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 BP1: Poster Session I |
Hide Abstracts |
Room: Adam's Mark Hotel Grand Ballroom I & II 9:30am |
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BP1.00001: TOKAMAKS, RFPS, AND OTHER CONFINEMENT DEVICES |
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BP1.00002: HBT-EP Active Mode Control Research G.A. Navratil, J. Hanson, A. Klein, Y. Liu, M.E. Mauel, D.A. Maurer, T.S. Pedersen, N. Stillits, J. Bialek, A.H. Boozer, O. Katsuro-Hopkins, D. Maslovsky, S.F. Paul, R. James The HBT-EP active MHD mode control program is studying advanced feedback control algorithms, ITER relevant internal feedback control coil configurations, and simultaneous control of internal and external MHD modes. The HBT-EP approach incorporates a segmented adjustable conducting wall, internal modular feedback control coils driven by a high-speed (10 microsec delay) MIMO digital control system for resistive wall modes (RWM), and edge rotation control using a biased electrode with supersonic nozzle fueling and lithium wall coating. Primary research thrusts are: (i) systematically study required feedback system gain as a function of control coil modularity and toroidal angle coverage compared with VALEN model predictions testing possible breakdown in basic ‘rigid mode’ model; (ii) test advanced feedback control techniques of adaptive filtering and equilibrium state estimation; (iii) study physics of RWM rotation stabilization by controlled variation of critical parameters (rotation, dissipation, and growth rate) using biased electrode ExB flow and control of ion charge exchange viscosity; (iv) use improved understanding and control capability to suppress tearing and kink type MHD modes simultaneously near the ideal wall limit. [Preview Abstract] |
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BP1.00003: High-speed Digital Mode Control Feedback on External Kink Modes in the HBT-EP Tokamak Alexander Klein, Thomas Pedersen, David Maurer, Michael Mauel, Gerald Navratil, Dmitry Maslovsky, Yuhong Liu, Nicolai Stillits, Jeremy Hanson Active feedback stabilization of external kink modes may be required in future economical advanced tokamak reactors. The HBT-EP experiment is uniquely equipped to test various passive and active kink mode stabilization schemes, as it incorporates a segmented moveable conducting wall and an assortment of magnetic sensor and control coils. In addition, active feedback control is accomplished using versatile high-speed digital (FPGA) processors capable of loop rates of 100 kHz. The versatility of the HBT-EP mode control system allows for a multitude of rapidly realizable experimental configurations to explore the limits of mode control feedback and digital feedback algorithms. We present results from discharges near the ideal wall limit involving poloidal sensors and control coils which directly face the plasma. Clear suppression of the external 3/1 kink mode has been achieved [1]. The effects of transfer function phase shifts, loop latency, and control coil coverage have been investigated. In feedback systems with substantial coverage gaps over the magnetic surfaces (such as that proposed for ITER), questions regarding mode rigidity and multi-mode sideband coupling arise. A investigation of these issues will also be presented. [Preview Abstract] |
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BP1.00004: Measurement of the Amplitude and Phase of External Kink Modes Using a Hall Probe Sensor Array on the HBT-EP Tokamak* Y. Liu, G.A. Navratil, D.A. Maurer, M.E. Mauel, T.S. Pedersen, A. Klein, N. Stillits In HBT-EP a one-dimensional high-spatial resolution 20 element Hall sensor array has been developed to directly measure the edge plasma perpendicular magnetic field and its fluctuations as a function of radius with 4 mm resolution. The array employs new small-area, high-sensitivity Indium Antimonde (InSb) Hall probes in combination with a high-density seven layer printed circuit board to provide for connections to supply Hall current, record the measured Hall voltage output signals, and mitigate inductive pickup. The array provides accurate knowledge of edge current profile and safety factor parameters, which are essential in the determination of tokamak equilibrium and stability. Using both the new internal Hall magnetic field measurements and external pick-up coil measurements allows accurate equilibrium reconstruction for stability analysis of HBT-EP plasmas. Plasma stability and equilibrium parameters are then used in the Fitzpatrick-Aydemir equations [1] to derive the RWM and plasma mode eigenvectors describing the external fluxes. In discharges with RWM activity, the measured mode structure in the vacuum region given by the model is consistent with Hall sensor array measurements. *Supported by U.S. DOE Grant DE-FG02-86ER53222. [1] R. Fitzpatrick and A. Y. Aydemir, Nuc. Fusion, \textbf{36,} 11, (1996). [Preview Abstract] |
