersect, this interaction can lead not only to pitch angle diffusion but also to heating of electrons mainly in perpendicular direction. We investigated this heating mechanism of the tail electrons. We studied the temporal evolution of the tail electrons temperature and energy of excited waves assuming constant amplitudes of pump LH waves. 1. Kadomtsev, B.B. and O.P. Pogutse, Sov. Phys. JETP, 26, 1146 (1967). 2. Shapiro, V.D. and V.I. Shevchenko, Sov. Phys. JETP, 27, 635 (1968). [Preview Abstract] |
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QP1.00042: Fusion Antenna Analysis Using the Modular Oak Ridge RF Integration Code (MORRFIC) Mark Carter Nonlinear and small scale effects occur in the RF near-fields of fusion antennas, including sheaths, ponderomotive effects, and gas build-up to self-generate plasma. Integrated modules in MORRFIC estimate the importance of, and the interplay between, these various effects. Modules include a linear RF Maxwell solver with azimuthally symmetric boundary conditions and plasma variations across and along field lines that can be used to estimate ponderomotive effects, sheath driving terms, and collisional dissipation. Other modules can be iterated with these solutions to study a weakly-ionized 2D model of collisional transport in RF self-generated plasma. A sheath mask is implemented to study different non-linear sheath models, and diagnostics are available to estimate non-linear effects. Models that resolve the sheath layer, with a time-averaged electron density inside the sheath, show strong absorption of RF power by confined electrons that sample the sheath. This absorption is influenced by the lower-hybrid and electron-plasma resonances and complicates the use of simplified sheath boundary conditions designed to relax the computational requirement to resolve the sheath. Imbalances in the RF voltage for sheaths connected along field lines are also found, pointing out the need for new theories to handle the DC rectification under these conditions. [Preview Abstract] |
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QP1.00043: Quasilinear Operator Development from Full-wave RF Plasma Simulations Lee Berry, R.W. Harvey, E.F. Jaeger Calculation of the plasma response to ICRF or LH fields is an important problem for plasma heating and current drive and requires iteration between RF-plasma and Fokker-Planck codes coupled by the quasilinear operator (QLO). Computation of the QLO using the fields from spectral codes such as AORSA poses complexities over typical from ray-tracing/Kennel-Engelmann solutions including a breakdown in the RPA because multiple Fourier modes describe a single plasma mode. We will present modified expressions for the QLO that addresses mode-correlation concerns. Another approach to calculating the QLO is to use time and toroidal angle averages of direct solutions to the Lorentz equations. Initial results indicate semi-quantitative agreement, taking into account that the Lorentz solutions are drift-orbit rather than field line averages. Progress on work to make more direct comparisons will be presented. [Preview Abstract] |
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QP1.00044: Amplitude and phase for ray splitting in linear wave conversion Andre Jaun, Eugene R. Tracy, Steve Richardson, Allan N. Kaufman Previously we have reported on the ray trajectories which arise in linear resonant plasma-wave conversion (mode conversion) in tokamak geometry, and have described the implementation of an algorithm to treat the associated ray splitting that occurs with conversion [1].~ We have shown how an incident ray gives rise to transmitted, converted, and reflected rays, noting the important effect of the poloidal magnetic field in the conversion of a magnetosonic wave to ion-hybrid waves [2]. We now report on the implementation of our algorithms for determining the amplitude and phase of each of these rays. Far from conversion regions, eikonal methods are used to transport amplitude, phase, and polarization on each ray. These solutions fit smoothly onto local approximations of the wave field as rays enter, split, and emerge from a conversion region in ray phase space.~Our aims are to expeditiously obtain energy-deposition profiles, as well as current-drive and flow-drive profiles, and to replicate the results of a full-wave treatment, using the much simpler set of ODEs associated with ray-tracing methodology. In this poster we describe the present status of our code development. [1] E R Tracy, A N Kaufman, A Jaun, Phys LettA 290 (2001) 309. [2] A N Kaufman, E R Tracy, A Jaun, A J Brizard, BAPS /DDP04 Poster PP1.085. [Preview Abstract] |
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QP1.00045: Time-Domain modeling of tokamak RF edge physics with the VORPAL code David Smithe, John R. Cary, Chet Nieter, Johan Carlsson, Cynthia Phillips, Dan D'Ippolito We present progress in the development of time-domain tokamak RF edge physics modeling. The target application for this modeling effort has transitioned to Tech-X's VORPAL simulation tool. This new tool provides significant advantages for speeding up the development process. Recent progress on this tool (see paper by Nieter et al., this conference) provides curved surfaces modeling. An existing fluid model can provide an algorithmic foundation for the semi-implicit linear magnetized plasma dielectric model. Demonstrated large scale parallel operation of VORPAL is also an important advantage. Additional theoretical development includes the introduction of a time-domain RF sheath model. A concern for RF heating of tokamaks is parasitic power loss that can occur in the plasma sheaths at the metallic and dielectric surfaces within the vessel. However, these effects can be very dependent on geometry specifics, necessitating a 3D modeling tool, such as VORPAL. We present a time-domain equivalent algorithm of the frequency-domain model in the reference, LRD-05-104, D. A. D'Ippolito et al., which estimates RF power loss to sheaths. [Preview Abstract] |
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QP1.00046: A quasi-optical ray tracing code for EC absorption and current drive Daniela Farina A new code GRAY has been developed for the quasi-optical (QO) propagation of a Gaussian beam of EC waves and the relevant absorbed power and driven current in a generic tokamak equilibrium [D. Farina, IFP-CNR Int. Rep. 2005, FP 05/1]. In the framework of the complex eikonal approach [E. Mazzucato, Phys. Fluids, 1, 1855 (1989)], the beam propagation is described by a set of mutually interacting rays. Several theoretical and numerical issues have been addressed and solved, mainly concerning the accurate solution of the complex dispersion relation. A fast numerical algorithm for the solution of the imaginary part of the QO dispersion relation has been implemented. Along each ray, EC wave absorption is computed solving either the weakly or the fully relativistic dispersion relation for EC waves (up to any order in Larmor radius expansion), and EC current drive by means of a neoclassical response function for the current [D. Farina, IFP-CNR Int. Rep. 2003, FP 03/5]. The code has been benchmarked against other existing codes, and used for calculations of EC driven current in ITER plasma. [Preview Abstract] |
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QP1.00047: New capabilities of TOPICA code: lower hybrid antennas and full toroidal plasmas V. Lancellotti, D. Milanesio, R. Maggiora, V. Kyrytsya, L. Valitutti, G. Vecchi, J.C. Wright, P.T. Bonoli TOPICA (TOrino Polytechnic Ion Cyclotron Antenna) code is a numerical suite aimed at the performance prediction and analysis of plasma-facing antennas. It is capable of handling real-life 3D antenna geometries (with housing, Faraday screen, etc.) as well as a realistic plasma model, including measured density and temperature profiles. Thanks to the approach underlying the code (i.e. the formal splitting of the problem into two parts: the vacuum region around the antenna and the plasma region inside the toroidal chamber), TOPICA can be extended to deal with lower hybrid (waveguide grill) antennas, as well as toroidal plasma. TOPICA has been upgraded to simulate and design lower hybrid (waveguide grill) antennas. On the other hand, to include plasma curvature effects, TOPICA can adopt the plasma impedance matrix computed independently via the fully toroidal TORIC plasma code. This way TOPICA both provides more accurate antenna parameters and yields the proper input (i.e. the electric field in front of the Faraday shield) to self-consistently run TORIC in a subsequent plasma analysis. In this work an account for the new capabilities of TOPICA will be presented. [Preview Abstract] |
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QP1.00048: Photo-neutron Production on HT-7 Superconducting Tokamak Yubao Zhu, Juequan Chen, Yanzhang Fu Experimental studies of photo-neutron production on HT-7 superconducting tokamak are presented. Time-resolved and spatial-distributed neutron fluxes are obtained using several polyethylene moderated BF$_{3}$ and $^{3}$He proportional counters as well as ZnS(Ag) scintillator. Comparisons of neutron production between helium and deuterium discharges are performed. Beside the commonly observed photo-neutron at the early times of plasma start-up and the late disruption stage, remarkable photo-neutrons are also observed on the discharges plateau period under low plasma density regime and non-inductively current driven conditions. The magnitude and time-evolution of neutron flux correlate very well with hard X-ray and $\gamma $ emissions. Photo-neutron flux distribution has a characteristic of toroidal asymmetry, which implies the localization of photonuclear reactions. The analyses confirm that photo-neutron productions are closely related to plasma density, loop voltage, MHD instability, energetic particles, impurity population and plasma-wall interactions. [Preview Abstract] |
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QP1.00049: High Field Induced Chemistry on Sharp Metallic Tips and Its Effect on Field Emission Currents C.H. Castano Giraldo, D.N. Ruzic, M. Aghazarian, J.B.O. Caughman Sharp tips of sub-micron radius, suitable to produce field emission, are being used to induce chemical reactions under a high electric field. We are studying the effect of vacuum chemical reactions on field emitters, and thus the emission current. Our final prospect is to achieve a better vacuum insulation capability to prevent breakdown in high voltage (HV) applications, such as in high-power antennas for fusion research. At present, concentrated field emission limits voltage hold off by about one order of magnitude below the theoretical predicted limit of several GV/m. Preliminary experiments in which chlorine (originating from CCl$_{4})$ reacted with copper (broad electrode) improved our voltage hold off from 50 to 94.7 MV/m, pointing to the possibility that chemistry can play a substantial role in HV systems. Different materials including nickel, copper, tungsten, and stainless steel are being tested. [Preview Abstract] |
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QP1.00050: Plasma Response to Waves in Arbitrary Magnetic Field Geometry Brent Goode, John R. Cary, L.A. Berry We examine the effect that complicated magnetic geometries and collisions have on the propagation and absorption of radio frequency waves in a plasma. This is accomplished by calculating a conductivity tensor for the plasma using the method of integration along characteristics. The geometric effects included are magnetic field curvature, perpendicular gradients, quadratic parallel gradients, and the full effect of linear parallel gradients. Previous theories treated only the lowest order effects of linear parallel gradients and treated collision in an arbitrary manner. The effect of higher order parallel gradients and collisions is to reduce or eliminate instances where the imaginary part of the Z function is negative. Other new term have no significant effect. One dimensional calculations of wave propagation and absorption in a tokamak are performed using the AORSA1D code. The result of these calculations demonstrate a clear change in power absorption due to the proper treatment of collisions and higher order effects of linear parallel gradients. [Preview Abstract] |
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QP1.00051: A Comparison of Fully Relativistic and Non-relativistic Ray-tracing of Electron Bernstein Waves E. Nelson-Melby, R.W. Harvey, A.P. Smirnov, A.K. Ram Electron Bernstein waves (EBW) can be important for heating and driving current in high density, high temperature plasmas. Mode- converted EBWs are especially of interest in overdense ($\omega_{pe}>\omega_{ce}$) machines such as spherical tokamaks (ST), because few other types of plasma waves can propagate inside such a plasma. EBWs usually damp near cyclotron resonances, where full relativistic effects are important. This would be especially true in an ST reactor, such as the ARIES-ST study, where $T_{e0} \simeq 16 keV$. Recently, a fully-relativistic, high-frequency dispersion relation has been added to the ray-tracing code GENRAY [1]. This portion of the code was written by A.K. Ram and E. Nelson-Melby. Recent work shown at the 16th RF Topical Conference [2] have shown that there can be significant differences between the non- and fully-relativistic EBW dispersion relations. Ray- tracing of EBWs mode-converted through the O-X-B scenario at the edge of the ARIES-ST tokamak will be presented. [1] A.P. Smirnov and R.W. Harvey, Bull. APS 40, Ab. 8p35 (1995). [2] A.K. Ram, J. Decker, and Y. Peysson, J. Plasma Phys., accepted for publication (2005); E. Nelson-Melby et. al., Poster B-08. (2005). [Preview Abstract] |
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QP1.00052: Electron Cyclotron Emission from Nonthermal Distributions R.W. Harvey, A.P. Smirnov, N.M. Ershov, E. Nelson-Melby, S. Coda, G. Taylor, M.E. Austin, R. Prater The GENRAY ray tracing code incorporates a solution of the RF energy transport equation (emission and absorption along WKB rays) including the effects of nonthermal electron distribution functions. Distributions are from self-consistent RF solutions of the bounce-averaged Fokker-Planck equation using the CQL3D 2V-1R code. We present computed spectra for two experimental situations: (1) EBW emission from electron distributions in NSTX due to future EBWCD experiments. In this case, the calculated transport of the EBW emission from overdense (omega{\_}pe $>$ omega{\_}ce) NSTX plasma to the plasma edge accounts for the effects of BXO mode conversion whereby EBW waves transform to X-mode, then O-mode near the omega{\_}pe=1 surface; and (2) EC emission in present low density DIII-D ECH experiments. A 27 keV central ECE temperature is calculated, in close agreement with the experimental value, for a plasma with 6.5 keV Thomson scattering temperature. Acknowledgment: USDOE Grants DE-AC03-99ER54463 and DE-FG03-02ER54684, and CRPP-EPFL. [Preview Abstract] |
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QP1.00053: Determining the electric field in a plasma by measuring the deflection of an ion beam Xi Chen, Paul Matthew Schoch, Kenneth A. Connor A design study has been completed on reconstructing the electric field in magnetically confined plasma by measuring the deflection of a non-confined heavy ion beam. The concept is to send an ion beam through a plasma device where the magnetic field $\mathord{\buildrel{\lower3pt\hbox{$\scriptscriptstyle\rightharpoonup$}}\over {B}} $ is well known and to measure the beam position change at a detector. Either the injection angle or the energy of the ion beam can be changed to allow multiple measurements with differing amounts of plasma penetration. Calculations using a simple geometry show that the reconstruction is reasonable when the electric field $\mathord{\buildrel{\lower3pt\hbox{$\scriptscriptstyle\rightharpoonup$}}\over {E}} $ is relatively strong (at least 1 percent of $\mathord{\buildrel{\lower3pt\hbox{$\scriptscriptstyle\rightharpoonup$}}\over {v}} \times \mathord{\buildrel{\lower3pt\hbox{$\scriptscriptstyle\rightharpoonup$}}\over {B}} )$. Results of the design study are presented for both a slab and a cylindrical geometry. The electric field is reconstructed from a series of measurements using a simple layer model. This diagnostic is simple and inexpensive compared to standard electric field diagnostics such as a Heavy Ion Beam Probe. Future work will be to model an existing device such as HSX. [Preview Abstract] |
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QP1.00054: Development of the Motional Stark Effect with Laser-Induced Fluorescence (MSE-LIF) Diagnostic Elizabeth L. Foley, Fred M. Levinton The motional Stark effect with laser-induced fluorescence (MSE- LIF) diagnostic is under development to extend the MSE magnetic pitch angle diagnostic to lower fields ( $<$ 0.5 T) and enable measurement of magnetic field magnitude as well as direction. The technique involves injecting a low energy-spread neutral hydrogen beam (30 kV, 30 mA) into plasma, and using a collinear laser to excite transitions from the n=2 to n=3 atomic states in the beam atoms. The subsequent fluorescence from the same transition (Balmer-alpha, near 650 nm for the Doppler-shifted beam) is observed, and its splitting and polarization due to the E = v X B electric field in the beam frame is used to determine the background magnetic field magnitude and direction. This poster will present recent results from MSE-LIF development, including magnetic field measurements at very low field ($<$ 0.01 T) in neutral gas based on an enhanced LIF phenomenon, a comprehensive collisional-radiative model which determines the population fractions in the n=1, 2 and 3 states of the beam as the states mix in applied magnetic and electric fields, as well as upgrades to the experimental apparatus that will enable measurements in intermediate fields (0.01 - 0.2 T) in plasma. [Preview Abstract] |
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QP1.00055: Imaging bolometer development for application to fusion reactor diagnostics B.J. Peterson, A.G. Alekseyev, S. Konoshima, N. Ashikawa, H. Parchamy, M. Sasao, M. Isobe, Y. Miura The imaging bolometer concept is based on a thin foil which absorbs the broad-band radiation and/or energetic particles from the plasma. The resulting temperature change in the foil is measured by an infrared camera located outside the vacuum vessel. Development of imaging bolometers is being carried out for applications in bolometry and lost alpha diagnosis for fusion reactors. In the case of an imaging bolometer, placing the foil behind a pinhole camera provides a two-dimensional image of the plasma radiation. In the case of a lost alpha diagnostic the foil is placed behind multiple layers of thin foils with one dimension being used for energy discrimination and the other layer being used for pitch angle discrimination. The work described includes the operation of imaging bolometers on the Large Helical Device and the JT-60U Tokamak, calibration experiments, testing prototype lost alpha diagnostic detectors on an ion beam facility and the design of an imaging bolometer and a lost alpha diagnostic for ITER. [Preview Abstract] |
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QP1.00056: Measurements of Energetic Ions injected by Tangential Neutral Beam Injectors Using a Hybrid Probe Kenichi Nagaoka, Mitsutaka Isobe, Kouji Shinohara, Masaki Osakabe, Keisuke Matsuoka, Shoichi Okamura Understanding of energetic ion behavior is one of the most important problems for burning plasmas, for example ITER. So far, most experimental researches for energetic ion confinement have been limited at peripheral region of the core plasmas. We will propose the new technique to measure energetic ions in the core plasma, and discuss the first results of the demonstration to measure energetic ions injected by neutral beam injectors (NBI) in the compact helical system (CHS). The Langmuir probe including thermocouple, which is named hybrid probe, is a directional probe limiting the ion collecting surface. The difference of co- and counter-streaming ion flux measured by the hybrid probe is considered to be dominated by energetic ion component in NBI heated plasmas, while it is generally utilized to measure ion flow velocity in plasmas. The demonstration of this method has been performed in two plasmas; one is neutral beam modulation and the other is a discharge with MHD bursts excited by energetic ions. The results of hybrid probe measurements have been compared with a neutral particle analyzer. [Preview Abstract] |
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QP1.00057: A Convenient Method of Calibrating Relative Sensitivity of Multi-Channel Thomson Scattering Diagnostic System Kazumichi Narihara, Ichihiro Yamada, Hiroshi Hayashi LHD is equipped with a 200-channel Thomson scattering diagnostic, which can yield electron temperature {\textbf{\textit{Te}}} and density {\textbf{\textit{ne}}} profiles along the major radius passing the magnetic axis. Though the {\textbf{\textit{Te}}} profiles are of fairly good quality, the {\textbf{\textit{ne}}} profiles are far from satisfactory due to incomplete sensitivity calibration of each polychromator. In order to elaborate this issue more efficiently, we are developing a versatile method of calibration: To simulate light coming from the scattering volumes in a plasma and passing the view window, we used light diffusively reflected from a BaSO$_4$-coated plate set on the surface of the viewing window in the airside, which is illuminated by 10 ns pulse light from an OPO with wavelength scanned between 600-1060 nm. Combining the scattering length and the solid angle for each spatial-channel with the polychromator-sensitivity thus obtained, we can obtain the coefficients necessary for deducing {\textbf{\textit{ne}}} profile. We will examine the {\textbf{\textit{ne}}} profiles at the position where the structures are observed on {\textbf{\textit{Te}}} profiles. [Preview Abstract] |
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QP1.00058: Microprobe Development at the Basic Plasma Science Facility Patrick Pribyl, Walter Gekelman, Noam Katz, Mio Nakamoto, Janet Stillman, Franklin Chiang Sub-Debye length phenomena remain largely unexplored in laboratory experimental plasma physics. At BaPSF we are developing a variety of techniques to explore this regime. Development falls into two categories: first, several multi-tip Langmuir probes have been assembled by hand. These consist of up to four tips arranged in a row, composed of glass capillary tubes each pulled to a narrow tip and threaded with 25 micron wire. The closest tip to tip spacing is about 40 microns. Second, MEMS techniques have been used to fabricate a series of microprobes having regular spacing at 20 microns tip to tip, with pairs separated by 60 microns. These efforts afford high precision, although the probes are fairly short. Since the target structures exist on a Debye length scale, the number of electrons involved in the LAPD plasmas of interest might be less than 10$^{6}$, for a charge of the order of 100 pC. Contributing to the problems, some of these structures are predicted to move at a significant fraction of the electron thermal speed, requiring sub-nanosecond time resolution. Consequently fast amplifiers must be located quite close to the probes; the 2 GHz amplifiers and the probe construction techniques are discussed. [Preview Abstract] |
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QP1.00059: Microwave Imaging Reflectometry on NSTX Z.G. Xia, C.W. Domier, Y. Liang, N.C. Luhmann, Jr., J. Wang, L. Yang, E. Mazzucato, H. Park A 3-D Microwave Imaging Reflectometry (MIR) instrument is proposed for NSTX. Reflections from multiple, extended plasma cutoff surfaces are imaged onto a 2-D mixer array (8x2 or 8x4 elements, depending upon the size of the viewing window). Through the simultaneous launch and collection of up to 16 probe frequencies covering a frequency span of 40-70 GHz, the result is a 3-D visualization (up to 8x4x16 or 512 channels) of plasma density fluctuations including IRE (sawtooth), EPMs and ``fishbones,'' CAE/GAE and TAE/rTAE modes. The resultant 3-D images of turbulence will address the details of ITG, zonal flows and streamers. Technical details regarding the MIR system design, including a number of implementation options (funding dependent), will be presented. [Preview Abstract] |
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QP1.00060: Performance and Planned Upgrades to the 2-D ECEI Diagnostic on TEXTOR C.W. Domier, N.C. Luhmann, Jr., M. Spillane, J. Wang, Z.G. Xia, H. Park, E. Mazzucato, I.G.J. Classen, M.J. van de Pol, A.J.H. Donne In collaboration with PPPL and FOM, UC Davis has developed and installed a 128 channel 2-D Electron Cyclotron Emission Imaging (ECEI) instrument on TEXTOR. In its present form, each array element of the 16 channel mixer array measures plasma emission at 8 simultaneous frequencies to form a 16x8 image of the plasma electron temperature distribution. Details regarding the ECEI system design and current status will be presented along with recent TEXTOR plasma data. A planned upgrade in 2006 will increase the plasma coverage from the 16 cm (vertical) x 6 cm (horizontal) to 18 cm x 12 cm. Designs will also be presented of a proposed 576 channel (24x24 image) system which is capable of visualizing temperature fluctuations over an area of 24 cm x 24 cm. [Preview Abstract] |
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QP1.00061: Frequency Measurements of the 110 GHz Gyrotron used for Fast Ion CTS Diagnostics at TEXTOR P.P. Woskov, S.B. Korsholm, F. Meo, E.L. Tsakadze, J.W. Oosterbeek, K. Jakubowska, J. Scholten, C. Tito High resolution frequency measurements during TEXTOR plasma operation have been carried out of the 110 GHz gyrotron as used for fast ion collective Thomson scattering (CTS) diagnostics. A pickoff sample of the gyrotron beam was frequency downshifted by both homodyne and heterodyne methods and fast Fourier transformed (FFT) by a 300 MHz bandwidth digitizing oscilloscope. The gyrotron was operated in a macro pulse of 100, 2 ms pulses distributed over about a 1 s time period during TEXTOR plasma flat top. The frequency was sampled near the beginning and end of the gyrotron macro pulse. The gyrotron frequency was found to be relatively clean and reproducible from plasma shot to plasma shot except for $\pm $ 18 MHz components at about -40~dB in some data. During a single macro pulse a downward frequency drift of 14~MHz was observed. An instrumentally limited instantaneous linewidth of $<$100 kHz at half maximum and $<$200 kHz at -40 dB was also observed. The TEXTOR ICRH system did not fire reliably during these measurements to asses its effect on the gyrotron spectrum. [Preview Abstract] |
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QP1.00062: Preliminary results of the new fast ion CTS systems at TEXTOR and ASDEX Upgrade S.B. Korsholm, H. Bindslev, F. Meo, S. Michelsen, P.K. Michelsen, S.K. Nielsen, E.L. Tsakadze, J. Egedal, P.P. Woskov, J. Hoekzema, F. Leuterer, E. Westerhof Upgraded and new fast ion collective Thomson Scattering (CTS) diagnostics using high power gyrotrons have been implemented on TEXTOR and ASDEX Upgrade tokamaks, respectively. Commissioning of the systems is currently underway and first results have been obtained from TEXTOR. The slowing down of fast ions due to the switching off of the NBI will be discussed, and the measured time evolution will be compared to numerical simulations. The commissioning activities on ASDEX Upgrade of the 105 GHz receiver and the new 1 MW, 10 s gyrotron will be described. Fast ion physics will be a key research goal on ITER, where confined alpha particles will play a dominant role. CTS using gyrotrons has the potential to satisfy this prime ITER diagnostic need for measuring the spatially localized velocity distributions of confined fast ions. [Preview Abstract] |
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QP1.00063: Local measurement of the plasma emission using the Zeeman effect in TRIAM-1M tokamak Taiichi Shikama, Shinichiro Kado, Hideki Zushi, Atsushi Iwamae, Satoru Tanaka The position of plasma emission is measured using the Zeeman patterns in the spectral shape in the TRIAM-1M super-conducting tokamak. From the spectral measurement which covers the poloidal cross section, it is shown that the beryllium-like oxygen ions are distributed in the region slightly inside the separatrix and neutral hydrogen and helium atoms are localized in the boundary region of the poloidal limiter shadow. For obtaining the better spatial resolution, the $\sigma $ components of the emission spectrum are resolved by attaching a linear polarizer in front of the object lens, and the Zeeman pattern in the spectral shape is calculated using the quantum mechanical method. In the boundary region, low temperature atoms having inward flow velocity of about 1 to 4 km/s across the magnetic field line are observed for hydrogen and helium. The time resolved measurement of position of emission and flow velocity suggests that the change in the position of magnetic axis leads to the change of the plasma-wall interaction such as recycling rate or inward neutral flow velocity which might be driven by the radial neutral pressure gradient. [Preview Abstract] |
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QP1.00064: Improvement of X-ray Imaging Crystal Spectrometers for KSTAR Sang Gon Lee, J.G. Bak, M. Bitter, U.W. Nam, M.K. Moon, J.K. Cheon The X-ray imaging crystal spectrometers for the KSTAR tokamak will provide spatially and temporally resolved spectra of the resonance line of helium-like argon (or krypton) and the associated satellites from multiple lines of sight parallel and perpendicular to the horizontal mid-plane for measurements of the profiles of the ion and electron temperatures, plasma rotation velocity, and ionization equilibrium. The spectrometers are consisted of a spherically bent quartz crystal and a 10 cm x 30 cm large 2D position-sensitive multi-wire proportional counter. A 2D detector with delay-line readout and supporting electronics has been fabricated and tested on the NSTX tokamak at PPPL. Position resolution and count rate capability of the 2D detector are still need to be improved to meet the requirements. Hence, a segmented version of the 2D detector is under development to satisfy the requirements. The experimental results from the improved 2D detector will be presented. [Preview Abstract] |
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QP1.00065: Feasibility study on local three-dimensional soft x-ray tomographic imaging for KSTAR plasmas Junghee Kim, Seung Hun Lee, W. Choe To obtain a 3-D internal soft x-ray emissivity structure of tokamak plasmas at a specific shot time, a direct inversion from 2-D detector arrays is required. The direct local 3-D soft x-ray imaging will be a powerful diagnostic tool for monitoring core plasmas. In order to pursue it, we introduce in this paper our sophisticated design of the system matrix and the newly modified fast MEM (maximum entropy method) without prior knowledge. Because the spatial accessibility of 2-D cameras in KSTAR is very limited, the posterior probability density function of the fast MEM inversion is modified and the geometrical characteristics around the local sought-for plasma are considered to calculate 3-D system matrix. A partly toroidal sought-for emission region is divided into several thousands of trapezoidal voxels instead of using cubical voxels. All tests for this study were performed on KSTAR-like 3-D emission phantoms and the reconstruction region is about a quarter of the whole plasma volume when the tangential cameras are installed at the same horizontal port of KSTAR. We expect that the tangential camera system for the local 3-D tomography can be consisted of one or two miniaturized CCD, MPGD (micro-pattern gas detector), 2-D photodiode arrays, and so on. [Preview Abstract] |
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QP1.00066: Initial design of the soft x-ray tomographic diagnostic array system for KSTAR plasmas W. Choe, Junghee Kim, Seung Hun Lee, J.K. Rhee A soft x-ray (SXR) tomographic array system is a powerful diagnostic tool for observing inner structure and various physical activities of tokamak plasmas. The SXR diagnostic system for KSTAR is based on the six photodiode detector arrays arranged in a poloidal plane with total of 192 detector channels. The newly developed fast MEM (maximum entropy method) combined with SVD (singular value decomposition) was applied to the tomography test with KSTAR-like emission phantoms. The spatial setup of the components was optimized by geometrical calculation and ray-tracing for accurate SXR analyses. The radiation-resistive AXUV-16ELG detector array was chosen for surviving high dose of radiation during the long pulse operation in KSTAR, and the high-speed ($\sim $600 kHz) in-vacuum preamplifier with high gain ($\sim $10$^{5})$ was developed and tested to observe high speed MHD activities. The spatial optimization of the detector in each array will be adjusted by the miniaturized remote motion-control system, which consists of a small in-vacuum motor and a controllable PXI module. Moreover, in order to maintain the performance of the detector, a controllable thermo-electric cooler and a shutter will be mounted on the array for long-pulse experiments on KSTAR. [Preview Abstract] |
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QP1.00067: Plasma Imaging on KSTAR via Simultaneous MIR/ECEI Z. Shen, C.W. Domier, N.C. Luhmann, Jr., J. Wang, H-C. Wu, Z.G. Xia, H. Park A plasma imaging diagnostic is proposed for the KSTAR tokamak to image electron density fluctuations via Microwave Imaging Reflectometry (MIR), and electron temperature profiles and fluctuations via Electron Cyclotron Emission Imaging (ECEI). Both systems will be inherently 2-D in nature, and capable of simultaneously imaging fluctuations over an extended portion of the KSTAR plasma. System details, including preliminary optical and electronics designs, will be presented along with a discussion of a number of implementation options which include plasma coverage, spatial resolution, number of channels (nominal 32x32=1024 for ECEI, 16x16=256 for MIR), tuneable vs. instantaneous RF bandwidths. [Preview Abstract] |
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QP1.00068: Microwave Imaging Reflectometry in LHD Soichiro Yamaguchi, Yoshio Nagayama, Shigeru Inagaki, Rostyslav Pavlichenko, Yuichiro Kogi, Atsushi Mase The Microwave Imaging Reflectometry (MIR) has been developed in Large Helical Device (LHD) to obtain the the 2-D imaging of the electron density fluctuation in order to investigate of the microturbulence and the magnetohydrodynamic instability. We use the X-mode since its cut off surface is more perpendicular to the illumination direction than the O-mode in LHD. The reflection wave propagates through the imaging optics into the heterodyne detection system. In the previous setup the imaging antenna was fixed and the reflection wave could not be detected frequently in the plasma experiments. The simulation of the wave propagation suggests that the reflection wave does not enter the imaging optics due to tilting of the reflecting surfaces in most operation condition. The new optics has the first mirror, which is driven by supersonic actuators. By steering the mirror to the optimum angle the reflection wave can be received at the detector. It is expected to enable observations in wider range of plasma parameters. [Preview Abstract] |
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QP1.00069: Wideband PAA Technology for MIR Y. Liang, C.W. Domier, N.C. Luhmann, Jr., Z.G. Xia, L. Yang, H. Park Advanced millimeter-wave imaging technology is under investigation at UC Davis in support of Microwave Imaging Reflectometry (MIR). Foremost of these new technologies is a beam shaping phased antenna array (PAA) system for use in MIR on shaped plasmas such as NSTX. Using microelectromechanical systems (MEMS) delay lines, a true time delay controlled PAA is being developed as an artificial lens with voltage-controllable focal length for launching the illumination beam. Control of the ``lens'' permits the curvature of the illumination beam to be matched to that of the target plasma over a wide range of frequencies. An NSTX design will be presented along with preliminary testing results. [Preview Abstract] |
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QP1.00070: LASER ACCELERATION AND RADIATION GENERATION |
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QP1.00071: 2D Particle-in-cell simulations of laser pulse propagation in plasma channels Rodolfo Giacone, Dimitre Dimitrov, David Bruhwiler, John Cary, Cameron Geddes, Eric Esarey, Wim Leemans In the absence of optical guiding, diffractive spreading of a laser pulse imposes a severe limitation on the acceleration length and maximum electron energy in the laser wake field accelerator (LWFA). Optical guiding of a laser pulse via plasma channels can overcome these difficulties. Energy efficient coupling of laser pulses into and through plasma channels is very important for optimal LWFA performance. We have run parameter studies on channel guiding using the PIC code VORPAL. We considered the effects that density ramp length and the position of the laser pulse focus have on coupling into the channel. We also considered enhanced leakage of laser energy transversely through the channel walls and the effects of tunneling ionization of a neutral gas on the guided laser pulse. The results of our simulations show that density ramps longer that one Rayleigh length have a negative effect on pulse-channel coupling and produce higher energy loss. When the pulse is focused on the channel entrance, large spot size oscillations result in increased energy leakage. Refocusing the pulse can reduce spot oscillations resulting in a reduction of leakage by a factor of three. Using power spectral diagnostics, we are able to separate pump depletion from energy leakage in the channel. In all cases the observed pump depletion is roughly four times larger than expected from 1D theory. [Preview Abstract] |
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QP1.00072: Evolution of the laser pulse profile and spectrum in particle-in-cell simulations of laser-plasma accelerators D.A. Dimitrov, R.E. Giacone, D.B. Bruhwiler, J.R. Cary, P. Messmer, C. Nieter, C. Geddes, E. Esarey, W. Leemans Propagation of intense laser pulses in plasmas has demonstrated accelerating electric fields of the order of 100 GV/m and the production of high quality electron beams with energies near 100 MeV. Moreover, laser pulse guiding (by either self-focusing or plasma channels) is necessary to further increase the electron energy in laser-plasma accelerators. The laser wakefield excitation mechanism produces specific signatures in the frequency spectrum and intensity profile of the laser pulse. Thus, it is of considerable interest to determine the frequency spectrum and intensity profile of a guided laser pulse interacting with plasma. We discuss how the calculation of the spectrum of the laser intensity as a function of frequency or wavelength was implemented in the VORPAL PIC code and then applied to study the mode structure of laser pulses propagating in specific plasma density profiles. These results will be compared to laser-plasma accelerator experiments at LBNL and elsewhere. [Preview Abstract] |
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QP1.00073: Wigner diagnostics for the photon accelerator Frederico Fiuza, Jorge Santos, Joao Dias, Samuel Martins, Ricardo Fonseca, Luis Silva The wake driven by a short intense laser pulse propagating in a transparent plasma can be used to up-shift or downshift another ultrashort electromagnetic pulse co-propagating with the wake (the photon accelerator). Photon acceleration/deceleration can be a powerful diagnostic for the structure of the wake, and it provides a mechanism to tune the frequency of short laser pulses. We have performed detailed fully relativistic one-dimensional, two-dimensional, and three- dimensional particle-in-cell simulations with osiris 2.0 of the photon accelerator, over a wide range of realistic laboratory conditions for the neutral gas background, the plasma background, the wake field driver, and the probe. Our study relies on the systematic use of the Wigner transform for the electromagnetic field, thus allowing for a complete diagnostic of the frequency modulations in the probe pulse. We demonstrate controlled tunability of the probe pulse, as well as the detailed control of the laser chirp. [Preview Abstract] |
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QP1.00074: Nonlinear laser pulse and plasma wave evolution in laser wakefield accelerators Eric Esarey, C.B. Schroeder, B.A. Shadwick, W.P. Leemans The evolution of short laser pulses in underdense plasmas is analyzed for arbitrary laser intensity. Some of the new results derived include expressions for the nonlinear group velocity of the laser pulse, the nonlinear phase velocity of the wakefield, the nonlinear frequency shift, and the laser pulse envelope distortion. New scaling laws are presented for the pump depletion length and the electron dephasing length. Analytical results are compared to numerical calculations based on fluid models. In the low laser intensity limit, the depletion length is much longer than the dephasing length, thus implying that some form of phase matching is necessary. In the high intensity limit, the depletion and dephasing lengths are approximately equal, thus allowing for high efficiency coupling of laser energy to the wake and the accelerated particles. Implications for an optimized design of a 1 GeV accelerator stage are discussed. [Preview Abstract] |
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QP1.00075: High power laser guiding in a capillary discharge waveguide at LBNL Bob Nagler, Csaba Toth, Wim P. Leemans, Anthony Gonsalves, Simon Hooker, Catalin Filip, Tom Cowan We will present progress on guiding experiments of a high power, short pulse laser in a capillary discharge waveguide. In the experiments, capillaries are used that are produced by laser machining 200-400 micron slots in sapphire plates. The capillary is then filled with hydrogen gas (1-9 10$^{18}$cm$^{3})$ and fully ionized by an electric discharge. Ohmic heating and diffusion create a parabolic plasma density profile that is ideally suited to guide the laser beam. Past experiments have shown guiding efficiencies of up to 90{\%}, with clean Gaussian mode profiles [1]. We will present progress towards guiding record-high intensities (10$^{18 }$W/cm$^{2}$ and above) in these structures using the LOASIS laser system at LBNL, focusing on a regime where relativistic guiding effects start playing a role. Applications of this waveguide as an accelerating structure in a Laser Wakefield Accelerator will be discussed. This work is supported by US DoE, DE-AC02-05CH11231 and in part by the Research Councils UK, Basic Technology Program (GR/R88090), DOE/NNSA under UNR grant DE-FC52-01NV14050, NSF, and AFOSR. [1] D. J. Spence, A. Butler, S. M. Hooker, J. Opt. Soc. Am. B, \textbf{20}, p138, 2003 [Preview Abstract] |
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QP1.00076: Blow-out and self injection in the laser wake field accelerator Jorge Vieira, Ricardo Fonseca, Luis Silva, Wei Lu, Michail Tzoufras, Frank Tsung, Warren Mori Recent experimental results showed that electron acceleration via laser wake field is an effective way to accelerate electrons to the hundred of MeV range in high quality beams. Numerical experiments also show that, with present day laser technology, it is feasible to accelerate electrons to the GeV range, in the same regime (the blow out regime), by making use of a parabolic plasma channel to guide the laser. We develop a theoretical model for wave breaking of relativistic plasma waves in the presence of an intense laser pulse, and self injection in a channel, based on the Dawson model for wave-breaking [1], that allowed us to determine the optimized conditions for blow-out and self injection in this regime. The model is compared with 2D and 3D PIC simulations. Scalings derived from the model allow for the design of accelerating stages of high quality self injected beams with energies to 10 GeV with new future laser technology. [1] J.M. Dawson Phys. Rev. 113, 383 (1959) [Preview Abstract] |
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QP1.00077: Relativistic bi-stability and adiabatic excitation of a large plasma wake Oleg Polomarov, Gennady Shvets The Dynamical Bi-Stability (DBS) [1] is caused by the relativistic nonlinearity of a high-amplitude plasma wave. When the laser beam intensity exceeds a detuning-dependent threshold, a long adiabatic beatwave can leave behind a large relativistic wake due to DBS. The Dynamical Bi-Stability is explained in terms of a representative Hamiltonian particle, describing the plasma wave, which undergoes a complicated nonlinear dynamics in the proper phase space. This dynamics includes separatrix crossings and phase space bifurcations, and are governed by an adiabatic evolution of the Hamiltonian of the representative particle [2]. We generalize DBS to the most interesting regime of short pulses and strong relativistic beat-wave amplitudes. Examples of the large wake-field comparable or larger in amplitude to the cold wave-braking electric field are given. Comparisons with other methods of adiabatic generation of the plasma wake (e.g. auto-resonant excitation) are made. [1]. G. Shvets, ``Beat-wave excitation of plasma waves based on Relativistic Bi-Stability,'' Phys. Rev. Lett. 93 195004 (2004). [2]. S. Kalmykov, O. Polomarov, D. Korobkin, J. Otwinowski J. Powerand, G. Shvets, ``Novel techniques of laser acceleration: from structures to plasmas'' submitted to Proc. Roy. Soc. [Preview Abstract] |
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QP1.00078: Resolution requirements for modeling of laser wakefield accelerators using particle-in-cell codes Estelle Michel, Bradley A. Shadwick, Carl B. Schroeder, Cameron G.R. Geddes, Eric Esarey, Wim P. Leemans, Hartmut Ruhl, Tom Cowan We investigate the spatial resolution required in particle-in-cell (PIC) codes for modeling laser-driven plasma-based accelerators. The grid size needed to reduce numerical noise in the PIC simulations, which manifests as macro-particle phase-space error, is determined. We show that the laser period must be resolved in the transverse direction as well as in the longitudinal direction to represent correctly the motion of the electrons in the plasma. The transverse resolution requirement is significantly higher than typically used (e.g., resolution of the transverse laser pulse shape), but is necessary to resolve the electron quiver motion in two- and three-dimensional simulations. The impact of the spatial resolution used in the PIC simulations on the momentum spread and subsequent spurious trapping in a plasma wave is investigated. This is a critical issue, since trapping of plasma electrons in laser-driven plasma wakefields is routinely modeled with PIC codes. [Preview Abstract] |
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QP1.00079: Developing Multi-timescale in PIC CODE OSIRIS Xiaodong Wang, Tom Katsouleas An idea of advancing beam and plasma with different time scales is proposed in this paper. Because beam particles usually response much slower than plasma particles, large time steps can be used to update beam particles to save computation time. In this paper, we will describe how to apply this multi-timescales method in particle-in-cell (PIC) code OSIRIS. Simulation results for SLAC E164 experiment parameters are given and show high degree of accuracy while gain 4-5 times saving in computing time. The limitation of this method is also studied. The maximum time saving is decided by driver beam energy and size of simulation box. [Preview Abstract] |
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QP1.00080: Self-Consistent Hamiltonian Model of Beam Transport in a Laser-Driven Plasma Accelerator B.A. Shadwick, G.M. Tarkenton, C.B. Schroeder Starting from the two species (bulk and beam electron) Vlasov--Maxwell system, we develop a self-consistent Hamiltonian model of beam transport in a background plasma where the beam is described by phase-space moments. The formalism used, based on the Hamiltonian structure of the Vlasov--Maxwell system, is a direct extension of that previously used to derive our warm fluid model.\footnote{B. A. Shadwick, G. M. Tarkenton and E. H. Esarey, Phys.\ Rev.\ Lett.\ \textbf{93}, 175002 (2004).} The bulk plasma model is independent of the moment model for the beam; in practice we find that a fluid description of the bulk plasma is appropriate. We present a detailed study of beam propagation in a resonant laser-wakefield accelerator. We discuss optimization of the system with regard to energy gain and beam quality. We comment on the implications for GeV-class accelerator stages. [Preview Abstract] |