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BP1.00005: External Kink Control in Low Rotation Plasmas: HBT-EP Experiments and VALEN Modeling D.A. Maurer, J. Bialek, A.H. Boozer, D. Maslovsky, M.E. Mauel, G.A. Navratil, T.S. Pedersen, S.F. Paul Optimized kink mode feedback is predicted to suppress these long- wavelength MHD instabilities up to the ideal-wall beta limit when (i) the sensors used to detect the instability are decoupled from the active control coils, (ii) the control coils couple more strongly to the plasma than to the surrounding conducting wall, and (iii) control loop algorithms have low latency, are noise immune, and have sufficient bandwidth. To quantify the effect of changes in kink mode rotation frequency on control algorithm optimization, plasma and kink rotation is externally adjusted on HBT-EP by applying a voltage to an insertable electrode. The biased electrode has induced large changes in plasma rotation, including the complete reversal of the rotation direction of external m/n = 3/1 kink perturbations. To aid in optimized feedback design in these low rotation plasmas, a method has been developed to extract the single circuit theory coupling constants of Boozer \footnote{A. H. Boozer, Phys. Plasmas 11, 110 (2004).} directly from the VALEN inductance formalism. Comparison of these low dimensional models of kink mode dynamics will be discussed and compared to VALEN predictions and HBT-EP biased probe experiments. [Preview Abstract] |
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BP1.00006: The Effects of Neutral Damping on Resistive Wall Mode Physics R. James, K. Becker, J. Hanson, A. Klein, M.E. Mauel, D.A. Maurer, G.A. Navratil, T.S. Pedersen, N. Stillits The physics of the dissipation mechanism responsible for rotational stabilization of the resistive wall mode (RWM) is an object of intense current research. On the HBT-EP tokamak there is experimental evidence that edge neutral damping is a significant dissipation mechanism that affects tearing mode behavior [1]. We describe initial progress made towards investigating neutral particle damping effects on RWM dynamics in HBT-EP plasmas. We will report on initial calculations using \href{http://psfcwww2.psfc.mit.edu/people/labombard/}{LaBombard's} (MIT) 1D space, 2D velocity kinetic transport model for atomic and molecular deuterium penetration to quantify profiles of these neutrals within the plasma [2]. In addition, the design, construction, and implementation of a 16-channel linear photo-detector array will be described, and its use to measure D$_{\alpha }$ emission, plasma fluctuations, and neutral penetration profiles. Initial estimates of the RWM dissipation parameter based on these modeling and measurement efforts will be discussed. \newline 1 E. D. Taylor, \textit{et al.,} Phys. Plasmas \textbf{9}, 3938 (2002) \newline 2 B. LaBombard,MIT PSFC RR-00-9, (2000). [Preview Abstract] |
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BP1.00007: Kalman Filters to Reduce Noise Effects during External Kink Control M.E. Mauel, J. Farrington, J. Bialek, O. Katsuro-Hopkins, A. Klein, D.A. Maurer, G.A. Navratil, T.S. Pedersen Magnetic feedback control of the resistive wall mode in tokamaks use derivative (and proportional) gain in order to optimize stabilization\footnote{M. Okabayshi, \textit{et al.}, Phys. Plasmas, {\bf 8}, 2071 (2001).}$^,$\footnote{Y. Liu, \textit{et al.}, Nuc. Fusion, {\bf 44}, 232 (2004).} and to adjust the phase response during control of rotating kinks.\footnote{A.Klein, \textit{et al.}, Phys. Plasmas, {\bf 12}, 040703 (2005).} Derivative gain amplifies noise and can lead to large and undesirable fluctuations in the feedback control current. In this poster, a recipe is presented for the implementation of a Kalman filter that tracks kink mode dynamics as recently described.\footnote{M. E. Mauel, \textit{et al.}, Nuc. Fusion, {\bf 45}, 285 (2005).} Numerical simulations demonstrate the use of the control algorithm for various configurations of magnetic field sensors and control coils used in the HBT-EP device. By properly tracking both the wall and plasma modes, feedback control is maintained up to the ideal wall limit in rotating discharges in the presence of measurement noise. [Preview Abstract] |
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BP1.00008: Computational Modeling of the HBT-EP ICRF Heating System J.M. Hanson, A. Klein, Y. Liu, M.E. Mauel, D.A. Maurer, G.A. Navratil, T.S. Pedersen, N. Stillits, M.D. Carter, R.W. James We describe computational modeling of the HBT-EP dual strap, inside launch ICRF heating system. RF heating is applied at 4.5MHz to deuterium discharges as a means of Beta enhancement. Experiments to date have shown weak antenna-plasma coupling. In order to better quantify loading measurements of the HBT-EP ICRH antenna and aid in optimizing and understanding plasma antenna coupling, two computational models have been developed to calculate antenna parameters. The first is a simple, 2D current stick model that provides information about the local vacuum magnetic field and spectral mode structure generated by the antenna. Results from this model indicate that coupling may be improved by adjusting the antenna's radial position or toroidal size. The second model is implemented using the RANT3D code\footnote{M. D. Carter, \emph{et al.} Nuc. Fusion, \textbf{36}, 209 (1996).}. RANT3D solves Maxwell's Equations given the 3D geometry of the antenna and an impedance matrix for the plasma edge. Progress made using RANT3D to confirm earlier results and suggestions for improving the current antenna design will be presented. [Preview Abstract] |
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BP1.00009: Biased Electrode H-modes on the HBT-EP Tokamak N. Stillits, J.M. Hanson, A. Klein, Y. Liu, M.E. Mauel, D.A. Maurer, G.A. Navratil, T.S. Pedersen An attractive route to higher performance plasmas for small tokamaks is through H-mode confinement improvements brought about by electric fields generated using an electrode inserted into the plasma edge. We have installed a simple mushroom-cap Molybdenum electrode to bias the HBT-EP plasma edge up to 400V with respect to vacuum chamber ground, and bias-induced H-modes are now routinely obtained. In order to characterize the edge plasma parameters further during biasing experiments, a multi-pin triple probe array has been constructed to allow radially and temporally resolved measurements of the evolving electron temperature, density, and plasma potential profiles at four spatial points in the edge. In recent experiments, H-mode characteristics have been measured by means of this multi-pin triple probe array. From this data, a cause and effect relationship between the plasma turbulence, flow shear, and gradients in, e.g., plasma temperature and density is examined. Furthermore, comparisons with quantitative theories are performed. Analysis of these data and results will be presented. [Preview Abstract] |
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BP1.00010: Experiment of beam particle self interaction in JT-60U K. Okano, R. Hiwatari, T. Suzuki, N. Umeda, K. Masaki, K. Tobita, T. Fujita Circulating fast ions generated by NBI will affect the beam stopping cross-section of the neutral beam itself through the interaction between the neutrals and the fast ions, i.e. ``beam-particle self-interaction (BPSI).'' Our estimation has shown that the BPSI effect will be detectable in JT-60U by using 350keV N-NB injection. The beam power and energy were 1.5 MW and 350 keV, respectively. In order to estimate the shine-through power, heat transport in the facing tile on the N-NB line has been analyzed. Without the BPSI effect, the shine-through power should be in proportion to the beam power and therefore to the beam current Iacc. It is found that, however, the time evolution of the tile temperature is not reproduced with the shine-through power in proportion to Iacc in a low density plasma (1x10\^{}19m\^{}-3). Assuming that the shine-through fraction decreases exponentially about by 35{\%} within several hundred msec, the time evolution of tile temperature has been well reproduced. This time scale is close to the build-up time of fast ion component due to N-NB injection ($\sim $200 msec), and the reduction rate of shine-through is consistent to the estimated range by the theory. Therefore, we conclude that the reduction of beam shine-trough observed in this experiment is due to the BPSI effect. [Preview Abstract] |
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BP1.00011: Characterisation of Improved H-mode at ASDEX Upgrade and Extrapolation to ITER Yong-Su Na, O. Gruber, C.F. Maggi, A.C.C. Sips, A. Staebler, J. Stober High confinement and stability is obtained simultaneously in a stationary regime so- called, Improved H-mode at ASDEX Upgrade. This regime is established by various tokamak devices under the common name, ITER Hybrid Scenario. This scenario can provide higher fraction of bootstrap current compared to standard H-modes, therefore allow a long pulse operation in ITER. In this article improved H-modes are characterised systematically by comparing with standard H-modes in similar experimental conditions. Improved H-modes are extrapolated to ITER with ASTRA code calculations, in order to investigate the potential for achieving significant fusion power and fusion gain with increased plasma duration. Interpretive simulations are performed for ITER employing kinetic profiles, H98(y,2) and normalized beta from a typical improved H-mode at ASDEX Upgrade. Predictive modelling for ITER is performed using boundary conditions for improved H-modes at ASDEX Upgrade. [Preview Abstract] |