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QP1.00081: Long time simulation of LHC beam propagation in electron clouds Bing Feng, Ali Ghalam, Elena Benedetto, Frank Zimmermann, Viktor Decyk, Warren Mori, Thomas Katsouleas In this paper we present three new simulation results of single-bunch instabilities caused by interaction of a proton beam with an electron cloud for the Large Hadron Collider (LHC). Our simulation was done using the QuickPIC code [1]. 1) We include the effect of dispersion in the equation of motion in the x direction, 2) We investigate the effect of the space charge of the beam on itself, and 3) we extend earlier modeling by an order of magnitude (from 50ms to 500ms) of beam circulation time. If dispersion is included the plane of instability changes. However, the total emittance is kept approximately the same. The effect of space charge is to change the emittance growth by less than a few percent. Results from longer runs suggest that the long term growth of electron cloud instability of the LHC beam cannot be obtained by extrapolation of the results of short runs. [Preview Abstract] |
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QP1.00082: Generation of high energy ions and electrons by the interaction of an intense laser pulse with plasmas Hae June Lee, Min Sup Hur High-energy ion beam generation from the interaction of an ultraintense femtosecond laser pulse with an overdense plasma slab combined with an underdense preplasma has been studied by using fully electromagnetic and relativistic particle-in-cell simulations. It was observed that the forward ion acceleration from the front and rear surfaces can be enhanced with a proper preplasma profile. The maximum and mean energy, the energy distribution functions of the accelerated ions, and Doppler shift of the reflected laser pulse are investigated with the variation of of laser intensity, pulse duration, and preplasma profiles. In addition, the electron beam generation has been observed from the interaction of the laser pulse with underdense plasmas. Moreover, the effect of the obliquely incident laser pulse is analyzed and the possibility of X-ray generation is discussed. [Preview Abstract] |
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QP1.00083: X-ray yield enhancement from solid targets coated with wavelength scale spheres. Hernan Sumeruk, Daniel Symes, Irina Churina, Alex Belolipetski, Stefan Kneip, Jamie Landry, Tom Donnelly, Todd Ditmire X-ray yield enhancement from laser solid interactions has previously been demonstrated for a variety of coating and target preparations. We investigate an additional method for enhancing the x-ray yield and possibly temperature relying on the field enhancement around a sphere as calculated using the Mie theory. We deposited uniform sized polystyrene spheres on a glass substrate. We irradiated the target with 15 mJ of 400 nm 100fs laser light focused to an intensity of approximately 10$^{17}$ W/cm$^{2}$. We observed an increase two orders of magnitude in x-ray yield from the sphere covered glass as compared to the plain uncoated glass. [Preview Abstract] |
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QP1.00084: Demonstration of saturated tabletop soft x-ray lasers at 5 Hz repetition rate in transitions of Ne-like ions with wavelengths near 30 nm Yong Wang, Miguel A. Larotonda, David Alessi, Bradley M. Luther, Mark Berrill, Vyacheslav N. Shlyaptsev, Jorge J. Rocca Recent experiments have demonstrated that the laser pump energy required to operate collisional soft x-ray lasers in the gain saturated regime can be significantly reduced by directing the heating pulse into the plasma at grazing incidence for a more efficient energy deposition [1-2]. Optimization of the incidence angle led to gain-saturated operation at 5Hz repetition rate in several transitions of Ni-like ions at wavelengths ranging from 18.9nm to 13.2nm [3]. We report saturated high repetition rate laser-pumped table-top soft x-ray lasers in Ne-like ions at wavelengths near 30nm. Gain-saturated lasers operating at 5Hz repetition rate were obtained in Ne-like Ti at 32.6nm and in Ne-like V at 30.4nm heating plasmas with laser pulses of $\sim $1J and 8ps impinging at 20$^{o}$ grazing incidence. Average powers $>$ 1$\mu $W were measured. Strong lasing was also observed in Ne-like Cr at 28.6nm. 1. R. Keenan et al, Phys. Rev. Lett., 94, 103901, (2005). 2. B. M. Luther et al, Opt. Lett., 30, 165, (2005). 3. Y. Wang et al, submitted to Phys. Rev. A, (2005). [Preview Abstract] |
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QP1.00085: Modeling of efficient soft x-ray lasers in transitions of nickel-like and neon-like ions. M. Berrill, I. Ellis, D. Alessi, J.J. Rocca, V.N. Shlyapsev We have modeled the plasma physics and amplification process in efficient soft x-ray lasers generated by rapid heating of plasmas by a short (8 ps) optical laser pulse of $\sim $ 1 J energy impinging at grazing incidence. This geometry allows for the efficient pumping of high repetition rate soft x-ray lasers [1-3]. The two temperature model includes all hydrodynamic equations written in 1.5D, as well a complete atomic model and multi-cell radiation transport. The equations that yield a full description of the plasma are solved using a finite difference method in a Lagrangian coordinate scheme. A post processor performs ray tracing to calculate output beam characteristics and intensities. The results corresponding to lasers in the 13.9 nm line of Ni-like Ag and the 32.6 nm and 30.1 nm of Ne-like Ti are discussed in comparison with experiments. Work supported by the NSF EUV ERC, Award EEC-0310717. \newline \newline 1. R. Keenan et al, Phys. Rev. Lett., 94, 103901, (2005) \newline 2. B. M. Luther et al, Optics Lett., 30, 165, (2005) \newline 3. D. Alessi et al, Opt. Express, 13,. 2093, ( 2005) [Preview Abstract] |
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QP1.00086: Investigation of the interaction of an electron beam and metallic grating in a Smith-Purcell free electron laser (SP-FEL) Tengiz Svimonishvili, Christopher Watts, Edl Schamiloglu, Steve Brueck Smith-Purcell (SP) radiation is produced when an electron beam passes over a metallic periodic structure and a continuous spectrum of modes associated with the beam is scattered by the grating. Most of the scattered modes are evanescent, while some may propagate. The SP radiation wavelength is found to be proportional to the grating period, $L$, and inversely proportional to beam velocity, $V$. Therefore, the SP radiation wavelength can be varied by changing $L$ and $V$. Based on recent experimental reports and theoretical calculations, a compact SP-FEL promises to be an attractive and affordable source of THz radiation. The major drawback of the traditional SP-FEL approach is that the electron beam must propagate very close to the grating surface. Furthermore, as one scales the concept to the THz regime, the metallic gratings become more lossy. We had proposed to improve the coupling of the beam to the grating by taking advantage of surface plasma waves in the grating. Here, we evaluate several models for the SP-FEL, including the effects of coupling to the surface plasma wave. Parametric scans are made to suggest optimal regimes to test the concept of a SP-FEL invoking surface plasma wave coupling. [Preview Abstract] |
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QP1.00087: High-Resolution Radiographs Produced by Single X-Ray ($\lambda$2.9 Angstroms) Pulses from UV Laser and Xe Cluster Interactions Ping Zhang The ability to produce high spatial resolution radiographs of tiny objects with short pulse x-rays is of high interest in the fields of x-ray micro-imaging and high field physics. This work reports the taking of radiographs exhibiting a resolution better than 10 microns of fruit flies and ants with a single x-ray (about 4.5 keV) pulse. The x-ray pulse was produced from Xe(L) 3d-2p hollow atom transitions excited by a high power UV laser pulse (180 fs, 248 nm, 600 mJ) interacting with Xe clusters. The emission that contributed to the x-ray imaging is concentrated in the wavelength of 2.9 Angstroms. [Preview Abstract] |
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QP1.00088: Experimental Observations of High Intensity UV Femtosecond Laser Interactions with Clusters Xiangyang Song High-intensity, ultraviolet (248 nm), femtosecond (about 180 fs) laser radiation was focused into a gas jet to study the spectral shifts and phase modulations at intensities up to $10^{18}W/cm^2$. Forward-scattered laser spectra were measured using a spatially resolved spectrometer; the peak pulse intensity envelope and phase were measured by the FROG (frequency-resolved optical gating) technique. In Ar targets, it was found that the forward Raman spectrum was blue-shifted and its bandwidth moderately broadened. Pulse shortening and phase modulation were also observed. [Preview Abstract] |
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QP1.00089: Spectroscopic Analysis of Plasma Formation in Flash X-ray Radiographic Diodes at SNL* M.D. Johnston, K. Hahn, D. Rovang, S. Portillo, J.E. Maenchen, D. Droemer, B.V. Oliver, D.R. Welch, E. Schamiloglu, Y. Maron Investigations are underway to study plasmas in flash x-ray radiographic diodes at Sandia National Laboratories. Studies were conducted on the RITS-3 accelerator (5.25MV and 120kA) and are being planned for the new RITS-6 accelerator (10MV and 120kA). Plasma spectroscopy is being employed as a primary method for obtaining information on the plasma conditions during electron beam propagation. Plasmas are believed to affect the impedance and beam focusing/transport behaviors of radiographic diodes. To date, studies have been conducted on four diode configurations: the plasma-filled paraxial, the standard paraxial, the self magnetic pinch, and the immersed Bz. Line and continuum emission has been observed for all diodes allowing determinations of electron and ion densities, temperatures, charge states, and expansion velocities to be made. Diagnostics include a gated, intensified multichannel plate camera with a multifiber input, and a multichannel plate intensified streak camera, each combined with a 1 meter Czerny-Turner monochromator. *Sandia is a multi-program laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under Contract DE-AC04-94-AL85000 [Preview Abstract] |
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QP1.00090: Electron cavitation and dynamic channel formation in underdense plasma George Petrov, Jack Davis, Alexander Velikovich Irradiation of clusters by high-intensity lasers leads to the generation of energetic X-rays ($\sim $KeV) useful for many applications in biology, material science and plasma diagnostics. If the laser intensity is sufficiently high ($\sim $10$^{17}$ W/cm$^{2})$ the electrons promptly leave the cluster and form an underdense plasma. Further, under favorable conditions, electron cavitaion occurs and the X-rays can propagate into the channel with minimal losses. Since the presence of the channel is critical for the X-rays propagation, we studied the channel formation in pre-formed underdense plasmas, irradiated by a high intensity laser. We found that the channel formation proceeds on a time scale of $\sim $10-100 fs, depending on the laser beam diameter. The channel typically lasts for as long as the laser pulse is on and closes shortly after the rear of the laser pulse has passed due to Coulomb attraction from the ion core. The front and the rear of the laser pulse propagate in very different environment: while the front of the pulse propagates through high-density media (unperturbed electron density), the rear of the pulse propagates essentially in vacuum. Results will be shown for Xe plasma from small Xe clusters (20 A radius), subject to ultra-high laser intensity of 10$^{20}$ W/cm$^{2}$. [Preview Abstract] |
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QP1.00091: Microwave-plasma interaction in the Undulator Induced Transparency regime and its applications Mikhail Tushentsov, Gennady Shvets A numerical modeling of a plasma-microwave interaction in the Undulator Induced Transparency (UIT) regime is presented. UIT is a phenomenon originating from the coupling between the transverse and longitudinal electromagnetic waves (EM) in a magnetized plasma in the presence of the static helical magnetic undulator, which eliminates the absorption of an EM wave at the cyclotron frequency. A radical transformation of the plasma properties in the UIT regime results in slowing down of the EM wave propagation and the extreme compression of the wave energy. We are envisioning a microwave pulse compression in the plasma with the subsequent rapid change of plasma or undulator parameters. This UIT-based high power switching technique is of interest to microwave electronics. Another signature of the UIT regime is that the phase velocity of the plasma wave can be made opposite to the phase and group velocity of the incident microwaves which opens up the possibility to design a plasma-based Backward Wave Oscillators (BWO). The direct application of UIT to electron and ion acceleration is suitable due to the fact that the polarization of the compressed waves is primarily longitudinal and their phase velocity is controllable by the undulator period. Our numerical simulations extend the UIT concept to three dimensions and use an experimentally feasible plasma and magnetic field configuration. [Preview Abstract] |
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QP1.00092: Laser Wakefield Acceleration of Photons C. Murphy, R. Trines, R. Bingham, J.T. Mendonca, P. Norreys, A. Reitsma, J. Gallacher, D. Jaroszynski, K. Krushelnick, S. Mangles, A. Thomas, Z. Najmudin Theory and simulation have for some time been able to predict that light can be shifted in frequency by a moving refractive index gradient. This has been observed experimentally in the case of a relativistic ionisation front. The effect has been considered for diagnosis of wakefields such as the one postulated for an electron accelerator. Here we present the first observation of photon acceleration from a laser-produced wakefield. In our experiments, the Astra laser at the Rutherford Appleton Laboratory was focused into jet of helium. The spectrum of the transmitted light was measured. We observe a blue shifted portion of the light which cannot be explained by acceleration from the ionisation front but can be explained by photon acceleration in a wakefield. A photon kinetic code has been implemented and can qualitatively reproduce the measured spectrum. The feasibility of using this effect as a wakefield diagnostic is discussed. [Preview Abstract] |
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QP1.00093: Role of field ionization in magnetic field generation from the Inverse Faraday Effect Samuel Martins, Ricardo Fonseca, Luis Silva, Frank Tsung, Waren Mori The diversity of mechanisms that can generate high intensity magnetic fields and the difficulty in identifying the different phenomena responsible for the measured fields have been a source of strong controversy. One of the possible mechanisms is the Inverse Faraday Effect (IFE), a magneto-optical effect, where a longitudinal B field is generated by a circularly polarized beam propagating in a medium with free electrons. One of the mechanisms that have not been fully taken into account in existing models is the ionization that occurs when the laser interacts with the neutral gas to generate the plasma, in particular when the ionization rate is comparable to the laser frequency, the scenario relevant for ultra-intense lasers. In order to quantitatively study the role of this mechanism, we develop a model for IFE that includes ionization. The model is then compared with three-dimensional particle-in-cell simulations in osiris 2.0. An overall good agreement is verified between theory and simulations. We conclude that gases with higher ionization potentials generate higher magnetic fields in the longitudinal direction, leading to enhancements one order of magnitude higher than in the pre-ionized scenario. [Preview Abstract] |
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QP1.00094: Two dimensional particle simulations of Raman backward amplifier Min Hur, Ilmoon Hwang, Hyyong Suk, Ryan Lindberg, Andy Charman, Josh Rembaum, Jonathan Wurtele We carried out two-dimensional particle simulations of the Raman backward amplifier. The particle code is based on the one-dimensional averaged-PIC (aPIC) code [1]. From the speculation that the longitudinal ponderomotive driving by the two counter-propagating lasers is quite dominant over the transverse one, the two-dimensional version of the aPIC can be easily built up by putting many one-dimensional aPIC solvers in parallel. The solvers are coupled by the diffraction terms of the lasers, which enables one to simulate the transverse effects in the Raman backward amplifier. One of the most important issues regarding the transverse effects is the focusability of the amplified pulse. Previous simulations [2-3], which are based on the fluid model, show that the focusing phase of the seed laser is preserved well during the amplification process. However, there has scarcely been kinetic studies on the same problem. Various simulations from the fully kinetic two-dimensional aPIC are presented. We discuss the kinetic effects (electron trapping) on the focusablity of the amplified seed. [1] M.S. Hur, G. Penn, J.S. Wurtele, and R. Lindberg, Phys. Plasmas vol. 11, p. 5204 (2004). [2] A.A. Solodov, V.M. Malkin, and N.J. Fisch, Phys. Plasmas vol. 10, p. 2540 (2003). [3] G.M. Fraiman, N.A. Yampolsky, V.M. Malkin, and N.J. Fisch, Phys. Plasmas vol. 9, p.3617 (2002). [Preview Abstract] |
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QP1.00095: Generation of radiation from interacion between ultra short pulse high power laser and plasma Noboru Yugami, Kazuhiro Kobayashi, Kenichi Ninomiya The generation of electromagnetic wave from the interaction between short pulse laser and plasmas are studied. The Ti:Sapphire laser (0.2 TW/100 fs) was forcused on neutral gas ($\rm N_2$ : 7.5 Torr) using a lens with a focal length 250 mm. By the interaction between short pulse and plasma, the electromagnetic wave was generated. The frequency of the observed electromagnetic waves was in the microwave range ($\sim$ 100 GHz). The radiation pulses of this microwave were detected by the microwave circuit element, constructed by the horn antenna and crystal the detectors. The pulse duration was typically 200 ps (FWHM). It has the polarization in the radial direction and emitted in the conical direction. The emission of the radiation is due to the electron oscillation, because the direction and its intensity were changed by the applied magnetic field. [Preview Abstract] |
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QP1.00096: Current-Voltage Characteristic of Nanosecond - Duration Relativistic Electron Beam Andrey Andreev, Mikhail Fuks, Edl Schamiloglu The pulsed electron-beam accelerator SINUS-6 was used to measure current-voltage characteristic of nanosecond-duration thin annular relativistic electron beam accelerated in vacuum along axis of a smooth uniform metal tube immersed into strong axial magnetic field. Results of these measurements as well as results of computer simulations performed using 3D MAGIC code show that the electron-beam current dependence on the accelerating voltage at the front of the nanosecond-duration pulse is different from the analogical dependence at the flat part of the pulse. In the steady-state (flat) part of the pulse), the measured electron-beam current is close to Fedosov current [1], which is governed by the conservation law of an electron moment flow for any constant voltage. In the non steady-state part (front) of the pulse, the electron-beam current is higher that the appropriate, for a giving voltage, steady-state (Fedosov) current. \newline \newline [1] A. I. Fedosov, E. A. Litvinov, S. Ya. Belomytsev, and S. P. Bugaev, ``Characteristics of electron beam formed in diodes with magnetic insulation,'' \textit{Soviet Physics Journal (A translation of Izvestiya VUZ. Fizika)}, vol. 20, no. 10, October 1977 (April 20, 1978), pp.1367-1368. [Preview Abstract] |
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QP1.00097: Magnetron with axial output of radiation M. Fuks, E. Schamiloglu As the limiting case of a magnetron with diffraction output [1], we consider a magnetron with axial extraction of electromagnetic energy through a cylindrical waveguide joined to the resonant system. A short length of waveguide with small cross section (less than cut off for the radiated wave) is placed between the resonant system and the output waveguide to obtain the optimal Q-factor. This magnetron is very compact due to its symmetrical design, even when the magnetic system is included. Importantly, any synchronous mode can be used as the operating mode because of identical loads for all the cavities. Computer simulations of the well-known A6 magnetron geometry, using the 3D fully relativistic particle-in-cell code MAGIC, demonstrate stable operation for the 2$\pi $-type of oscillations, with radiation power of about 0.7 GW and radiation frequency of 3 GHz. Electron efficiency is about 15{\%} when and the applied voltage is 350 kV. Application of a cathode [2] that consists of several individual longitudinal emitting strips arranged in a cylindrical geometry, provides fast start of oscillations. [1] M. Fuks and N. Kovalev, Program and abstracts of XI Int. Conf. On High Power Electromagnetics, Tel-Aviv, 1998, 23. [2] M. Fuks and E. Schamiloglu, Papers of USA-Japan Meeting on Plasma Science, Hawaii, 2004, 71-76. [Preview Abstract] |
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QP1.00098: Cathode Priming vs. RF Priming for Relativistic Magnetrons W.M. White, B.W. Hoff, R.M. Gilgenbach, Y.Y. Lau, M.C. Jones, V.B. Neculaes, N. Jordan, P. Pengvanich, R. Edgar, T.A. Spencer, D. Price Magnetron start-oscillation time, pulsewidth and pi-mode locking are experimentally compared for RF priming versus cathode priming on the Michigan-Titan relativistic magnetron (-300 kV, 2-10 kA, 300-500 ns). Cathode priming [1, 2] is an innovative technique first demonstrated experimentally at UM. In this technique, the cathode is fabricated with N/2 emitting strips or N/2-separate cathodes (for an N-cavity magnetron), which generate the desired number of spokes for pi-mode. Cathode priming yields 13{\%} faster startup with more reproducible pi-mode oscillation. Radio Frequency (RF) priming is investigated as the baseline priming technique for magnetrons. The external priming source is a 100kW, 3\textbf{$\mu $}s pulsewidth magnetron on loan from AFRL. RF priming reduced startup delay by 15{\%} and increased pulsewidth by 9{\%}. [1] M.C. Jones, V.B. Neculaes, R.M. Gilgenbach, W.M. White, M.R. Lopez, Y.Y. Lau, T.A. Spencer, and D. Price, Rev. Sci. Inst., 75, 2976 (2004) [2] M.C. Jones, Doctoral Dissertation, University of Michigan, 2005 [Preview Abstract] |
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QP1.00099: Magnetic Priming of a Relativistic Magnetron B.W. Hoff, R.M. Gilgenbach, W.M. White, N. Jordan, R. Edgar, Y.Y. Lau, V.B. Neculaes, M.C. Jones, P. Pengvanich, T.A. Spencer, D. Price Magnetic priming utilizes N/2 azimuthal variations in the axial magnetic field of an N-cavity magnetron to prebunch N/2 spokes for $\pi $-mode. [1] Positive results have been obtained in magnetic priming of the UM/Titan, relativistic magnetron (-300kV, 2-10kA, 0.3-0.5$\mu $s). Priming fields were created by three, axial, mu-metal wires within the cathode. Modeled magnetic field data were imported into 3-D MAGIC PIC and run for the A6 relativistic magnetron. Simulations showed faster startup and enhanced pi-mode control compared to the unprimed baseline. Initial experiments were performed in the UM/Titan magnetron with 3, 4 cm-long mu-metal wires embedded in the cathode, centered beneath the emission region. This primed magnetron yielded increased $\pi $-mode shots (57{\%} primed vs. 35{\%} unprimed) and statistically significant decreases in startup time (114 ns primed vs. 156 ns unprimed) and time to peak power (241 ns primed vs. 277 ns unprimed); mean peak power increased (11 MW primed vs. 6.5 MW unprimed, measured from 1 of 3 outputs). Additional concepts include longer cathode wires and wires in the anode. [1] V.B. Neculaes, R.M. Gilgenbach and Y.Y. Lau, US Patents 6,872,929 and 6,921,890 [Preview Abstract] |
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QP1.00100: Characteristics of an A6 Magnetron Using Transparent Cathode Sarita Prasad, H.L. Bosman, M.I. Fuks, E. Schamiloglu Relativistic magnetrons are attractive for many applications as the most compact sources of high power microwaves. However, the rather slow build-up of oscillations, which is dependent on the magnitude of the azimuthal electric field E$_{\theta }$ near the cathode surface, is a major drawback for applications in fields like nanosecond radar systems. The transparent cathode is a new cathode design in which longitudinal strips are removed from a hollow cylindrical cathode. This allows the E$_{\theta }$ field to penetrate to the center of the cathode, thus increasing the E$_{\theta }$ amplitude at the cathode surface. Computer simulations were performed with the 3D fully electromagnetic particle-in-cell code MAGIC, using the well-known A6 magnetron geometry. Dependences of the microwave output on the magnitude of the voltage and its rise time (for respectively instant turn-on and slow turn-on, relative to the electromagnetic fill time of the resonant system), show faster start of oscillations with the transparent cathode than with the traditional solid cathode. Magnetron operation with different configurations of the transparent cathode was studied to determine the optimal conditions for fast excitation of desired modes. These results together with dispersion characteristics will be presented. [Preview Abstract] |
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QP1.00101: Long-Term Operating Experience with High-Power Gyrotron Oscillators Kevin Felch, Monica Blank, Philipp Borchard, Patrick Cahalan, Steve Cauffman, Tak Sam Chu, Howard Jory High-power, megawatt-class gyrotron oscillators have now been used in electron cyclotron heating (ECH) experiments for several years. The long periods of sustained operation have provided important information about the design limits that had initially been placed on the key elements of the gyrotron. In particular, observations made on recent 110 GHz, 1 MW gyrotrons used in ECH experiments on DIII-D at General Atomics indicate that several of the important components of the device, including the electron guns, interaction cavities and diamond output windows, have performed quite well, while analyses of the electron beam collectors on some of the devices indicate that design limits have often been exceeded. Observations made on these gyrotrons will be summarized and plans to address problem areas will be discussed. [Preview Abstract] |
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QP1.00102: Development of Circuit Models for Extractor Components in High Power Microwave Sources John Luginsland, Jack Watrous, Keith Cartwright, Tim Fleming, Mike Haworth The state-of-the-art in High Power Microwave (HPM) sources has greatly improved in recent years, in part due to advances in the computational tools available to analyze such devices. Chief among these advances is the widespread use of parallel, particle-in-cell (PIC) techniques. Despite these advances, however, parallel PIC software could be greatly supplemented by fast-running parametric codes specifically designed to mimic the behavior of the source in question. These tools can then be used to develop zero-order point designs for eventual assessment via full PIC simulation. Building on the extensive literature from the vacuum electronics community, this poster will investigate the circuit models associated with the purely electromagnetic components of the extractor in the absence-of-space charge. Specifically, three-dimensional, time-domain computational electromagnetics (AFRL's ICEPIC software) will be used to investigate the modification of the resonant frequencies and mode-quality factors as a function of slot and load geometry. These field calculations will be reduced to circuit parameters for potential inclusion in parametric models and the fidelity of the resulting description will be assessed. [Preview Abstract] |
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QP1.00103: Evolution of Modes in Magnetically Insulated Crossed-Field Diodes Keith Cartwright, Tim Fleming, Chris Fichtl, Chris Lenyk The time-dependent behavior of electron sheaths in a magnetically insulated B$>$B$_{Hull}$ anode-cathode gap with crossed electric and magnetic fields is studied. The crossed-field, diode is modeled for various magnetic fields by means of multidimensional (1d and 2d), self-consistent, electromagnetic, particle-in-cell (PIC) simulations. The transient behavior of the system is examined in detail and is divided into three separate stages: cycloidal flow, collapse of cycloidal flow and sheared (near-Brillouin) flow. It has been shown in 1d planar geometry that the cycloidal flows collapse into a steady, near-Brillouin flow.\footnote{ P. J. Christenson, et.al., \textit{Phys. Plasmas}, 3(12):4455--4462, Dec 1996.} Our 2d electromagnetic PIC simulations (both planar and cylindrical) show that cycloidal flows also collapses into a flow that is dominated by the E cross B drift, but is neither steady nor stable. The growth of the kinetic mode is faster than that of either magnetron or diocotron fluid instability. After the kinetic mode saturates, the fastest growing fluid mode grows to dominate the system. A slow wave structure (SWS) is added to the anode that matches the wavelength and frequency of the fastest growing fluid instability. The SWS is then perturbed so that wavelength and/or frequency does not match the smooth bore diode growth rate and the region of `lock-in' to the SWS is found. This work is supported by a grant from AFOSR. [Preview Abstract] |
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QP1.00104: Characterization of materials (with low secondary electron emission yield) for use in high-power microwave devices Prashanth Kumar, Tengiz Svimonishvili, Christopher Watts, Lester Bowers, Herman Bosman, Mark Gilmore, Edl Schamiloglu, John Gaudet Secondary electron emission (SEE) results from bombarding materials with electrons, atoms, or ions. The amount of secondary emission depends on factors such as bulk and surface properties of materials, energy of incident particles, and their angle of incidence. When studying SEE, one is interested in determining the \textit{true} secondary electron emission yield (as opposed to reflected incident electrons), defined as the number of secondary electrons produced per incident primary electron. The goal of our research is to identify novel materials with a very low SEE yield coefficient that will make high-power microwave devices more efficient. To this end, we have employed a low-energy electron gun (5 eV to 2000 eV) to characterize different materials. Initial experiments are aimed at measurement of SEE with angular primary incidences, resolution of reflected primaries as well as comparison of experimental results with existing literature. These measurements are in the DC regime, but pulsed mode measurements are planned for the future to supplement the data. We also plan experiments to determine the effect of surface roughness on SEE yield. In addition, an ICEPIC (Improved Concurrent Electromagnetic Particle-in-Cell) simulation of SEE is performed in parallel. [Preview Abstract] |
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QP1.00105: Effects of Random Geometrical Perturbations in Slow Wave Devices P. Pengvanich, D. Chernin, Y.Y. Lau, R.M. Gilgenbach, D. Dialetis Motivated by the current interest in THz source, where miniature circuits are usually required, we evaluate the effects of geometrical random errors introduced during the manufacturing processes on the circuit characteristics. One such study was given in a previous paper [1], where a pi-section lumped element was used to model a periodic slow wave structure. In this paper, we extend the formulation of Ref. [1] to other geometries, such as the Smith-Purcell radiator or the traveling wave tube. The effects of the random manufacturing errors on the gain and efficiency in the electron-circuit interaction will be addressed. \newline \newline [1] D. Dialetis and D. Chernin, \textit{``Effect of Random Manufacturing Errors on Slow Wave Circuit Performance,''} 1997, IEEE Conference Record. [Preview Abstract] |
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QP1.00106: Photonic Crystal Structures for High Frequency RF Sources Gregory R. Werner, John R. Cary As microwave frequencies rise through the GHz range, wavelengths decrease to millimeters and shorter. Because the simplest radio frequency (RF) sources rely on the interaction between electron beams and the lowest-frequency modes of cavities or waveguides, small RF structures must be used to produce high frequencies. Smaller RF sources require smaller beams, from which it is more difficult to extract high powers. To use larger beams, high-power sources often have larger RF structures operating at high-order resonances. Besides allowing larger beams, larger structures carry more power at a given maximum field intensity (limited by breakdown). Photonic crystals may facilitate the design of high-order-mode structures. Photonic crystals are periodic dielectric structures that can reflect electromagnetic waves within a narrow photonic band gap (PBG); photonic crystals can therefore trap and guide waves at frequencies within the band gap. Waveguides and cavities constructed from hollowed-out photonic crystals may be designed to limit the number of trapped modes; ideally, only a single high-order mode would be trapped, allowing construction of conceptually simple, high-power, high-frequency sources. Photonic crystal structures may also prove advantageous at frequencies beyond the GHz range where dielectrics can withstand higher fields than metal surfaces. [Preview Abstract] |
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QP1.00107: HIGH INTENSITY SHORT PULSE LPI/HEDP SCIENCE |
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QP1.00108: Using Explicit PIC Results in Transport Codes B.F. Lasinski, A.B. Langdon, B.C. McCandless, C.H. Still, M. Tabak, R.P.J. Town The generation of hot electrons in short pulse high intensity laser plasma interactions is most completely studied in explicit Particle-in- Cell (PIC) codes such as our Z3. However the transport of these hot electrons through dense matter is most amenable to implicit PIC simulations with a code such as LSP. \footnote { D. R. Welch, {\it et al}, Nucl. Inst. Meth. Phys. Res.{\bf A 242}, 134 (2001). } Here we report on the direct transfer of the Z3 generated hot electrons into LSP simulations. The procedure involved are outlined. Results are compared to other methods for including these essential hot electrons in LSP simulations appropriate for current and future short pulse laser plasma experiments. [Preview Abstract] |
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QP1.00109: Advanced-Fuel Burning and Current Drive in a Degenerate Plasma Seunghyeon Son, Nathaniel Fisch In degenerate dense plasmas, the rates of electron physical processes, such as bremsstrahlung, inverse bremsstrahlung and Compton scattering, are much reduced due to the Fermi-Dirac statistics. In particular, ion-electron couplings (ion-electron collision and electron-ion collision) are vastly reduced. We apply this fact about ion-electron coupling in a degenerate plasma to two well-known plasma physics problems. First, we show that advanced fuel can be burned in degenerate plasma overcoming the bremsstrahlung losses. We discuss practical obstacles and prospects. Second, we obtain the current drive efficiency formula J/P in degenerate plasma. We investigate whether the current drive efficiency is correspondingly increased by virtue of the reduction of ion-electron coupling. We show that current carried by an electron is smaller in degenerate plasma than in classical plasmas due to ensemble averaging. Curiously, the current drive efficiency formula then does not formally reflect the reduced collision frequency. We also consider the ion beam current generation method and the minority-species current generation method. In these current generation methods, we obtain the current drive efficiency J/P that is considerably larger than that predicted by classical calculation. These finding will have applications in astrophysics, in inertial confinement fusion and in the generation of intense magnetic fields in dense matter. [Preview Abstract] |
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QP1.00110: Stimulated Brillouin scattering driven by broadband radiation Luis Silva, Jorge Santos, R. Bingham The interaction of intense radiation with plasmas is a problem of paramount importance in a wide range of scenarios. When the radiation pulse length is comparable or larger than the typical time scale of the ion dynamics, not only stimulated Raman scattering can occur, but also stimulated Brillouin scattering (SBS) plays an important role. In this work, we employ the formalism based on the Wigner description of the Klein- Gordon equation (see J. E. Santos, L. O. Silva, R. Bingham, this conference) to understand how the broadband features of the pump laser determine the growth rate of SBS. This formalism is based on a statistical description of the electromagnetic field, in the photon phase-space, thus allowing for the description of arbitrary fields, with random statistics or not. We explore the role played by a broadband pump field. For a monochromatic pump we recover the standard growth rates for SBS. Our model also yields the generalized dispersion relation for SBS with an arbitrary statistics of the field. The generalized dispersion relation is analyzed for simple photon distribution functions for which analytical results can be derived. Conditions on the pump bandwidth leading to SBS supression are also discussed. [Preview Abstract] |
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QP1.00111: Explosion of Layered Nanoplasmas Joana Martins, Marta Fajardo, Ricardo Fonseca, Luis Silva, Fabio Peano Ultra-intense laser pulses can drive Coulomb explosions in clusters. Recently, we have demonstrated the possibility to control these explosions by driving large-scale shock shells inside large clusters. We have explored the possibility for additional control of the explosion by considering clusters with heterogeneous compositions, either using layered targets or large clusters/liquid droplets with different compositions. These scenarios were explored with multi-dimensional particle-in-cell simulations with OSIRIS 2.0. Our results show that even for heterogeneous clusters, the dynamics of the explosion can be controlled. The presence of different ionization states and charge to mass ratios increases the complexity of the phenomena, leading to explosions occurring on different time scales for the different ion species, leading to modulations in the time-dependent spectrum of the accelerated ions. [Preview Abstract] |
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QP1.00112: Formation of laser plasma channels in a stationary gas Alexander Dunaevsky, Alexander Goltsov, Joel Greenberg, Szymon Suckewer, Ernie Valeo, Nathaniel Fisch Plasma channels with a density of $\sim $10$^{19}$ cm$^{-3}$ and nonuniformity of about $\pm $ 3.5{\%} can be formed in a non-flowing gas, contained in a cylindrical chamber. Channels with radius of 15 um and length about 2.5 mm were formed in He, N$_{2}$, Ar, and Xe by a 0.3 J, 100 ps laser pulses. Laser beam passed through the chamber along its axis via pinholes in the chamber walls. Ablative plasma on the pinholes, created by the wings of radial profile of the laser beam, plays an important role in the plasma channel formation and its uniformity. Uniform channel forms only at proper time delay and in optimal pressure range, which depend on the sort of gas. Interaction of the laser beam with the gas stream from of the front pinhole was found insignificant. Uniformity of the plasma channel may be improved slightly by low current glow discharge initiated in the. High current arc discharge, oppositely, leads to refraction of the laser beam near the front pinhole because of the formation of overdense plasma. [Preview Abstract] |
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QP1.00113: Controlling multiple plasma channels created by a high-power femtosecond laser pulse O.G. Kosareva, V.P. Kandidov, N.A. Panov, Q. Luo, S.A. Hosseini, W. Liu, S.L. Chin Femtosecond light filaments are comparatively long regions of the spatially and temporally localized radiation zones, which generate free electrons in the medium. At high pulse peak power multiple filaments are produced leading to stochastic plasma channels (Mlejnek et al.: PRL \textbf{83}, 2938 (1999)). In both atmospheric long-distance propagation (Sprangle et al., PRE \textbf{66}, 046418 (2002), Kasparian et al, Science \textbf{301}, 61 (2003)) and focusing the radiation into condensed matter important issues are production of elongated plasma channels, as well as high conversion efficiency to the white light. We control stochastic plasma channels by changing the initial beam size or shape. The result is the increase in the plasma density and white light signal. Control by regular small-scale perturbations allows us to suppress atmospheric turbulence in air and create an array of well-arranged filaments in fused silica. [Preview Abstract] |
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QP1.00114: Ultra-fast Measurements of Optically Induced Lattice Dynamics in LuMnO$_3$ Using Aluminum K-alpha X-Ray Diffraction. J. Workman, H.J. Lee, J. Roberts, Q. McCulloch, A.J. Taylor, D.J. Funk, J.S. Wark An experiment to study the structural dynamics at the ultra-fast time scale in optically-pumped samples is presented. Measurements of lattice dynamics in LuMnO$_3$ are presented and compared to calculations using dynamical diffraction theory modified for hexagonal crystal structure. Ultra-fast x-ray emission is used to measure Bragg peak shifts using diffraction and compared to calculations. Results are presented for optical pump energy densities of 8 and 20-mJ/cm$^{2}$. The experiment uses $\sim $150 mJ of a 100fs Ti:Sapphire laser to excite K-alpha x-ray emission in an aluminum wire with $\sim $1-2{\%} split off for the material pump. The x-ray emission is relayed using a spherical Quartz crystal to the sample target. Plans for experiments using Cu K-alpha emission to probe Fe samples will also be described. [Preview Abstract] |
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QP1.00115: Generation of 15-TW single-cycle laser pulse using cross-phase modulation in a relativistic plasma Shouyuan Chen, Matthew Rever, Donald Umstadter The compression of amplified laser light is a topic of much current interest, and one in which plasmas can play an important role. We present the theory of high-power, ultra- short pulse generation using cross-phase modulation in a relativistic plasma [1]. The analysis shows that the spectrum of the modulated pulse has a bandwidth of 400 nm, which indicates a broadening by more than 10 times. The pulse duration will be only one or two laser cycles if a Fourier-transform limited pulse can be obtained after compression. The power of the compressed pulse is adjustable and can reach 15 TW since plasma has no damage threshold. That is orders of magnitude higher than the power of ultra-short pulses generated by self-phase modulation in noble gases. This work was supported by the Chemical Sciences, Geosciences, and Biosciences Divisions of the Office of Science, U.S. Department of Energy and the National Science Foundation.\newline \newline References: \newline [1] S. Chen et al., ``Observation of Relativistic Cross-Phase Modulation in High Intensity Laser- Plasma Interactions,'' CLEO (JThD6, oral presentation), Baltimore, May 22-27, 2005. [Preview Abstract] |
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QP1.00116: Measurements of Optical Spectra in Short Pulse Laser-Solid Interactions at Moderately Relativistic and Relativistic Laser Intensities A.M. Niles, S. Wilks, H. Chen, R. Shepherd, Y. Ping, K. Widmann, S. Moon, K.B. Fournier, S.B. Hansen, H-K. Chung, A. Kemp We present a series of measured spectra, 300nm-950nm with 1nm resolution, for various target materials including: Al, Ti, Au, Si, Cu and Al coated Ti obtained during laser-solid interaction experiments performed at the ultra-intense short pulse laser facilities JanUSP (LLNL) and Vulcan (RAL). Laser intensities ranged from moderately relativistic, $\sim $10$^{17}$W/cm$^{2}$, to relativistic, $\sim $ 10$^{20}$W/cm$^{2}$. Measured spectra were sampled from within the specular reflection cone off of the target and delivered via discrete optics to a fiber coupled diffraction grating spectrometer. In addition to the expected higher integer harmonics, 2$\omega $ and 3$\omega $, of the laser frequency, several shots measured high levels of 3/2$\omega $, and 5/2$\omega $. These are attributed to two plasmon decay in the underdense plasma blow-off caused by the prepulse interacting with the solid before the main pulse. Density profiles for several shots will also be presented, in an attempt to correlate the presence of the 1/2 harmonics with long scale pre-plasmas. The work was performed under the auspice of the Department of Energy under Contract No. W-7405-Eng-48 and Laboratory Directed Research and Development (LDRD) Programs 04-LW-020 and 04-ERD-023. [Preview Abstract] |
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QP1.00117: Short Pulse Laser Absorption and Energy Partition at Relativistic Laser Intensities Yuan Ping, Hui Chen, R. Shepherd, G. Dyer, A. Faenov, J. Hunter, K. Widmann, T. Pikuz, H. Chung, K. Fournier, S. Hansen, S. Wilks We present the first absorption measurements at laser intensity between $10^{17}$ to $10^{20}$ W/cm$^{2}$ using an intergrating sphere and a suite of diagnostics that measures scale length, hot electrons and laser harmonics. A much-enhanced absorption in the regime of relativestic electron heating was observed. Furthermore, we present measurements on the partitioning of absorbed laser energy into thermal and non-thermal electrons when illuminating solid targets from $10^{17}$ to $10^{19}$ W/cm$^{2}$. This was measured using a sub-picosecond x-ray streak camera interfaced to a dual crystal von H\'{a}mos crystal spectrograph, a spherical crystal x-ray imaging spectrometer, an electron spectrometer and optical spectrometer. Our data suggests an intensity dependent energy-coupling transition with greater energy portion into non-thermal electrons that rapidly transition to thermal electrons. The details of these experimental results and modeling simulations will be presented. [Preview Abstract] |
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QP1.00118: Self-Consistent Short Laser Pulse Absorption and Particle Transport with KALOS Wojciech Rozmus, Mark Sherlock, Tony Bell A new version of the numerical code KALOS has been developed to solve the Vlasov-Fokker-Planck equation for electrons coupled self-consistently to Maxwell's equations to describe EM wave propagation. KALOS represents the electron distribution function in momentum space by an expansion in spherical harmonics. Its unique features make possible simultaneous investigations of laser energy absorption at the target surface and the transport of heat into the dense target. The collisional evolution of thermal particles, including the return current of cold electrons is also accurately modeled. We report here on results obtained in one spatial dimension for the absorption of femtosecond laser pulses in dense plasmas at moderate laser intensities ($\sim $10$^{16}$ W/cm$^{2})$. In this regime the electron distribution function is modified at the target surface by strong inverse bremsstrahlung heating and inside the target by non-SH heat flow. The code results are compared to theoretical predictions which treat the problem as the heating of an inhomogeneous hot-spot region coupled to a propagating electron heat wave into the dense target. We have studied this absorption and energy transport for different density gradient scales and laser intensities. To describe absorption and hot electron transport for oblique incidence we have used the standard approach involving a Lorentz transformation to a frame moving along the target surface. [Preview Abstract] |