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BP1.00012: Reconstruction of the electron distribution function during magnetic reconnection events in the TCV tokamak Igor Klimanov, Ambrogio Fasoli, Timothy Goodman, Laurie Porte The sawtooth crash is a well known example of magnetic reconnection in tokamaks. Despite its importance, several aspects of the physics behind it are still under investigation. An important question is the modification of the charged particle distribution by the associated magnetic reconnection. Experiments have been performed on the TCV tokamak (R=0.88, a=0.25, Bt=1.5 T), which is equipped with 4.5 MW of ECH power (3MW - 2nd harmonic, 1.5 MW - 3rd). The main experimental observations have been made using the ECE diagnostic system, which includes two 24-channel radiometers, viewing the plasma from the high field side and the low field side simultaneously, overlapping in frequency. Spikes of non-thermal ECE associated with sawteeth have been observed in both auxiliary heated and ohmic discharges. An analysis of fast changes of the electron distribution function during the crash phase of the sawtooth instability will be presented. [Preview Abstract] |
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BP1.00013: Third Harmonic Extraordinary Mode Heating of L-mode and H-mode Plasmas on TCV Laurie Porte, Stefano Alberti, Gilles Arnoux, Yves Martin, Richard Pitts A third harmonic extraordinary mode (X3) electron cyclotron resonance heating (ECRH) system has been used to heat L-mode and H-mode plasmas on the TCV tokamak. The radiation is launched vertically along a line of constant magnetic field strength to maximize absorption. A real time control system has been deployed and is used to adjust the poloidal angle of the launch mirror to counteract changes in absorption and refraction during the pulse. Results from its application will be presented. Ohmic ELMy H-modes are routinely obtained on TCV, but access to stable, additionally heated ELMy H-modes has only been made possible with the addition of 1.5 MW ($\approx $ 3P$_{\Omega })$. Experiments in which X3 heating is applied to an established ohmic ELMy H-mode have produced stationary phases of large ELMs in which $\delta $W$_{e}$ / W$_{e} \quad \approx $ 10{\%} compared with $<$2{\%} in the ohmic phase such that f$_{ELM}\delta $W$_{ELM}$ remains approximately constant. It has also been possible to access regimes of H-mode like confinement which are ELM-free but quasi-stationary. These experimental results will be described. [Preview Abstract] |
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BP1.00014: Study the Transient Gas Flow Through Tubes Applicable to Disruption Mitigation in Tokamaks M. Bakhtiari, D.G. Whyte, P.B. Parks Recently the high pressure gas injection and conventional massive gas puffing in recent large tokamaks have been shown to be effective disruption mitigation techniques. Injected gas species usually include pure noble gases, hydrogen, or mixtures of them. In order to study such techniques we have developed a shock capturing code to simulate the transient multicomponent gas flows in a tube taking into account the friction forces from the tube on the flow. Validity of the code was confirmed by the available experimental results and the analytical calculations. For the high pressure gas injections such as those in DIII-D disruption mitigation experiments with directed gas tube it is shown that although the flow in the tube is at first supersonic the flow in the end of tube is eventually subsonic or transonic. It is also shown that injecting a mixture of hydrogen and a high-Z noble gas leads to faster and more effective density and radiation rises. The flow of a gas mixture of 2\% of Argon and 98\% of Hydrogen shown to be very similar to that of a pure hydrogen. Fast delivery of small amounts of argon would lead to a fast cold front penetration which consequently makes the hydrogen atoms penetration to be faster toward the central regions. [Preview Abstract] |