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QP1.00119: Multiple-stage ionization dynamics of carbon film irradiated by high power lasers Tomohiro Masaki, Yasuaki Kishimoto Ionization dynamics and energy transport in ultra-short (100fs) high power (5x10$^{19}$W/cm$^{2})$ laser-carbon film interaction is simulated by extended particle based integrated code (EPIC3D). In the code, optical field and electron impact ionization, and also collisional relaxation process are taken into account. We found two types of ionization dynamics, namely, a fast time scale convective propagation of the ionization front with C$^{4+}$ triggered by induced plasma waves, and a slow front with C$^{5+}$ and C$^{6+}$ triggered by high energy electrons near interaction surface. Thus, ionization dynamics in carbon film are found to evolve through multiple stages. We also found that the slow time scale ionization due to electron impact is tightly coupled to electron heat transport process and non-local high energy electrons which transfer the heat contribute to the ionization. It is also found that the ambi-polar electric field which is established to keep charge neutrality during heat transport accelerates ions in the carbon film. [Preview Abstract] |
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QP1.00120: Experiments of multi-keV x-ray production from pre-pulsed germanium foils Frederic Girard, Daniele Babonneau, Michel Primout, Jean-Paul Jadaud, Michel Naudy, Bruno Villette, Larry J. Suter, Robert L. Kauffman, Carmen Constantin, Mike C. Miller, Jacob Grun, John Davis Previous work with laser pre-exploded thin foils of titanium (He alpha at 4.7 keV) and copper (He alpha at 8.3 keV) showed high multi-keV x-ray conversion efficiencies. They are increased by a factor of more than 2 in comparison with solid materials and are close to gas targets. Experiments with a thin foil irradiated with 2 laser pulses (one delayed in time) lead to hot and underdense plasmas, which are efficient to produce multi-keV K-shell emission. Exploded thin foil experiments have been performed on the OMEGA laser facility at LLE (University of Rochester) to quantify the multi-keV x-ray output from germanium targets. X-ray power was measured by filtered diodes (DMX broadband spectrometer), which was fit to the germanium K-shell emission around 10.3 keV. Within the spectral bandwidth of 10 $<$ h$\nu$ $<$ 13 keV, a conversion efficiency enhancement by a factor of 2.2 is measured relative to the case without pre-pulse. [Preview Abstract] |
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QP1.00121: Satellite formation in high intensity laser-solid interactions due to the presence of non-thermal electrons H.-K. Chung, S.B. Hansen, K.B. Founier, S. Moon, S. Wilks, H. Chen, A. Niles, Y. Ping, R. Shepherd, K. Widmann, J. Hunter, T. Ditmire, G. Dyer, T.A. Pikuz, A.Y. Faenov The COMET laser in LLNL was used to study the energy distribution between thermal and non-thermal electrons created by short-pulse laser interacting with soild-density matter. The aluminum-coated titanium targets were illuminated by the ultrashort laser pulses with variable intensities of 10$^{17}$-10$^{19}$ W/cm$^{2}$. We have measured the time-dependent x-ray spectra of titanium K$_{\alpha }$ and the aluminum He-like 1s2p-1s$^{2}$ and Li-like satellites for tens of picoseconds. Data show that the titanium K$_{\alpha }$ line is broadened and shifted to higher energies and the Li-like satellites of aluminum He$_{\alpha }$ vary with time. Time-dependent collisional-radiative calculations were preformed to generate the time-resolved x-ray spectra of aluminum and titanium in the presence of a small fraction of non-thermal electrons in solid-density bulk electrons. We present the spectroscopic analysis to determine the plasma conditions of the front aluminum layer as a function of laser intensity, which will improve the understanding of the dynamics of non-thermal electrons and the heating of the thermal electrons. [Preview Abstract] |
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QP1.00122: Vlasov vs LSP Simulations of Laser-Plasma Interactions: Blue-Green and Green-Orange Interactions Vlad Savchenko, Bedros Afeyan, Kirk Won, Dale Welch, Mike Cuneo We use Vlasov-Maxwell simulations to model the interaction of crossing laser beams driving electron plasma waves and KEEN waves(1) in inhomogeneous plasmas. We have compared these results with LSP PIC code simulations as well to see the relative merits of these simulation tools. Our aim is to characterize the hot electron generation properties of these mechanisms and the subsequent X ray generation for radiographic applications. Short pulse high intensity regimes are compared to lower intensity and long interaction pulse cases highlighting the kinetic physics that occurs in each case. \newline \newline (1) B. Afeyan et al., Proc. IFSA (Inertial Fusion Sciences and Applications 2003, Monterey, CA), 213, B. Hammel, D. Meyerhofer, J. Meyer-ter-Vehn and H. Azechi, editors, American Nuclear Society, 2004. [Preview Abstract] |
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QP1.00123: Particle Trapping in Raman Backscatter Ryan Lindberg, Andrew Charman, Min Sup Hur, Jonathan Wurtele We present an approach to modelling the effects of particle trapping on coherent Raman backscatter (RBS) in plasmas with negligible linear Landau damping. Using the formalism of adiabatic separatrix crossing [1], we calculate the trapped particle fraction and their average action, and use this to approximate a kinetic detuning term for use in a three-wave model of RBS. 1. Tennyson, Cary, Escande. PRL 56, 2117 (1986). [Preview Abstract] |
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QP1.00124: Modeling the Dynamics of Reduced Mass Targets Heated by Ultra-Intense Lasers S.J. Moon, S.C. Wilks, R.I. Klein, A.J. MacKinnon, R. Town, P.K. Patel, H-K Chung, B.A. Remington, D.D. Ryutov, R. Shepherd We present simulations of the time-dependent behavior of a novel target design that allows high temperature solid density plasmas to be created using ultra-intense laser pulses. Simulations and experiments using targets composed of copper and tamped with aluminum showed that decreasing the target size led to a significant increase in temperature, compared to standard foil targets. In a recent paper (G. Gregori, et. al., CPP 2005.) analysis of time-integrated Cu K-alpha emission showed a maximum temperature of 220 eV in solid density Cu. The radiation-hydo code LASNEX is used to model the time-dependent behavior from a 100J 10ps high-intensity laser with a 7 $\mu $m FWHM focal spot. We discuss the time-dependent Cu K-alpha signal and time-integrated XUV emission. Future comparison of these predictions with recent experimental results obtained from the Rutherford Appleton Laboratory Petawatt laser will give us an indication of the behavior of the energy coupling of ultra-intense lasers, and the ability to heat solid-density matter to $\sim $keV temperatures. This work was performed under the auspice of the Department of Energy under Contract No. W-7405-Eng-48. This project was also funded by the Laboratory Directed Research and Development (LDRD) Program 04-ERD-028. [Preview Abstract] |
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QP1.00125: Numerical Investigation of Droplet-Based Heavy-Ion Generation P. Messmer, D.L. Bruhwiler, D.A. Dimitrov, C. Nieter, J.R. Cary, M. Richardson Basic research in nuclear physics and astrophysics requires the acceleration and collision of highly charged rare isotopes. Conventional ion source technology based on electron cyclotron resonance (ECR) cannot produce the necessary beam current for many of these rare isotopes. Laser ionization of micron-sized doped water droplets generates plasmas which can produce accelerated ions. The droplet and laser parameters can be tuned to ionize a precisely controlled number of heavy atoms. Laser-droplet plasmas therefore seem an attractive enhancement for the heavy ion current in ECR sources. Here we present results of particle-in-cell (PIC) simulations of the interaction of a strong laser pulse with $\mu$m-sized droplets, using the plasma simulation code VORPAL[1]. The code features perfectly matched layer boundary condition which allows to avoid spurious reflections off the simulation domain walls. Droplet ionization is modeled using the quasi-static ADK tunneling ionization model implemented in the ionpack library[2]. The simulation parameters are chosen close to the experimental parameters.\newline [1] Nieter, C, Cary, J.R., J. Comp. Phys, 196(2), 448, 2004.\newline [2] http://www.txcorp.com/technologies/IONPACK/ [Preview Abstract] |
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QP1.00126: Quasilinear theory of saturation of Landau damping in parametric interactions Nikolai Yampolsky, Nathaniel Fisch In a 3-wave interaction in plasma, in which one of the plasma waves is Landau damped in a plasma, the quasilinear saturation of the Landau damping can increase the parametric interaction between the waves. For short interaction times, effects on the bounce-frequency time scale are not important, and a simplified hydrodynamic model of quasilinear theory can describe saturation of Landau damping. As an application, regimes for the amplification of short laser pulses in hot collisionless plasma by means of backward Raman scattering were found. In those regimes the effectiveness of the seed pulse for the plasma wave generation can be reduced. This reduction of the seed pulse effectiveness can be a large and deleterious effect during the linear stage of the amplification when the pumped seed pulse stretches out. During the nonlinear stage, when the pumped seed pulse compresses, the reduction of the seed pulse effectiveness becomes less important. [Preview Abstract] |
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QP1.00127: Direct Measurements of the Dynamics of Self-Guided Femtosecond Laser Filaments in Air Daniel Gordon, Antonio Ting, Richard Fischer, Ilya Alexeev, Theodore Jones, Joseph Penano, Phillip Sprangle High power laser pulses propagating in air form self-guided filaments due to a balance between Kerr focusing and ionization induced defocusing. These filaments are difficult to characterize because they are intense enough to damage optics. Measurements were carried out using a novel diagnostic which allows quantitative measurements of filament characteristics despite the high intensity in the filaments [A. Ting et al., Appl. Opt. 44, p. 1474]. The diagnostic apparatus was aligned to a rail so that measurements could be carried out at a large number of axial positions thereby allowing the dynamics of the filament propagation to be studied. It is observed that the energy in a filament remains constant for several meters despite the fact that much of the filament's energy must be expended to create just one meter of plasma. This indicates that the filament energy is constantly replenished by the surrounding radiation. The conductivity of the plasma column created by the optical filament was also measured. Furthermore, the effect of varying the laser pulse length while holding the pulse energy constant was studied. [Preview Abstract] |
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QP1.00128: Absolute x-ray yields from laser-irradiated, Ge-doped aerogel targets K.B. Fournier, M.T. Tobin, J.F. Poco, K.S. Bradley, S.B. Hansen, C.A. Coverdale, D.E. Beutler, M.R. Severson, E.A. Smith, D.L. Reeder We have measured the production of $h\nu$ $\geq$~10~keV x rays from low-density, Ge-doped aerogel targets at the OMEGA laser (LLE, U. of Rochester). The targets were 1.2mm long by 1.5mm diameter Be cylinders filled with Ge-doped (20 atomic{\%}) SiO$_2$ aerogel. The doped-aerogel density was 4.8 or 6.5~mg/cc. These targets are a major advance over previous doped aerogels: instead of suspending the dopant in the SiO$_2$ matrix, the Ge atoms are incorporated directly in the matrix. Forty beams of the OMEGA laser ($\lambda$ = 351~nm) illuminated the cylindrical faces of the target with a total power of $\approx$~18~TW. The laser strongly ionizes the target ($n_e$/$n_{cr}$ $\leq$ 0.1--0.2), and allows the laser-bleaching wave to ionize supersonically the high-Z ions in the sample. Ge K-shell x-ray emission was spectrally resolved with a two-channel crystal spectrometer and recorded with temporal resolution with a set of calibrated photoconductive devices (PCDs). The heating of the target was imaged with a gated x-ray framing camera. 2-D rad-hydro calculations predict rapid and uniform heating over the target volume with minimal energy losses into hydrodynamic motion. The calculations predict 150--200~J of x-ray output with $h\nu$ $\ge$~10~keV, in agreement with the measurements.\newline *This work performed under auspices of the U.S. DoE by LLNL under Contract W-7405-ENG-48. [Preview Abstract] |
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QP1.00129: Opacity Effect on Extreme Ultra-Violet Radiation from Laser Produced Tin Plasmas Shinsuke Fujioka, Tsuyoshi Ando, Hiroaki Nishimura, Katsunobu Nishihara, Keiji Nagai, Takayoshi Norimatsu, Noriaki Miyanaga, Yasukazu Izawa, Kunioki Mima, Atsushi Sunahara, Yoshinori Shimada, Akira Sasaki Opacity effects on the extreme ultra-violet (EUV) emission from laser-produced tin (Sn) plasmas have been experimentally investigated. EUV light source for a microlithography is getting a lot of attention as a challenging application of the laser-produced high-Z plasmas. Laser produced Sn plasma is an attractive 13.5 nm light source due to its compactness and its high emissivity. Opacity as well as emissivity of the laser-produce Sn plasma is quite high for the 13.5 nm light, therefore the 13.5 nm light from deep inside region of the Sn plasma is absorbed strongly during transporting through a surrounding plasma, and is unavailable. The opacity is a critical parameter to investigate optimum conditions for the EUV generation, however no reliable database had been available. Absolute opacity structure of a uniform Sn plasma, whose electron temperature is 30 - 40 eV, has been measured in the EUV range (10 - 20 nm of wavelength) for the first time. Experimental results indicate that control of optical thickness of the laser-produced Sn plasmas is essential to obtain high efficient and narrow band EUV radiation from the laser-produced tin plasmas. [Preview Abstract] |
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QP1.00130: Diagnosing Ignition Failure Modes on the Early NIF Ignition Implosion Campaign S.P. Hatchett, C.J. Cerjan, S.W. Haan The first ignition implosion attempts in the NIF Ignition Campaign are planned for late 2010. A number of non-ignition shots, almost all at less than full energy, will lead up to these, but the number of initial ignition attempts at $>\sim $1 MJ must be very limited --- a situation quite unlike previous non-ignition implosion campaigns. We plan for success, ignition, but must have in place a suite of diagnostics capable of revealing the cause(s) of non-ignition. These causes include basic physics uncertainties such as thermal conductivities, hydrodynamic processes that we cannot calculate in full detail, and drive asymmetries that may only be apparent at full laser energy with cryogenic capsules. The capsules are sufficiently small that many possible ultimate causes and combinations of causes can produce modest ($\sim $10{\%}) drops in some figures of merit, such as average fuel $\rho $R, that will cause ignition failure. The diagnostic environment is harsh, the suite of diagnostics is limited, and the number of lines of sight is very limited. We will describe the proposed suite of diagnostics and discuss what we know about the diagnostic signatures of various failure modes. We will describe the implied requirements on sensitivity and dynamic range for the various diagnostics. Finally, we will describe a draft response matrix. \textit{This work was performed under the auspices of the U.S. Department of Energy by the University of California, Lawrence Livermore National Laboratory under Contract No. W-7405-Eng-48.} [Preview Abstract] |
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QP1.00131: A Jetting Instability Due to Surface Defects in Indirectly Driven ICF Targets P.J. Christenson, R.A. Vesey, M.E. Cuneo, D.A. Steinman The effects of small, dome-like structures on the outer surface of indirectly driven ICF targets is examined. This work was motivated by the experimental observation that dense material appeared to be at the center of a deuterium gas filled capsule early in the capsule compression. A possible explanation for the observation is that micro-dome defects of the order of a half micron in amplitude and five microns in radius, thousands of which are seen on capsule surfaces, cause jetting of ablator material into the capsule allowing dense material to converge at the center prior to peak compression, but leaving the ablator layer mostly intact. 2-D Lasnex simulations of various ICF targets have been performed to examine the dependence of jetting and target performance on defect amplitude. The minimum defect amplitude that allows a measurable thermonuclear neutron count was computed to determine a desirable surface smoothness. [Preview Abstract] |
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QP1.00132: Sensitivity of Ignition Capsule Performance to Hydrogen Equation of State Abe Fetterman, Steve Haan, Mark Herrmann, Marty Marinak, Dave Munro, Jay Salmonson There has been some disagreement between experiments measuring the shock Hugoniot density for hydrogen [M. D. Knudson, D. L. Hanson, J. E. Bailey, C. A. Hall, C. Deeney, and J. R. Asay, AIP Conf. Proc. 706, 81 (2004); G. W. Collins, P. Celliers, L. B. Da Silva, et al., Phys Plasmas 5, 1864 (1998)]. We did simulations of capsules intended to produce ignition on the National Ignition Facility [J. A. Paisner, J. D. Boyes, S. A. Kumpan, W. H. Lowdermilk, and M. S. Sorem, Laser Focus World 30, 75 (1994)] with several candidate equations of state spanning the range of recent experiments. Changing the equation of state shifts the optimal shock timing somewhat, but primarily affects the sensitivity of the yield to uncertainties in the shock timing. Even with the stiffest candidate equation of state, performance of the capsules is acceptable with somewhat tightened requirements on shock timing and levels. [Preview Abstract] |
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QP1.00133: Investigation of weakly deuterium doped tritium capsules as a technique to measure hot spot ignition properties. Melissa Douglas, Paul Bradley An important step in gaining confidence in the capability of hot spot ignition in cryogenic D/T capsules is the experimental characterization of the hot spot region. Currently this is difficult to accomplish because of the intense high energy of neutrons that can create substantial background noise, or worse, damage diagnostics, making X-ray imaging of the hot spot region as it forms difficult. One approach in minimizing the deleterious effects of neutrons in hot spot imaging calls for a reduction of deuterium in the baseline D/T (50/50 mixture) capsule. By limiting the number of D/T reactions, the neutron flux will be lowered as well as the neutron energies. The desired effect is thus to decrease the neutron flux to an ``acceptable'' diagnostic level while maintaining hot spot conditions, i.e. effective $\rho $r and heating, that would be found in the baseline capsule design. Here we computationally investigate the effects of modifying the D/T ratio and explore the feasibility of this proposed technique. [Preview Abstract] |
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QP1.00134: High Density Plasma Beam Target Fusion: An Alternative form of Inertial Confinement to Ignition Dah Yu Cheng A scaling law has been demonstrated for a plasma gun in a 30 m diameter space chamber. Using a specific combination of delay time between the gas valve opening and operation of a capacitor bank switch, and the capacitor bank's voltage, the deflagration gun (Ref. 1) is capable of producing high kinetic energy and at a high density beam. Using a convergent gun barrel it is possible to compress beam density by a factor of 100. A 10$^{17}$/c.c.beam was obtained. If the kinetic energy is at 280 Kev (fusion threshold) Neutron flux up to 5 x 10$^{19}$ could be produced in a 1 micro-second period. This represents 500 MJ of energy yield. If the classical fusion energy data were in error by a factor of 100 that still would yield 5 MJ of fusion energy. Results obtained from experiments in the 30 m diameter space chamber have demonstrated such a capability using a 120 KV capacitor bank with 200 KJ of stored energy (Ref. 1). A very small scale experiment has demonstrated a yield of 10$^{15}$ neutrons using less than 10 kJ of capacitor energy.\newline References:\newline 1. Cheng, D.Y. 1970 Plasma Deflagration and the Properties of a Coaxial Plasma Deflagration Gun. \textit{Nuclear Fusion}, 10, pp. 305- 317 [Preview Abstract] |
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QP1.00135: Electromagnetic beam filamentation with collisions Massimiliano Fiore, Michael Marti, Ricardo Fonseca, Luis Silva, Chuang Ren, Michail Tzoufras, Warren Mori In recent years, the Weibel instability has received an increasing interest due to its potentially important role in Astrophysics (GRBs) and in other fields of Plasma Physics, such as Fast Ignition. In the standard fast ignitor scenario the collisionless filamentation (Weibel) instability of the forward laser driven MeV electrons and their return current can occurs in the coronal region of the fuel pellet. As the collision frequency becomes non negligible in inner regions, the filamentation instability still occurs but with a significantly smaller growth rate and larger typical wavelengths, which can be comparable to the typical beam size, and whole beam instabilities arise. Using relativistic kinetic theory, we present a model to study the linear stage of the filamentation instability, including collisions through the BGK model. The dispersion relation is numerically solved. Simulations with OSIRIS 2.0 including binary collisions illustrate this regime of the filamentation instability. [Preview Abstract] |
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QP1.00136: Hierarchy of beam plasma instabilities up to high beam densities for Fast Ignition Scenarii Antoine Bret, claude deutsch We investigate the hierarchy of electromagnetic instabilities suffered by a relativistic electron beam(REB) passing through a plasma. We work in a fluid approximation and consider beam densities up to plasma ones. The hierarchy between instabilities is established in terms of only two parameters: beam relativistic Gamma factor and ratio Nb/Np of beam to plasma densities. For Nb/Np $<$ 0.53, most unstable modes are a mix of Filamentation and 2-Stream instabilities. Beyond that limit, Filamentation may be dominating, depending on beam Gamma factor. Growth rates behavior for Nb/Np $\sim $ 1 leads to conclude that a Fast Ignition Scenario REB could experience a very high instability level at earliest stages of its journey through supercompressed DT core. [Preview Abstract] |
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QP1.00137: Monte Carlo Simulations for Studying Hot-Electron Transport in Non-Degenerate Plasmas of Arbitrary Z C.D. Chen, C.K. Li, J.A. Frenje, F.H. Seguin, R.D. Petrasso, J. Myatt, J.A. Delettrez A Monte Carlo code is used to model the scattering and energy loss of fast electrons off ions and electrons in a cold non-degenerate plasma of arbitrary Z. For $Z = 1$ solids and plasmas, the effects of \ $e$--$e$ \ scattering are expected to be as important as \ $e$--$i$ \ scattering. These calculations will be compared to the results of analytic calculations studying the penetration, energy deposition, straggling and blooming of fast electrons. These calculations will be used to establish requirements for fast ignition and tolerable levels of electron preheat for direct and indirect ignition targets. This work was supported in part by LLE, LLNL, the U.S. DoE, the Univ. of Rochester, and the N.Y. State Energy Research and Development Authority. [Preview Abstract] |