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BP1.00015: Improved Profile Diagnostics for the UCLA Tokamak L. Schmitz, T.A. Carter, J.-L. Gauvreau, P.-A. Gourdain, D.J. LaFonteese, W.A. Peebles, R.J. Taylor Ohmic plasmas in the UCLA tokamak ($<$n$><$ 2.7x10$^{18}$m$^{-3}$, kT$_{e}$(0), kT$_{i}$(0)$<$ 250 eV) routinely exceed the Greenwald density limit and approach the beta limit with $\beta _{N}\sim $2. High quality profile data are important for planned detailed studies of density limit and beta limit physics. Equilibrium profiles are acquired using combined data from Thomson scattering (0 $<$ r/a $<$ 0.7), helium line ratio measurements with good spatial resolution (r/a $>$ 0.5, based on localized inboard/outboard gas puffing), and swept Langmuir (Mach probe) array data for r/a $>$ 0.8. Radial ion temperature profiles are obtained using a two-sided ion energy analyzer (r/a $>$ 0.8). Doppler broadening data (using He gas puffing) will be used to obtain ion temperature profiles further into the core plasma. The use of swept probe arrays in the closed flux surface region permits systematic studies of the plasma momentum balance for r/a $>$ 0.85. Spontaneous poloidal flow in the electron diamagnetic direction is observed in Ohmic plasmas and enhanced with ICRF heating. This rotation is believed to be driven by ion orbit loss. The edge electric field and poloidal flow can be reversed by electrode biasing. The scaling of the electric field and plasma rotation with neutral and ion collisionality (0.5$<\nu $*$_{edge}<$20) is investigated. Possible mechanisms for poloidal/toroidal flow generation are discussed. [Preview Abstract] |
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BP1.00016: Pinch Mitigation in the UCLA tokamak P.-A. Gourdain, J.-L. Gauvreau, D.J. Lafonteese, W.A. Peebles, L.W. Schmitz, R.J. Taylor The UCLA tokamak (R = 5 m, B = 0.25 T, A = 5, a = 1 m, I$_{p}$ = 60 kA) produces long-pulse ($<$ 5s), clean plasmas that exceed the Greenwald density limit and approach the \textit{$\beta $} limit. Ohmic discharges exhibit a significant negative radial electric field and associated poloidal rotation, which are thought primarily responsible for an observed particle pinch. Calculations indicate that this ``electric pinch'' is an order of magnitude stronger than the omnipresent Ware pinch. This particle accumulation produces a rather peaked density profile. Due to strong MHD activity, the density build-up slows and a large disruption follows, returning the core density back to initial values without terminating the discharge. A series of experiments aimed at explaining and controlling the pinch are presented. Direct mitigation of the pinch has been demonstrated using low power ICRF, a positively biased electrode or a perturbative magnetic field at the edge of the plasma. [Preview Abstract] |
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BP1.00017: Multi-Frequency ICRFDiagnostic of Tokamak Plasmas D.J. LaFonteese, R.J. Taylor, G.J. Morales Fast Alfven waves at twice the local ion-cyclotron frequency within the radial profile of a tokamak plasma carry information about the global properties of the plasma and are also sensitive to the value of the plasma parameters (e.g. density, ion temperature) at the location of the second-harmonic resonant layer. These properties suggest a new diagnostic capability that relies on the simultaneous launching of several fast waves with frequencies spanning the second harmonic resonance across the plasma radius. The cross correlations of the phase, amplitude, antenna loading, and resonant-Q value of toroidal eigenmodes should, in principle, allow the simultaneous sampling of plasma-profile features. The present system, as implemented in the UCLA Tokamak, launches 6 frequencies and samples the corresponding time series of the propagating magnetic field with small B-dot loops at three toroidal positions. Loading resistances four times larger than vacuum loading are observed at highest densities. Unusually long quiet-periods (300 ms) are revealed by the phase of some frequencies while others remain noisy. Research supported by U.S. DoE grant DE-FG03-86ER53225 [Preview Abstract] |
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BP1.00018: Global Energy Confinement Scaling Predictions for the Kinetically Stabilized Tandem Mirror Jane Pratt, Wendell Horton We study transport dynamics in the axisymmetric kinetically stabilized tandem mirror (KSTM), an attractive magnetic confinement device for achieving steady-state fusion burning. For a MHD stable system, we investigate radial transport models using Bohm and ETG diffusion. Numerical coefficients in these models are taken consistent with tokamak and stellarator databases, providing a conservative radial transport estimate because of the well-known confinement improvement at high $\beta$ as well as the absence of destabilizing toroidal curvature in a mirror machine. The plug mirrors create an ambipolar potential that controls