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QP1.00138: Stopping of Fast Electrons in Dense Hydrogenic Plasmas A. Solodov, R. Betti, J. Myatt The energy deposition and penetration depth of fast electrons in dense hydrogenic fuel are of great concern in the contexts of fuel preheat of standard ICF targets and heating by relativistic electrons in the fast-ignition scheme. We use the recent theory of electron stopping in hydrogenic plasmas, including scattering effects,\footnote{ C. K. Li and R. D. Petrasso, Phys. Rev. E \textbf{70}, 067401 (2004).} to determine the preheating effects on standard ICF targets, the penetration and blooming in fast-ignition fuel, and preheating of the recently proposed shock-ignition scheme.\footnote{ R. Betti and C. Zhou, ``High-Density and High-\textit{$\rho $R} Fuel Assembly for Fast-Ignition Inertial Confinement Fusion,'' this conference.} We also test the collisional electron transport model in the hybrid code LSP and compare it with the theoretical results of electron stopping and energy deposition. This work has been supported by the US Department of Energy under Cooperative Agreement ER54789 and DE-FC52-92SF19460. [Preview Abstract] |
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QP1.00139: Measurement of Super High Velocity of Laser-Driven Planar Target for Impact Fusion Ignition T. Sakaiya, H. Saito, H. Azechi, K. Otani, T. Shiota, D. Ichinose, K. Shigemori, M. Murakami, S. Fujioka, M. Nakai, H. Shiraga, H. Nagatomo, A. Sunahara, K. Mima, M. Karasik, J. Gardner, J. Bates, D. Colombant, J. Weber, S. Obenschain, Y. Aglitskiy A totally new ignition scheme of ICF, called ``Impact Fusion Ignition (IFI),'' has been recently proposed [Nucl. Instrum. Methods A. 67, 544 (2005)], in which a compressed main fuel is ignited by impact collision of a fragment of separately imploded fuel (impactor). A most critical requirement for IFI is to achieve a super high velocity ($>$1000 km/s) of the impactor. One then needs to substantially suppress Rayleigh- Taylor (RT) instability for a stable acceleration of the target. We have observed super high velocity by utilizing such a RT suppression technique as double ablation in high-Z doped targets [S. Fujioka et al., Phys. Rev. Lett. 92, 195001 (2004)]. We will present and discuss the first experimental results on IFI. [Preview Abstract] |
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QP1.00140: Short-Pulse High-Intensity Laser Generated Electrons Transport in solid targets Kramer Akli, M. Key, S. Hatchett, R. Snavely, G. Gregori, A. Mackinnon, D. Hey, J. King, R. Town, S. Wilks, R. Freeman, L. Van Woerkom, D. Clark, N. Patel, K. Highbarger, R. Weber, R. Stephens, P. Norreys, K. Lancaster, J. Green, C. Gregory, F. Beg, S. Chen, C. Stoeckel, W. Theobald Understanding the propagation and energy deposition of relativistic electrons generated by high-Intensity short-pulse laser is essential to fast ignition. We conducted a systematic study of the electron beam energy transport using the Vulcan laser facility in the United Kingdom. The laser Energy was 400 J delivered in either 1ps or 10ps. We used a 5 $\mu $m thick Molybdenum targets with two different layers. The Vanadium layer was 1 $\mu $m thick and located at the back of the target. The Nickel layer was 0.5 $\mu $m thick and located at various depths into the target. Highly Ordered Pyrolytic Graphite Crystal (HOPG) was used to obtain the X-ray spectra generated by the interaction of the laser pulse with the solid targets. The experimental results will be presented and the modeling will be discussed. This work was performed under the auspices of U.S. Department of Energy by The University of California, Lawrence Livermore National Laboratory under Contract No. W-7405-Eng-48. Some of this work was undertaken as part of a United Kingdom university collaboration funded by CCLRC. [Preview Abstract] |
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QP1.00141: Electron energy transport in recent petawatt laser plasma interaction experiments James Green Energy transport experiments have been performed using the Vulcan petawatt laser facility at the Rutherford Appleton Laboratory. Electron transport has been investigated by irradiating plain Cu, Al, and Al-Cu-Al sandwich targets, diagnosed using transverse probing images and K-alpha rear-side imaging. Comparisons will be shown with the global plasma expansion of thin foil targets observed in a transverse probe after the interaction pulse. It is likely that the electron refluxing pattern can be inferred from transverse probe images of thin Cu foils. Divergent flow has been observed from the K-alpha images and will be compared with the flow pattern observed with transverse optical probing. Computational modeling of the experimental results, incorporating density profiles measured interferometrically during the experiment will also be presented. [Preview Abstract] |
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QP1.00142: Measurements of electron and proton heating temperatures from XUV images at 68 eV in PW laser experiments Peimin Gu, Richard Freeman, BingBing Zhang Short pulse isochoric heating by electrons and protons has been studied in experiments using the Rutherford Appleton Laboratory (RAL) PW laser. An XUV imager at 68 eV recorded the spatial patterns of heating. Temperatures were deduced from absolute intensities and comparison with modeling using a hydrodynamic radiation and physics code (Lasnex). The experimental method including calibration of the XUV mirrors and CCD cameras will be discussed. Data analysis of both electron and proton temperature patterns from the XUV images will be presented. This work was part of fast ignition research in collaboration with colleagues from GA and LLNL and supported by the US Office of Fusion Energy Sciences. [Preview Abstract] |
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QP1.00143: Generation of fast electrons and electron transport in fast ignition plasma J. Tonge, M. Tzoufras, F.S. Tsung, W.B. Mori, C. Ren The characteristics of fast electron generation by the laser ignition pulse at the critical density layer of the target plasma and the transport of these electrons to a fast ignition target core are questions which bear on the viability of the fast ignitior concept. Results of 2.5D massively parallel full PIC simulations of scale Fast Ignitor targets (half size) and Fast Ignitor relevant plasmas are presented. With these simulations we explore mechanisms for fast electron generation by the ignition pulse and electron transport to the core. We also look at the effect of electron current termination at the core on the fast electron generation and transport. This determines whether the problem of fast ignition can be broken up into parts (fast electron generation, transport, and energy deposition at the core) for simulation or the problem must be treated as a whole. [Preview Abstract] |
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QP1.00144: Laser Driven Shock Experiments for Deuterium EOS Studies Jaechul Oh, Andrew Mostovych With the Nike KrF laser facility at the Naval Research Laboratory, we have conducted laser driven shock experiments along the primary Hugoniot of deuterium in the pressure range 25 $\sim $ 200 GPa (0.25 $\sim $ 2 Mbar). A streak camera was used to resolve the optical self-emission from the shocks and provide information about the shock temperatures. A NIST traceable lamp with photomultipliers was used to calibrate \textit{in situ} the device for the temperature measurements. Velocity interferometer system for any reflector (VISAR) measured the shock speed and the reflectivity at the shock front. The preheat effect on the shock formation is also investigated. The results from these measurements will be presented to evaluate various EOS models. This research was performed in Laser Plasma Branch, Plasma Physics Division, Naval Research Laboratory and was supported by the U.S. Department of Energy. [Preview Abstract] |
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QP1.00145: Material dynamics at extreme pressures and strain rates Bruce A. Remington Solid state experiments at very high pressures and strain rates are possible on high power laser facilities, albeit over small spatial and time scales. A shockless drive has been developed on the Omega laser that spans peak pressures of 0.1 -- 2 Mbar (10 - 200 GPa). VISAR measurements establish the high strain rates, 1.e6 -- 1.e8 1/s. Solid-state strength is inferred using the Rayleigh-Taylor instability as a ``diagnostic.'' Temperature, compression, and phase can be deduced from EXAFS measurements. Lattice response, phase, and grain size can be inferred from x-ray diffraction. Deformation mechanisms and integral response can be identified by examining recovered samples. We will review our work in laser-based materials science, then present our plan for reaching much higher pressures, P $>$ 10 Mbar (1000 GPa), in the solid state on the NIF laser facility. [Preview Abstract] |
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QP1.00146: Compression of diamond foils to metallic phase Keisuke Shigemori, Takeshi Shiota, Kazuo A. Tanaka, Masatake Yoshida, Kunihiko Wakabayashi, Hirofumi Nagao, Ken-ichi Kondo, Kazuto Otani, Hirosshi Azechi, Kunioki Mima It is suggested that carbon under extreme pressure ($>$ 14 Mbar) will transform to metallic phase of BC8 crystal structure. We demonstrated an experiment to compress diamond foils to such a phase with intense laser by means of the shock impedance mismatch technique. Diamond foils with buffer foam ($\sim $ 150 $\mu $m$^{t})$ and gold tamper (5 $\mu $m$^{t})$ were irradiated with laser ($\lambda $: 0.35 $\mu $m) at an intensity of 2 $\times$ 10$^{13}$ W/cm$^{2}$. We measured the particle velocity and the reflectivity of rear surface of the foils with a velocity interferometer system for any reflector (VISAR). We also measured the temperature at the rear surface with an optical pyrometer. The measured signal from the VISAR clearly shows increasing reflected light from the diamond foils at a certain time. Also measured from the pyrometer indicate that the temperature of the diamond was $\sim $ 8000K, which is within the non-melting phase. The shock breakout time and the temperature from the measurements are in good agreements with calculations from 1-D simulation code ILESTA. [Preview Abstract] |
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QP1.00147: Formation of Plasma Targets Suitable for Equation of State Experiments. John Benage, Cindy Christensen, James Cobble, Evan Dodd The measurement of the Equation of State (EOS) of materials in the dense-plasma state is difficult. The standard method for measuring EOS relies on the shock driven Hugoniot technique, where the material is initially at standard temperature and pressure and is shocked using a flyer plate. The locus of states produced using this technique is called the standard Hugoniot. However, the states produced do not fall into the regime of dense plasmas, where the EOS of the material is quite uncertain. We are developing a technique for measuring the EOS in a dense plasma, conditions far away from the standard Hugoniot. This technique requires that the initial condition of the material be at densities well below and temperatures well above standard. We have completed initial experiments producing and characterizing the plasma targets, a CH foam that has been heated to 1 eV using gold m-band x-rays created with the Trident laser. The measurements include visible spectroscopy and imaging along with x-ray radiography of this plasma target. Simulations of these initial measurements and of the laser drive necessary to produce a uniform shock in the material are also shown. The conditions that we calculate to be produced by this shock are then compared to models for the EOS of this material. [Preview Abstract] |
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QP1.00148: A Molecular Dynamics Post-Processor James Hawreliak, Bruce Remington, Hector Lorenzana, Daniel Kalantar, Eduardo Bringa, James Belak, Kai Kadau, Timothy Germann, Peter Lomdahl, Justin Wark, Jon Sheppard, Katarina Rosolankova, Huw Davies With the development of tera-scale computing facilities, modeling material behavior can now be done on the atomic scale. Non-equilibrium molecular dynamic simulations can calculate the position and interaction of millions atoms in a simulated material. By post-processing the simulation output files using Fourier transform techniques, it is possible to extract the long scale length ordering in the MD simulations. These techniques can be used on MD simulations of materials such as iron, titanium or copper to determine crystal structure and lattice kinetics as the crystal is strained. From the post-processed analysis a direct link can be made to the experimental measurements made using x-ray diffraction. [Preview Abstract] |
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QP1.00149: Determination of the Charge State Distributions and Electron Temperatures from X-ray Spectra in Au Plasmas at the OMEGA Laser M.J. May, M.B. Schneider, H.K. Chung, D.E. Hinkel, K. Widmann, R.F. Heeter The determination of the charge state distributions (CSD) of highly ionized Au in Non-LTE high-density plasmas ($\sim$ 10$^{21}$ cm$^{-3}$) is critical for benchmarking radiation-hydrodynamic physics codes. Predictive calculations of the CSD have produced widely varying results. Previous experiments on NOVA have had some success in guiding the models at 2 keV. We present Au CSD's inferred from reduced-scale hohlraum target experiments at the OMEGA Laser at much higher electron temperatures. Measurements of the 5f$\rightarrow$3d transitions in Ni- to Ar-like Au and the 3d$\rightarrow$2p transitions in Co- to Ne-like Au have been compared to atomic modeling from the Hebrew University Lawrence Livermore Atomic Code to infer the CSD and average ionization state (Z=58.5$\pm$1.3). Improvements in these measurements were possible due to a new spectrometer, the MSPEC, which can accommodate a variety of crystal mounts (e.g. elliptical and convex) and has a better spectral range and resolution than previous instruments. Comparisons of the experiments with CSDs calculated by FLYCHK indicated a plasma electron temperature between 7 and 8 keV. This work was performed by the University of California LLNL under the auspices of the DOE under contract W-7405-ENG-48. [Preview Abstract] |
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QP1.00150: Extreme ultraviolet emission from a laser-produced lithium plasma by use of quasi mass-limited target Takeshi Higashiguchi, Keita Kawasaki, Naoto Dojyo, Masaya Hamada, Wataru Sasaki, Shoichi Kubodera Usage of two laser pulses is an efficient way to produce EUV emission, since a density and temperature of a plasma formed by the first laser pulse are regulated by the second laser pulse. By adjusting the delay of the second pulse, one could maximize the EUV emission at 13.5 nm. A mass-limited lithium aqueous target with several tens $\mu$m diameter was formed in a vacuum chamber through a small capillary nozzle. A subpicosecond Ti:Sapphire laser at a wavelength of 800 nm produced a maximum energy around 30 mJ. The beam was divided by a Michelson interferometer, which produced two laser pulses with energies of 5 mJ. The pulse duration was adjusted around 300 fs (FWHM). Both beams were focused on a micro-jet using a lens with a focal length of 15 cm. The delay time between the two pulses was varied from 100 to 800 ps by use of an optical delay line. Clear enhancement of the EUV emission was observed when the delay between the two pulses was around 500 ps. The experimentally observed delay agrees well with that of a plasma to expand to its critical density of $10^{21}$ cm$^{-3}$. [Preview Abstract] |
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QP1.00151: BASIC PLASMA: SOURCES, ITG, GYROKINETICS, ZONAL FLOWS, AND CHAOS |
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QP1.00152: Construction of the HELCAT dual Helicon/Cathode Linear Plasma Device S. Will, M. Gilmore, C. Watts, J. Herrera, A.G. Lynn Construction is nearing completion on a large (0.5m diameter, 4m length) dual source linear plasma device. A 5kW helicon antenna and 6kW, 15cm oxide coated thermionic cathode are to be placed on opposite sides of the device. The helicon source produces plasmas of density 1-5$\times$10$^{13}$ cm$^{-3}$ and is currently operational. The cathode source has shown densities of up to 1$\times$10$^{12}$ cm$^{-3}$ and electron temperatures of $\sim $10eV in a previous device. The plasma is contained by an up to 2.2kG axial field produced by 12 large water cooled magnet coils. The device will be suitable to a wide range of basic physical experiments, including the study of Alfen waves, of interest to space plasmas. Biased concentric rings will be installed on the opposite end from the cathode (removable for helicon operation), which will be capable of generating shear flow for studying the possible active control of turbulent transport. [Preview Abstract] |
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QP1.00153: Multiple helicon source operation on Alpha Christopher Watts Helicon sources have been shown to be extremely efficient at generating high-density, albeit cold, plasmas. However, the density of plasma scales inversely with the radius of the source, and large area sources have generated only modest densities, $\sim $10$^{18}$ m$^{-3}$ peak. To overcome this limitation we have built a multi-source helicon array in an effort to make a large area, high density plasma. Alpha, Articulated Large-area Plasma Helicon Array, consists of a 4 m long, 0.5 m diameter chamber surrounded by 13 circular magnetic field coils creating a maximum value of 0.2 T on axis. Seven 13 cm diameter helicon antennae act as a single source creating a 40 cm diameter plasma. The helicon array can operate in one of two modes, creating either seven distinct plasma columns (to be used for turbulence and wave spreading studies) or an integrated uniform large diameter plasma column. [Preview Abstract] |
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QP1.00154: High Power Helicon 2 (HPH2) James Prager, Timothy Ziemba, Robert Winglee, John Slough, John Carscadden The high power helicon source, developed at the University of Washington and MSNW, is capable of depositing up to 100 kW of peak power into the plasma. High input powers are obtained using a low impedance, solid state switching power supply, which produces peak oscillating antenna currents up to 2.4 kA. Typical operational frequencies are from 0.3 to 1.1 MHz with optimum performance seen near 600 kHz. The HPH system has been operated in a pulsed manner with shot durations ranging from 30 $\mu $s to several milliseconds, with ambient magnetic field strengths (B0) ranging from 60 to 500 G. Measured source plasma densities in Argon are near 2x10$^{20}$ m$^{-3}$ with electron temperatures of 5-7 eV. Langmuir probe measurements, at the exit of the source and further downstream, show a peaked spatial profile. Both the time of flight and Mach probe measurements indicate a supersonic axial flow. The ion energy distributions show a dual peaked population flowing downstream from the discharge. Maximum sustained directed ion energies using Argon are near 55 eV. The results to be presented include ion energy distributions, spectroscopic data, power loading, and density profiles. [Preview Abstract] |
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QP1.00155: Measurements of wave fields and density profiles in a non uniform helicon plasma Charles A. Lee, Dan Berisford, Roger D. Bengtson We present local measurements of wave fields and electron density in a non uniform helicon plasma. We also present the design of new Langmuir and magnetic probes used to collect both axial and radial measurements in the long, cylindrical geometry of this helicon experiment. This experimental data will be used as input to models currently in development of power absorption mechanisms in a plasma using a helicon source. Our primary motivation is to compare the wave structure and damping of the electromagnetic waves with the models. [Preview Abstract] |
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QP1.00156: Measurements of electron density and energy content in the VX-30 helicon discharge Ella Sciamma, Roger Bengtson, Greg Chavers, Chris Dobson, Jonathan Jones, Franklin Chang-Diaz, Verlin Jacobson, Jared Squire We have observed plasma conditions at several locations in the VASIMR experiment, VX-30, a 20 kW helicon plasma expanding into a nozzle. A three frequency interferometer (70, 90, 110 GHz) provided electron densities at these locations. We made absolutely calibrated spectroscopic measurements of He I and He II lines in the UV, visible, and near IR. A comparison with a collisional radiative model suggested that the actual electron density distribution function was not a Maxwellian, but rather was significantly underpopulated at higher electron energies. We will present preliminary results comparing our measurements with a spectral model using a non Maxwellian distribution. [Preview Abstract] |
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QP1.00157: Electron kinetics in helicon discharge Guangye Chen, Laxminarayan Raja, Alexey Arefiev, Boris Breizman A self-consistent description of the helicon discharge requires power balance analysis, involving electron kinetics. A steady-state electron distribution is established when electron heating becomes balanced by electron energy losses on atom excitation. The rf-electric field and plasma density are the primary parameters that determine key features of the steady-state electron energy distribution function. The electron distribution has been studied in four different heating regimes using the Direct Simulation Monte Carlo method. We have found that the electron distribution is significantly non-Maxwellian in the dense gas regime, where electron-atom collisions dominate. In this regime, the electron distribution has also been calculated analytically, assuming that the rf-field is relatively weak, so that the quiver energy is small compared to the excitation threshold. The analytical solution has been used to benchmark the code and the computed distribution agrees well with the analytical distribution. The observed non-Maxwellian feature of the electron distribution function has a strong impact on the cost of ionization in the discharge. [Preview Abstract] |
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QP1.00158: Axial cavity for helicon waves in a nonuniform plasma Alexey Arefiev, Boris Breizman, Mikhail Pekker Plasma nonuniformity has a strong effect on the structure of helicon modes. In particular, a density gradient across the magnetic field supports a surface-type wave, localized in the strong gradient area [1]. This so-called radially-localized helicon mode has recently been observed experimentally in an axisymmetric plasma column [2]. The experimental results indicate that the plasma forms a cavity for the mode along the field lines, as the wave transit time along the column is much shorter than the wave damping time. In this work, we investigate the phenomenon of axial wave trapping analytically and numerically. We have shown that 1) a sharp drop in plasma density along the field lines causes a nearly complete reflection of the radially-localized helicon mode; 2) the reflection is accompanied by the excitation of a conventional whistler. Since the axial wavelength of the excited whistler is much greater than that of the radially-localized wave, only a small portion of the energy is transferred to the whistler. We have used these analytical results to validate our rf-field solver, which is designed to study waves in an axisymmetric plasma column. The solver will ultimately be used for self-consistent modeling of helicon plasma sources [3]. [1] B.N. Breizman and A.V. Arefiev, Phys. Rev. Lett. 84, 3863 (2000). [2] M.I. Panevsky and R.D. Bengtson, Phys. Plasmas 9, 4196 (2004). [3] A.V. Arefiev and B.N. Breizman, Phys. Plasmas 11, 2942 (2004). [Preview Abstract] |
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QP1.00159: Development of a Large Diameter Helicon Plasma Source for Development of the Motional Stark Effect with Laser-Induced Fluorescence Diagnostic John DeBardeleben, Elizabeth L. Foley, Fred M. Levinton While the Motional Stark Effect (MSE) diagnostic has been successful in measuring magnetic field pitch angle and radial electric field in tokamaks, there is need to extend its application to measurement of low field ($<$0.5T) as well as magnetic field magnitude. The use of laser-induced fluorescence on a diagnostic neutral beam is being pursued to this end (MSE-LIF). The test bed for the new diagnostic will be a large diameter helicon plasma source. The design goals are 10$^{13}$ cm$^{-3}$ peak density with $>$10 cm plasma radius. A neutral beam with low axial energy spread will be injected into the plasma. Hydrogen atoms within the beam will undergo collisions with the plasma, which must be of sufficient density and size for the beam to reach equilibrium excited state populations. A laser tuned to the H-alpha transition will excite electrons from n=2 to n=3. The resulting fluorescence can be analyzed to determine the magnetic field pitch angle and magnitude. This poster will describe the design and construction of the spiral antenna helicon plasma source. [Preview Abstract] |
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QP1.00160: Ion temperature in the ASTRAL helicon plasma source Robert Boivin, David Branscomb Ion temperature is measured in the ASTRAL (\textbf{A}uburn \textbf{S}teady s\textbf{T}ate \textbf{R}esearch f\textbf{A}ci\textbf{L}ity) helicon plasma source by means of a diode laser based Laser Induced Fluorescence (LIF) diagnostic. ASTRAL produces plasmas with the following parameters: n$_{e}$ = 10$^{10}$ to 10$^{13}$ cm$^{-3}$, T$_{e}$ = 2 to 15 eV and T$_{i}$ = 0.03 to 0.5 eV. A series of 7 large coils produce an axial magnetic field up to 1.3 kGauss. A fractional helix antenna is used to introduce rf power up to 2 kWatt. The 1.5 MHz bandwidth diode laser has a Littrow external cavity with a mode-hop free tuning range up to 15 GHz and with a total power output of about 15 mW. The wavelength is measured by a wavemeter and frequent monitoring prevents wavelength drift. For Ar plasma, the laser tuned at 668.61 nm, is used to pump the 3d$^{4}$F$_{7/2}$ Ar II metastable level to the 4p$^{4}$D$_{5/2}$ level. The fluorescence radiation between the 4p$^{4}$D$_{5/2 }$and the 4s$^{4}$P$_{3/2 }$levels (442.6 nm) is monitored by a PMT. Other diagnostics are presently installed on the plasma device. They included a RF compensated Langmuir probe which is used to measure both electron temperature and plasma density. A spectrometer which features a 0.33 m Criss-Cross Scanning monochromator and a CCD camera is used for spectroscopy studies of the plasma. [Preview Abstract] |
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QP1.00161: Helicon plasma produced using a flat spiral antenna Takao Tanikawa, Shunjiro Shinohara, Kyoichiro Toki A large volume (75 cm in diameter and 486 cm in axial length) helicon-plasma device has been developed, and the plasma characteristics in the device have been investigated [1]. The device utilizes a large flat spiral antenna (43 cm in diameter) for plasma production installed just outside a quartz-glass window at the end of the vacuum chamber. Unique properties of the device (such as, its high discharge efficiency and easily adjustable radial density profile) will be summarized. In addition, a new device (20 cm in diameter, and 100 cm in axial length) with a new type of flat antenna for plasma production has recently been completed. This antenna consists of four concentric broken circular elements. By changing the connection among the antenna elements, waves with a higher azimuthal mode number can be excited. Preliminary experimental results using this device will be presented. \par \noindent [1] S. Shinohara and T. Tanikawa, Rev. Sci. Instrum. {\bf 75}, 1941 (2004); Phys. Plasmas {\bf 12}, 044502 (2005). [Preview Abstract] |
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QP1.00162: Collisional-radiative modelling of an Ar helicon plasma discharge Stuart Loch, Michael Pindzola, David Branscomb, Robert Boivin We report on recent modelling results of emission observed from a helicon plasma, comparing theoretical and observed line intensities and line ratios of Ar, Ar$^+$ and Ar$^{2+}$. Our Helicon plasma is from the ASTRAL device at Auburn University, with spectral measurements from 275 nm through to 1015 nm. We concentrate on the Ar$^+$ ion stage, and present the results of a collisional-radiative model using various qualities of atomic data. In particular, we compare the modelling results using Plane-Wave Born, Distorted-Wave and R-matrix electron impact excitation data with those observed from the plasma. As part of the modelling work, we investigate the potential use of various lines as plasma diagnostic tools. [Preview Abstract] |
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QP1.00163: A New View on the Potential of H-minus Ion Volume Production Marthe Bacal The ``Volume production'' of hydrogen negative ions is due to dissociative attachment of low energy electrons to rovibrationally excited molecules. These molecules can be produced by electron collisions, and also through recombinative desorption from surfaces, e.g. the plasma grid of an ion source[1]. It is shown that the second process may be dominant at high plasma density. The surface treatment by dense plasma promotes a ``hot-atom reactions'' enhancing the recombinative desorption. Several examples of H-minus accelerator ion sources with performances of $\sim $100 mA/cm$^{2}$ confirm this behavior. Fusion-dedicated, cesium free, ion sources can thus be conceptualized on this principle.\newline \newline [1] M. Bacal, A. Hatayama and J. Peters, IEEE Trans. Plasma Sci. Dec,2005 [Preview Abstract] |
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QP1.00164: Discharge mode transition in a high-pressure RF capacitive discharge S.Y. Moon, D.B. Kim, J.K. Rhee, W. Choe $\alpha $ and $\gamma $ mode of a RF helium capacitive discharge were investigated at higher than 5 torr up to atmospheric pressure. The discharge source consisted of two parallel electrodes of same diameters of 60 mm for avoid the self-bias voltage. The discharge gap was fixed as 1 cm at (5 -- 200) torr and varied from 0.5 mm to 5 mm at atmospheric pressure. $\alpha $ and $\gamma $ modes and the mode transition were observed with a nearly 40{\%} voltage drop and a 55{\%} V-I phase angle decrease. The relation between the mode transition voltage and the multiplication of pressure and distance (\textit{pd}) looked similar to the Paschen curve. At atmospheric pressure, the mode transition occurred abruptly with an instantaneous arc generation, different from a smooth transition at lower pressures. At less than 3 mm gap, an abnormal glow discharge occurred, showing a linear current increase with respect to the voltage. At 3 mm gap, $\alpha $ mode excited as a normal glow discharge with a constant current density (17 mA/cm$^{2})$. At over 5 mm gap, either $\gamma $ mode was excited or the discharge was extinguished. It means there is a critical (\textit{pd})$_{cr}$ value for $\alpha $-mode generation at atmospheric pressure, like at lower pressures. From the experimental result and a simple electrical circuit model, we conclude that the transition between two modes resulted from the $\alpha $-sheath breakdown. [Preview Abstract] |
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QP1.00165: ECR-driven multicusp H$^{-}$ volume source operated in pulsed or cw mode Panayiotis Svarnas, Jacque Breton, Phillipe Auvray, Marthe Bacal Electron cyclotron resonance (ECR) driven multicusp H$^{-}$ volume hybrid source [1, 2] operates in continuous (cw) or pulsed microwave (2.45 GHz) mode up to 3 kW. The hydrogen plasma is produced between 1 and 7 mTorr by seven elementary ECR sources housed in the magnetic multipole chamber ``Camembert III'' [3]. This ECR configuration could be applied both to accelerator and fusion ion sources. Negative ion or electron extracted currents and plasma characteristics are studied in both modes with electrical measurements, electrostatic probe and photodetachment. The role of the plasma electrode bias in the values of the extracted currents is major. H$^{-}$ current is maximized for a bias voltage close to plasma potential. An optimum pressure at 4-5 mTorr yields enhanced H$^{-}$ density in the center of the chamber, under cw regime. Finally, the post-discharge formation of H$^{-}$, in the pulsed mode, is observed. [1] A.A. Ivanov Jr., C. Rouille, M. Bacal, Y. Arnal, S. Bechu, J. Pelletier, Rev. Sci. Instrum. 75(5), 1750 (2004) [2] M. Bacal, A.A. Ivanov Jr., C. Rouille, P. Svarnas, S. Bechu, J. Pelletier, AIP Conf. Proc. No 763 (Kiev, Ukraine) (2004) [3] C. Courteille, A.M. Bruneteau, M. Bacal, Rev. Sci. Instrum. 66(3), 2533 (1995) [Preview Abstract] |
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QP1.00166: Convergence Study of Long-Term ITG Turbulence Simulations with the GTC Code S. Ethier, W.W. Lee, W.M. Tang The 3D gyrokinetic particle-in-cell code GTC has been successfully employed in studying microturbulence in toroidal fusion devices for several years now [1]. Recently, new GTC simulations of ITG turbulence using the adiabatic electron approximation but including the usually ignored ion velocity-space nonlinearity showed a faster evolution towards steady-state than the original simulations without this extra term [2]. In this work, several simulations of this type are carried out to assess the convergence of the steady-state transport for an increasing number of particles at a fixed device size. The resulting ion thermal conductivity, entropy production, and perturbed velocity-space distribution will be presented.\newline [1] Z. Lin, T. S. Hahm, S. Ethier, and W. M. Tang, \textit{Phys. Rev. Lett.} \textbf{88}, 195004 (2002). \newline [2] W. W. Lee, ``Steady State Global Simulations of Microturbulence,'' \textit{Bull. Am. Phys. Soc}. \textbf{49}, no. 8, 135 (2004). [Preview Abstract] |
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QP1.00167: Revisiting Gyro-Kinetics as a Transport Theory, I of II G. G. Plunk, Eric Wang, Steve Cowley A derivation of the non-linear Gyro-kinetic equation using Method of Multiple Scales is presented. Results are obtained for slab and toroidal field geometries. This approach assumes three disparate frequency scales: the ion cyclotron frequency, turbulent frequency, and transport rate. Also assumed are two important spatial scales: the ion larmor radius ($\rho_i$) and plasma size ($L \sim n/|\nabla n|$). The fundamental expansion parameter is $\epsilon = \rho_i/L$. The Fokker-Planck equation is expanded in orders of $\epsilon$, up to the Gyro-kinetic equation. Finally, a physical interpretation of Gyro-kinetic equation is given. [Preview Abstract] |
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QP1.00168: Revisiting Gyro-Kinetics as a Transport Theory, II of II Eric Wang, G. G. Plunk, Steve Cowley Given the Gyrokinetic equation, we present the remaining equations needed to obtain closure. This includes applying the proper ordering to quasi-neutrality and Ampere's Law. The time evolution of the equilibrium density and temperature are derived. The relationship between collisions, entropy production, and heating is described using entropy balance and energy conservation equations. Toroidal geometry of the equilibrium magnetic field is assumed. [Preview Abstract] |
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QP1.00169: Transport, noise, and conservation properties in gyrokinetic plasmas Thomas Jenkins, W.W. Lee, J.L.V. Lewandowski The relationship between various transport properties (such as particle and heat flux, entropy production, heating, and collisional dissipation) [1] is examined in electrostatic gyrokinetic simulations of ITG modes in simple geometry. The effect of the parallel velocity nonlinearity on the achievement of steady-state solutions and the transport properties of these solutions is examined; the effects of nonadiabatic electrons are also considered. We also examine the effectiveness of the electromagnetic split-weight scheme [2] in reducing the noise and improving the conservation properties (energy, momentum, particle number, etc.) of gyrokinetic plasmas. \newline \newline [1] W.\ W.\ Lee and W.\ M.\ Tang, Phys.\ Fluids {\bf 31}, 612 (1988). \newline [2] W.\ W.\ Lee, J.\ L.\ V.\ Lewandowski, T.\ S.\ Hahm, and Z.Lin, Phys.\ Plasmas {\bf 8}, 4435 (2001). [Preview Abstract] |
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QP1.00170: Modulational Instability Effects on the Transition to Collisionless ITG Turbulence Roman Kolesnikov, John Krommes Transition to collisionless curvature-driven ITG turbulence has been discussed via the application of systematic dynamical systems analysis\footnote{R. Kolesnikov and J. Krommes, Phys.\ Rev.\ Lett. \textbf{94}, 235002/1 (2005).} to a low-dimensional truncated model [including a drift wave (DW), a zonal flow (ZF), and a DW sideband]. This method allows one to calculate the Dimits shift of $\nabla T$ due to ZF generation. However, although the center-manifold dynamics demonstrate the basic physics of the Dimits shift, the lowest-order truncation does not saturate above the point of ZF destabilization. In this work we study the effects of long-wavelength envelope modulations\footnote{R. Kolesnikov and J. Krommes, Phys.\ Plasmas (submitted).} on saturation. We show that the system can undergo the transition to turbulence via the Benjamin--Feir mechanism. Collisional and collisionless scenarios are contrasted, and a collisionless version of the Ginzburg--Landau equation is derived. While a collisional system becomes modulationally unstable at linear marginality, collisionless systems exhibit that behaviour at the point of the Dimits-shift destabilization, the regime of suppressed turbulence being stable to long-wavelength modulations. [Preview Abstract] |
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QP1.00171: ITG driven turbulent transport in the toroidal plasma including external heating Kazuhiro Miki The dynamics of zonal modes such as zonal flows and zonal pressures and also zonal magnetic fields plays an important role in regulating the turbulent transport. Specifically, the zonal pressure which correspond to a profile relaxation sometimes cause an intermittent transport and blow-off events. Such a profile relaxation, which may also link to the zonal flows, is sustained by external heating, leading to a quasi-steady state. So far many simulations have been done by initially assuming a linearly unstable profile. Here, we investigate the turbulent transport based on our gyro-fluid global toroidal code$^{1}$ by introducing the effect of external heating. In order to model experimental discharges, the plasma pressure is allowed to gradually evolve from linearly stable profile to unstable one, crossing a critical gradient. We found that the ratio of the zonal flow energy to that of turbulent fluctuations depends on the structure of the zonal pressure and also heating profile. It is found that the zonal flow level when the profile relaxation is allowed becomes higher than that in the case of the fixed profile. $^{1}$N. Miyato, Y. Kishimoto, and J. Li, Phys. Plasmas \textbf{11,} 5557 (2004). [Preview Abstract] |
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QP1.00172: Gyrokinetic simulations of a Z pinch Paolo Ricci, Barrett Rogers, William Dorland We present nonlinear gyrokinetic simulations of small-scale turbulence in a Z-pinch magnetic geometry using the GS2 code. We explore the dynamics of the turbulent transport as a function of various parameters in the system, including the collisionality and the strength of the plasma gradients. In good agreement with analytic calculations, at sufficiently steep gradients we observe the ideal interchange instability. At weaker gradients somewhat below the ideal threshold, turbulence is driven by the so-called entropy mode, which has peak growth rates at $k_\perp\rho_i \sim 1$. The particle transport driven by this mode is very strongly dependent on the strength of the gradients and the collisionality. The break-up of the linear modes by the Kelvin-Helmholtz instability generates strong ExB (zonal) flows along the symmetry axis that, in the absence of collisions, can greatly reduce or even essentially eliminate the transport. In the presence of sufficiently strong collisions, however, these zonal flows can be strongly damped, and a large enhancement of the transport is observed. [Preview Abstract] |
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QP1.00173: Drift waves in helically symmetric stellarators Tariq Rafiq, Chris Hegna The local linear stability of electron drift waves and ion temperature gradient modes (ITG) is investigated in a quasihelically symmetric (QHS) stellarator and a conventional asymmetric (Mirror) stellarator. While the eigenfunctions have a similar shape in both magnetic geometries, they are slightly more localized along the field line in the QHS case. The most unstable electron drift modes are strongly localized at the symmetry points (where stellarator symmetry is present) and in the regions where normal curvature is bad and magnitude of the local magnetic shear and magnetic field is minimum. Modes are found more affected by the normal curvature than by the geodesic curvature. The threshold of stability of the ITG modes in terms of $\eta_i$ is found to be 2/3 in this fluid model consistent with the smallest threshold for toroidal geometry with adiabatic electrons. Optimization to favorable drift wave stability has small field line curvature, short connection lengths, the proper combination of geodesic curvature and local magnetic shear, large values of local magnetic shear and the compression of flux surfaces in the bad curvature region. In most of these studies, a simplified adiabatic electrons response is used. Progress on a more rigorous treatment of the electrons will be reported that accounts for toroidally and helically trapped particles and landau resonances in stellarator geometry. [Preview Abstract] |
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QP1.00174: Progress on simulating high-n energetic particle driven instabilities with the GEM code Yang Chen, Scott Parker, Guoyong Fu GEM is an explicit $\delta \! f$ particle code with kinetic electrons and EM perturbations \footnote{Y.~Chen and S.~E.~Parker, J. Comp. Phys. 189 (2003) 463}. The code has been recently extended to handle general toroidal equilibrium magnetic field configuration and arbitrary equilibrium density and temperature profiles. The perturbed magnetic field is given by $\delta {\bf B}_\perp=\nabla A_\parallel\times{\bf b}$, with $A_\parallel$ given by the parallel Ampere's law. The electric potential is obtained from the quasi-neutrality condition. It has been shown \footnote{H.~Qin, ~W.~M.~Tang and G.~Rewoldt, Phys. Plasmas 5 (1998) 1035} that this gyrokinetic model recovers the MHD equation for the shear Alfv\'en modes, and previous numerical studies have also shown that this model accurately describes the slab shear Alfv\'en waves and the Kinetic Ballooning Modes. Work is underway to implement energetic particle species, with experimentarily relevant equilibrium distributions, such as the slowing-down distribution. The primary coupling of the hot particles to the bulk plasma comes from the hot particle term in the quasi-neutrality condition. Possible extensions of the model, such as the inclusion of the perturbed parallel magnetic field through the pressure ballance equation, will be discussed, with an emphasis on the solution method. [Preview Abstract] |
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QP1.00175: Gyrokinetic equations for the non-linear simulation of toroidal tearing modes Simon Allfrey, Steven Cowley, Bill Dorland The standard gyrokinetic ordering is given by $\omega/\Omega_i \sim k_\|/k_\perp \sim e\phi/T_e \sim \rho_i/L_n \sim \delta B/B \sim \mathcal{O}(\epsilon)$, $k_\perp \rho_i \sim \mathcal{O}$$(1)$. We derive equations with a modified electromagnetic gyrokinetic ordering appropriate for the description of tearing modes in toroidal geometry. While the radial wave-number of the perturbation remains of the same order as the ion gyroradius, the perpendicular variation \textit{within} the magnetic surface is one order lower in $\epsilon$. An `inner' solution to these equations, in the region of the rational surface, is matched to and `external' MHD solution encapsulated by a quantity $\Delta'$ [Furth \textit{et al} 1963]. These equations will form the basis of numerical simulations of magnetic island evolution. The eventual application of this formulation will be the study of the non-linear interaction of turbulence and evolving island structures. [Preview Abstract] |
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QP1.00176: Effects of Plasma Shaping on Nonlinear Gyrokinetic Turbulence E.A. Belli, G.W. Hammett, W. Dorland The effects of flux surface shape on the gyrokinetic stability and transport of tokamak plasmas have been studied using the GS2 code. Studies of the scaling of nonlinear turbulence with shaping parameters have been performed starting with a representative JET-like flux surface and artificially varying elongation, triangularity and their radial gradients together using the Miller analytic equilibrium formalism\footnote{R. Miller, et al, Phys. Plasmas \bf{5}, 973 (1998).} to approach the circular limit via linear interpolation. Both linearly and nonlinearly, high elongation coupled with high triangularity was found to be stabilizing on the ITG turbulence. However, while shaping had little effect on the linear critical temperature gradient, high shaping resulted in a larger upshift of the nonlinear critical gradient due to enhanced zonal flows. The effects of electromagnetic dynamics coupled with shaping are also presented. For electromagnetic runs, beta is also varied with shaping to keep the Troyon-normalized beta fixed while also holding $q_{95}$ fixed. Preliminary results indicate that beta strongly effects the electron transport and may lead to unsaturated transport in some cases, even well below the linear ballooning limit. [Preview Abstract] |
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QP1.00177: Discrete Particle Noise and Its Effects on Particle-In-Cell Simulations of Plasma Turbulence G.W. Hammett, W.M. Nevins, A.M. Dimits, D.E. Shumaker, W. Dorland In order to understand the differences between gyrokinetic particle simulations and gyrokinetic continuum simulations of Electron Temperature Gradient (ETG) turbulence, we have investigated the role of discrete particle noise. A detailed theory of the spectrum of noise fluctuations in a gyrokinetic particle simulation has been developed. With no free parameters, this theory agrees very well both with the fluctuation spectrum and the transport levels observed at late times in gyrokinetic particle simulations when noise dominates. The theory also matches the simulations well as the average squared weight (and thus the fluctuation energy) is varied by a factor of 500 in noise restart tests. The theory is based on Krommes' calculation of the gyrokinetic noise spectrum\footnote{J.A. Krommes, Phys. Fluids B {\bf 5}, 1066 (1993).}, extended to include the effects of numerical filtering, finite-size particles, and a resonance-broadening type of renormalization of the dielectric shielding and of the test particle trajectories. The noise builds up in time in present $\delta f$ algorithms and eventually becomes large enough in typical particle simulations to suppress ETG turbulence, thus explaining why they give lower transport levels at late times than observed in continuum simulations. [Preview Abstract] |
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QP1.00178: Geodesic Acoustic Modes in a Tokamak Plasmas A. Smolyakov, X. Garbet, M. Ottaviani, G. Falchetto Recently geodesic acoustic modes (GAM) have been identified in a number of experiments and numerical simulations. GAM are rotational modes that occur as a result of the compression of poloidal rotation in a toroidal geometry. In ideal MHD, they correspond to two neutral ($\omega=0$) modes that are degenerate in a cylindrical plasma immersed in the axial magnetic field. These modes, uniform poloidal rotation and uniform ($m=0$, $k_z=0$) slow compression, become GAM in the toroidal case. We study GAM both with kinetic and fluid theory, such as that used to study the ion temperature gradient driven turbulence. We also investigate the transition of GAM into the slow modes in the classical theory of the neoclassical rotation. Effects of Landau damping, higher order toroidal coupling, and excitation of GAM by fast particles are studied. Results of fluid numerical simulations of ion temperature gradient driven turbulence exhibiting zonal flow and GAM generation will be presented. [Preview Abstract] |
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QP1.00179: Coupling of zonal flows and geodesic acoustic modes: effects on turbulent transport Paolo Angelino, Alberto Bottino, Roman Hatzky, Sebastien Jolliet, Olivier Sauter, T.M. Tran, Laurent Villard The zonal E$\times $B flow (ZF) associated with axisymmetric electrostatic perturbations has a stabilizing effect on turbulence. In toroidal systems, ZFs show an oscillatory behavior due to coupling with poloidally asymmetric perturbations, called geodesic acoustic modes (GAMs). Recent studies show a dependence of GAMs on the safety factor q. ZFs show larger oscillations in high q regions than in low q regions. This affects the turbulent transport because the oscillatory ZFs are less effective in suppressing the turbulence than the steady ones. Simulations with the code ORB5 give new insight into the mutual interactions between ion temperature gradient modes (ITGs), ZFs and GAMs. ORB5 is a global nonlinear electrostatic gyrokinetic PIC code. It has the capability to simulate MHD numerical equilibria, keeping into account the ZFs full geometrical coupling. A q value scan at constant shear has been performed, showing GAM oscillation suppression with increasing plasma current and the anomalous heat flux decrease. Effects of the q profile shear are also investigated. [Preview Abstract] |
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QP1.00180: Effect of external plasma flows on the interaction between turbulence and convective cells Ken Uzawa, Yasuaki Kishimoto, Jiquan Li It is widely recognized that large scale structures, such as zonal flows, streamers and also long wavelength Kelvin-Helmholtz modes are nonlinearly generated from maternal turbulence through modulational instability process and play a crucial role in regulating the transport in tokamaks. In order to control the transport, it is desirable to control such structures and/or modulational process. One of control parameters may be \textit{mean flow} which intrinsically exists in tokamak plasmas. Besides the direct influence on the transport through vortex decorrelation, the mean flow may indirectly change the zonal flow generation by acting on the modulational process itself. In this work, we theoretically investigate the characteristics of zonal flow generation due to the electron temperature gradient (ETG) turbulence in the presence of long wavelength ITG driven zonal flow. This was done by extending our previous modulational analyses[1]. We have numerically analyzed the influence of mean flow on zonal flow generation. The main result is that the zonal flow generation is suppressed by the presence of the mean flow. [1]J. Li, Y. Kishimoto, Physics of Plasmas, 9, 1241 (2002) [Preview Abstract] |
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QP1.00181: Drift wave and Zonal flow Interaction Roscoe White, Liu Chen, Fulvio Zonca Nonlinear equations for the slow space time evolution of the radial drift wave (DW) envelope and zonal flow (ZF) amplitude within the coherent 4-wave drive wave-zonal flow model of Chen, Liu and White are investigated. The amplitudes are expanded in the set of linear eigenmodes, which reduces the problem to a coupled set of scalar time dependent amplitudes. The competition between linear drive/damping drift wave spreading due to linear and nonlinear group velocity and nonlinear energy transfer between DW and ZF determines the saturation levels of the fluctuating fields. This system exhibits chaotic behavior and itermittency, depending on system size and proximity to marginal stability. \\ References: \\ Liu Chen, Zhihong Lin, Roscoe White, Fulvio Zonca, Nuclear Fusion 41, 747, (2001). \\ Liu Chen, Zhihong Lin, Roscoe White, Physics of Plasmas 7, 3129 (2000) \\ Z. Lin, S. Ethier, T.S. Hahm, W. M. Tang, Phys Rev Lett. 88, 195004, (2002). \\ Fulvio Zonca, Roscoe White, Liu Chen, Physics of Plasmas 11, 2488 (2004). [Preview Abstract] |
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QP1.00182: Fractional diffusion models of transport in magnetically confined plasmas Diego del-Castillo-Negrete A class of models based on the use of fractional derivative operators is proposed to describe transport in magnetically confined plasmas. Fractional operators incorporate in a unified framework non-Fickian transport, non-Markovian (``memory'') effects, and non-diffusive scaling. Recently, this formalism was applied to study transport in pressure-gradient-driven plasma turbulence [1]. Here we incorporate finite-size domain effects, boundary conditions, sources, spatially dependent diffusivities, and general asymmetric fractional operators. The model is applied to describe, at a phenomenological level, non-diffusive, non-local transport processes observed in fusion plasmas, including anomalous confinement time scaling, ``up-hill'' transport, pinch effects, and on-axis peaking with off-axis fuelling. [1] D. del-Castillo-Negrete, et al., Phys. Plasmas 11, 3854 (2004); Phys. Rev. Lett. 94,065003 (2005). [Preview Abstract] |
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QP1.00183: Nondiffusive transport in drift/Rossby waves in zonal flows modeled by a fractional diffusion equation Kyle Gustafson, Diego del-Castillo-Negrete, William Dorland Nondiffusive transport is a problem of theoretical and practical interest in plasma physics and fluid dynamics in general. Recently, it has been shown that fractional calculus is a natural and powerful tool for describing, within a unified framework, this type of transport in plasma turbulence [1]. Here we explore the use of this formalism to study test-particle transport by drift/Rossby waves in zonal flows, following [2]. The drift/Rossby wave Hamiltonian gives rise to asymmetric L\'evy flights and non-Gaussian probability density functions (pdfs) for particle displacements. We further examine the behavior of the transport asymmetry in parameter space. Also, we demonstrate quantitatively that a fractional diffusion equation provides a description of nondiffusive transport in this system. Lastly, we discuss preliminary ideas concerning how a similar approach can be applied to describe complex nondiffusive plasma transport problems, such as self-consistent gyrokinetic-Maxwell systems. [1] D. del-Castillo-Negrete, et al., Phys. Rev. Lett. {\bf 94}, 065003 (2005). [2] D. del-Castillo-Negrete. Phys. Fluids {\bf 10}, 576 (1998). [Preview Abstract] |
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QP1.00184: Fractional renormalization of plasma turbulence Raul Sanchez, Ben A. Carreras, David E. Newman, Vickie E. Lynch, Boudewijn Ph. van Milligen Several phenomenological models for plasma transport have been recently proposed based on fractional differential operators [1]. These operators provide with a mathematical framework that may outperform the standard diffusive paradigm when it comes to capturing the strange phenomena observed in tokamak plasmas: canonical profiles, anomalous scalings, etc. The reason is simple. They are designed to model transport mechanisms lacking characteristic scales as those apparently pointed to as relevant by the observed phenomenology. It is however not clear how fractional operators (non-local and non-Markovian in nature) can be formally derived from the usual turbulent plasma equations. In this contribution we will establish such a link by developing a scheme reminiscent of quasilinear theory. A careful analytical and numerical analysis of the conditions under which this procedure can be justified also results in a deeper understanding of the meaning of fractional operators in this context.\\ {\bf REFERENCES}: \noindent [1] B.A. Carreras et al, Phys. Plasmas {\bf 8}, 5096 (2001); B.Ph. van Milligen et al, \emph{ibid.} {\bf 11}, 2272 (2004); D. del-Castillo-Negrete et al, \emph{ibid.} {\bf 11}, 3854 (2004); R. S\'anchez et al, \emph{ibid.} {\bf 12}, 056105 (2005) [Preview Abstract] |
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QP1.00185: Verification of fractional quasilinear renormalization theory using drift-wave turbulence simulations D.E. Newman, R. Sanchez, B.A. Carreras, V.E. Lynch, B.Ph. van Milligen A very recent renormalization scheme for turbulent transport has been formulated in terms fractional differential operators [1]. In this contribution, we test it against numerous tracer particle transport experiments carried out in simulations of drift-wave turbulence in slab geometry [2]. The simplified geometry allows that simulations be carried out for a sufficiently large number of decorrelation times so that the long-term dynamics captured by these operators can be made apparent. By changing the relative dominance of the polarization and ${\rm\bf E}\times{\rm\bf B}$ nolinearities artificially, we tune at will the degree of homogeneity and isotropy of the system. Additionally, externally-driven sheared flows can also be considered. This wide spectrum of options creates a superb environment to test the strengths and weaknesses of the fractional renormalization formalism. With it, the potential for application to more realistic geometries such as those in state-of-the-art tokamak turbulence codes will be assessed.\\ \noindent{\bf References}\\ {\small \noindent [1] {R. S\'anchez, B.A. Carreras, D.E. Newman, V. Lynch and B.Ph. van Milligen, submitted (2005)\\} \noindent [2] {D.E. Newman, P.W. Terry, P.H. Diamond and Y. Liang, Phys. Fluids B \textbf{5}, 1140 (1993) } } [Preview Abstract] |
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QP1.00186: Turbulent Spreading of Energy in Hasegawa Wakatani Model O.D. Gurcan, P.H. Diamond, T.S. Hahm It is well known that the Hasegawa-Wakatani system, which describes the evolution of drift waves, including the effects of non-adiabatic electron response, ``nonlinearly'' conserves energy. Here we discuss the problem of spatial spreading of energy, and conclude that the energy is conserved locally, if electron response is adiabatic. If zonal flows are included (for the case of adiabatic electron response), the spreading rate of total energy, depends on zonal flow damping, rather than the zonal flow amplitude. This derivation is based on a Poynting type theorem $\partial_t[\varepsilon+\varepsilon_{ZF}]+\nabla\cdot(v_r[\varepsilon+\varepsilon_{ZF}])$, which imply that the zonal flows spread along with the turbulence rather than ``lead'' in the spreading process. For the hydrodynamic limit (which naturally includes the zonal or other types of large scale flows) we derived closed form expressions for the nonlinear, spatial energy flux, using a two-scale version of weak turbulence theory. This is a rigorous derivation, which includes all possible resonant triad interactions. We conclude that the spreading of internal energy is stronger, for the hydrodynamical limit. This is not in contradiction with the fact that the large scale structures (not necessarily zonal flows) may be important in turbulence spreading. In fact the dual cascade resulting in large scale flows, but small scale density fluctuations, maybe considered most effective in spreading the density fluctuations. [Preview Abstract] |
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QP1.00187: Potential Vorticity Conservation and Homogenization : Unifying Concepts for Self-Consistent Models of Drift Wave Turbulence P.H. Diamond, O.D. Gurcan, T.S. Hahm, P.K. Kaw We identify a local, exact inviscid invariant of the 3D Hasegawa-Wakatani system, including dynamics (i.e. instability). This invariant is the total potential vorticity (PV). Consideration of PV dynamics facilitates a unified picture of several themes frequently encountered in the modelling of turbulence and transport. Most interesting, however, is the observation that in the absence of sources and sinks, conservation of the mean PV flux forces the sum of the particle flux and vorticity flux to be constant. Thus, recall that the vorticity flux is simply the gradient of the Reynolds stress, the constancy of PV flux implies that a jump in the flow shear amplification across a layer in which the density flux drops. This suggests that the density flux is 'inverted' to a vorticity flux and shear amplification. We also discuss generalizations of mixing length theory, upper bounds on driven sheared flow, and extensions to more complex models. [Preview Abstract] |
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QP1.00188: Time variability of growth rates and transport fluxes in fixed-gradient turbulence Sangeeta Gupta, P.W. Terry, D.A. Baver Bursty transport fluxes are typically observed in local flux measurements, and naturally emerge from models with evolving gradients. In fixed-gradient models, transport fluxes are often regarded as stationary, as are growth rates. We examine time varying growth rates and transport fluxes in simple fixed gradient turbulence models, including ion temperature gradient (ITG), Rayleigh Taylor (RT), and trapped electron mode (TEM). We seek to understand the conditions under which the fixed gradient turbulence models give non-stationary transport fluxes. In the models studied, linearly damped/neutral eigenmodes are excited nonlinearly. We observe that the energy input rate fluctuates in time; however, the transport fluxes fluctuate from positive to negative only in ITG turbulence. A non-stationary energy spectrum is also observed. We will present detailed analyses of time variability in growth rates, transport fluxes, and energy spectrum, and study underlying components such as auto and cross correlation of different eigenmodes. [Preview Abstract] |
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QP1.00189: Study of L-H transitions and transport barrier propagation for a model of coupled heat and particle fluxes with random forcing Mikhail Malkov, Patrick Diamond Accurate transition criteria for the edge and internal transport barriers in Tokamaks are critical problems in magnetic confinement research. A popular model suggested earlier by Hinton and Staebler (Hinton and Staebler, 1993, Ph. Fl. 5, 1281) includes coupling between the nonlinear fluxes of particles and heat in a form of two diffusion equations. Because of the mathematical degeneracy of these equations there is ambiguity in the stationary transition criteria. Both superdiffusive regularization scheme and variational formulation point at the equal area Maxwell rule as a primary transition criterion. However, inclusion of pressure curvature and time dependence alter these results. We study the effect of random particle deposition and heat source to resolve this ambiguity. [Preview Abstract] |
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QP1.00190: Accretion Theory of the Spontaneous Rotation Phenomenon, Consistency with Recent Experiments and Edge Blobs Transport Model M. Lontano, B. Coppi The accretion theory\footnote{B. Coppi, \textit{Nucl. Fusion} \textbf{42}, 1 (2002)} of the ``spontaneous'' rotation of toroidal plasmas, the first to be based on attributing the source of angular momentum near the edge of the plasma column, has found further support in recent experiments carried out by the Alcator C-Mod\footnote{B. LaBombard, 2005 Sherwood Meeting Paper 03-OTR} and the D-IIID\footnote{J.S. de Grassie et al., Paper IAEA-CN-116/EX/6-4Rb} machines. The second basis of the theory involves the intrinsic coupling between spontaneous rotation and thermal energy transport and is consistent with the observation that the variation in the rotation velocity from the L-regime to the H-regime is related to the scaling$^2$ for the threshold to attain the H-regime. Theoretically, the analysis of non-symmetric spectra, relative to the sign change of the ratio of the poloidal to the toroidal wave number has been pursued considering travelling modes driven by the ion temperature gradient in the presence of an inhomogeonous toroidal velocity. The idea that blobs formed at the edge of the plasma are responsible for the ejection of the angular momentum toward the surrounding material wall is being pursued in collaboration with Myra et al. (paper at this meeting)\\ $^*$Supported in part by CNR (Italy) and the US DOE. [Preview Abstract] |
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QP1.00191: Computing Eigenvalues of Symbolic Kinetic Equation A.B. Rechester, R.B. White \def\crlf{\par\vskip-\parskip} Methods of symbolic dynamics provide a novel way of reconstructing the coarse grained phase space dynamics from the analysis of a single turbulent fluctuating variable X(t). Symbolic Kinetic Equation (SKE) described the time evolution of a coarsed grained phase space probability distribution function P$_{\ell ,n }$[1,2]. $ P_{\ell ^\prime,n+1 }=\sum\limits_\ell P_{\ell ,n} \times \Gamma (\ell \rightarrow \ell ^\prime)$; Here n = 0,1,2,$\ldots$,N is a decretized time, index $\ell$ labels different coarse grained phase space volumes and $\Gamma (\ell \rightarrow \ell ^\prime $) is the probability of a transition from state $\ell $ to the state $\ell^\prime $. Stationary solution of SKE is called invariant distribution function P$_ {\ell }$. It corresponds to the largest eigenvalue $\lambda$ = 1. In this paper we are computing the whole spectrum of eigenvalues $\lambda_{i }$ of SKE, which describe the approach to the stationary state. We would like to demonstrate that $\lambda_{i}$ are invariant of the dynamics, that is they are the same for different fluctuating variables. We are using fluctuating variables X$_{i}$(t) generated by analytic model of drift wave turbulence[3] and real tokamak experimental data.\crlf \noindent [1] A.B. Rechester and R.B. White, Phys. Lett. \textbf{A 156}, 419 (1991). \crlf \noindent [2] M. Lehrman and A.B. Rechester, Phys. Rev. Lett. \textbf {87}, 164501 (2001). \crlf \noindent [3] L. Chen, Z. Lin, R. White, Physics of Plasmas {\bf 7}, 3129 (2000). [Preview Abstract] |
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QP1.00192: Numerical Studies of Dissipative Phenomena Due To Generalized Parallel Ion Stress Michael Addae-Kagyah, Eric Held Parallel ion stress related phenomena in fusion-grade plasmas, such as wave damping and flow damping, are studied using the NIMROD [1] code. The study constitutes a tentative validation of the effects of the generalized parallel ion stress on the transport properties and physics of plasma systems. Consequently, this work focuses on numerical simulations of plasma systems in which closures for stress related forces is provided by the generalized parallel ion stress tensor and heat flow recently derived and presented by Held [2]. The integral nature of this generalized tensor enables it to accurately capture non-local, long scale-length contributions to relevant fluid transport quantities, for arbitrary regimes of collisionality. The main applications implemented are NIMROD simulations of wave and flow damping, using a local anisotropic stress in alternation with a nonlocal anisotropicstress (the integral closure). Specifically, sound waves and plasma flow damping rates are are extracted from evolving resistive MHD (and adiabatic) equations in slab geometry. Profiles of damping rate versus temperature (and other parameters) are plotted and analyzed for both single and multiple scale-length flow perturbations. $^{1}$C.R. Sovinec, et al., J. Comput. Phys. 195, 355, (2004). $^{2}$E.D. Held, Phys. Plasmas \textbf{10, } 4708 (2003). [Preview Abstract] |
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QP1.00193: Breakup of Central Meanders and Outer Shearless Tori in the Standard Nontwist Map K. Fuchss, A. Wurm, A. Apte, P.J. Morrison The standard nontwist map, a simple model for degenerate Hamiltonian systems, e.g., magnetic field lines in toroidal plasma devices with reversed magnetic shear profile, is a useful tool for studying the breakup of shearless orbits in such systems in a numerically feasible way. In the past, the breakup of several shearless orbits with noble winding numbers has been studied for this map. [1,2] Here, we conduct similar breakup studies in a parameter range where a shearless orbit of meander shape is encountered, a type of orbit that is exclusively found in nontwist systems. Further, in contrast to previous studies, multiple shearless orbits can exist for certain winding numbers. The breakup of such additional ``outer orbits'' is examined here as well. \newline \newline [1] D.~del Castillo-Negrete, J.M.~Greene, and P.J.~Morrison, Physica D 91, 1 (1996); [2] A.~Apte, A.~Wurm, and P.J.~Morrison, Chaos 13, 421 (2003) [Preview Abstract] |
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QP1.00194: Spatiotemporal Measurements of Fluctuations in a DC Discharge B. McGeehan, D. Olivan, S.A. Cohen, E. Torbert Periodic and chaotic fluctuations in a restricted DC discharge in neon have been measured with capacitive probes placed along the discharge column and by monitoring the discharge current. These fluctuations, in the frequency range 1-50 kHz, show multiple period-doubling bifurcations that eventually lead to chaos as the discharge current is varied. From these bifurcations the Feigenbaum constants have been extracted. A spatiotemporal analysis of the system shows the characteristics of these fluctuations as well as the dispersion relation. The analysis makes use of biorthogonal decomposition, fourier transforms, and other routines to extract space, time, and frequency information needed to obtain the Lyapunov exponents and to reconstruct the phase maps of the system. [Preview Abstract] |
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QP1.00195: Recent Studies of Reconnection and Transport in the Standard Nontwist Map A. Wurm, K. Fuchss, A. Apte, P.J. Morrison Area preserving and symplectic maps have applications in many areas of physics, e.g.\ the study of magnetic field lines and particle orbits in toroidal plasma devices. An example is the {\em standard nontwist map} which has been used to model magnetic field lines in reversed magnetic shear profiles. In this map the so-called twist condition is violated along a curve called the shearless curve. Here we report on recent studies$[1]$ of separatrix reconnection, a global bifurcation that changes the phase space topology in the vicinity of the central barrier, and its consequences for global transport. $[1]$ A.~Wurm, A.~Apte, K.~Fuchss, and P.J.~Morrison, Chaos. {\bf 15}, 023108 (2005) [Preview Abstract] |
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QP1.00196: Plasma self-organization by maximum entropy production Y.-B. Kim Understanding turbulence saturation mechanism in magnetically confined plasma is one of the most important but unsolved problems in plasma physics research. The following hypothesis has been proposed as a possible turbulence saturation mechanism in confined plasma. The confined system filled with plasma, turbulent electromagnetic field and trace amount of neutral particles, e.g., magnetically confined thermonuclear system, will approach to the state of global maximum entry production. This hypothesis determines unique equilibrium plasma profiles without knowing detailed underlying turbulence dynamics in certain cases. This approach is different from the conventional picture of transport; in which source is balanced by linear thermodynamic forces and then transport coefficients are determined from either microscopic theory or experiment. The definition and evolution of entropy in this complex system is introduced and global entropy production rate is maximized under the constraint of particle, momentum, and energy conservation. Results from analytical and numerical calculus of variation will be discussed. [Preview Abstract] |
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QP1.00197: Ionization wave chaos in a glow discharge plasma Ralph B. Wilson IV, Nirmol K. Podder, Anastasia V. Tarasova The temporal dynamics of self-excited ionization waves in an argon plasma is investigated through the use of high voltage electrical probes and Langmuir probes. The fluctuations in floating potentials and ion saturation currents are measured at a fast sampling rate. From these fluctuations, phase space trajectories are reconstructed using various embedding dimensions and time delay parameters. Various chaos identifying parameters, such as correlation dimension and Lyapunov exponent are computed. A detailed analysis of these parameters over a range of plasma conditions is used to determine any interdependence, and thus a means for controlling chaos in the plasma. [Preview Abstract] |
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