end losses; radial losses are driven by drift wave turbulence, which lowers the electron temperature via ETG radial transport losses. Taking into account the Pastukhov energy and particle end losses, we analyze radial transport equations. For mirror ratio $R_m=9$ and a large density ratio between plug and central cell regions, we use an ion potential of $\phi_i / T_i = 7.8 $ for high axial confinement. Radial profiles, time profiles, and energy confinement times are calculated for a test reactor facility ($L = 30$ m, $B=3$ T, $a= 1.5$ m). We study the conditions necessary for density and temperature to saturate, and investigate whether the dependence of electron radial transport on increasing electron temperature stabilizes the thermal instabilities reported in Hua and Fowler. Work supported by DOE grant DE-FG02-04ER54742. [Preview Abstract] |
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BP1.00019: Generation and Sustainment of a Plasma Magnetic Sail John Slough, Louis Giersch Plasma sail propulsion based on the plasma magnet is a unique system that taps the ambient energy of the solar wind with minimal energy and mass requirements. The coupling to the solar wind is made through the generation of a large-scale ($\sim >$ 30 km) dipolar magnetic field. Unlike the original magnetic sail, the coil currents are conducted in a plasma rather than a superconducting coil with the mass of the sail is reduced by orders of magnitude. Unlike a solid magnet or sail, the plasma magnet expands with falling solar wind pressure to provide constant thrust. The plasma magnet consists of a pair of polyphase coils that produce a rotating magnetic field that drives the necessary currents in the plasma to inflate and maintain the large-scale magnetic structure. The plasma magnet is deployed by the Lorentz self-force expanding outward until the expansion is halted by the solar wind pressure. The results from the initial experiments demonstrated that the appropriate high $\beta $, high current plasma can be created and sustained in the geometry suitable for the space application of the concept. Sufficient current was generated in the plasma magnet to produce a plasma magnetosphere of sufficient pressure to push out well beyond the 30 km scale required. Work is now underway to measure the thrust imparted to the plasma magnet by a large scale surrogate solar wind source. [Preview Abstract] |
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BP1.00020: Overview and Recent Results from the ZaP Flow Z-Pinch U. Shumlak, B. Nelson, C. Adams, J. Buller, D. Den Hartog, R. Golingo, S. Jackson, A. Jennings, A. Madson, J. Newman, D. Palm, J. Pasko, J. Proctor, D. Schmuland, T. Shreve The ZaP Flow Z-Pinch Experiment at the University of Washington investigates sheared flow stabilization in an otherwise unstable configuration. An axially flowing Z-pinch is generated with a coaxial accelerator coupled to a pinch assembly chamber. Magnetic probes measure the fluctuation levels of the azimuthal modes m = 1, 2, and 3. The plasma is magnetically confined for an extended quiescent period where the mode activity is significantly reduced. Multichord Doppler shift measurements of impurity lines show a large, sheared flow during the quiescent period and low shear profiles during periods of high mode activity. The plasma has a sheared axial flow that exceeds the theoretical threshold for stability during the quiescent period and is lower than the threshold during periods of high mode activity. The Z-pinch plasmas are globally stable for approximately 2000 growth times. The end of the quiescent period corresponds to a drop in plasma density and a decrease in plasma acceleration. Recent experimental results suggest a means to extend the experiment to quasi steady state operation. [Preview Abstract] |
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BP1.00021: Spectroscopy Diagnostics on the ZaP Flow Z-Pinch Experiment C.S. Adams, U. Shumlak, B.A. Nelson, R.P. Golingo, S.L. Jackson The ZaP Flow Z-Pinch Experiment investigates the stabilizing effects of sheared flow on plasma instabilities. Z-pinch stability is evaluated based on magnetic mode activity determined by arrays of magnetic probes. The pinch is considered to be quiescent when the value of the normalized m=1 mode is below a heuristic limit of 0.2. A suite of spectroscopic diagnostics determine the emissivity, temperature, and velocity characteristics of the Z-pinch. An Ion Doppler Spectrometer (IDS) determines the time evolution of the intensity, Doppler shift, and Doppler broadening of impurity ion emission, enabling the time evolution of the ion temperature and velocity to be calculated. An Intensified CCD (ICCD) spectrometer measures the spatial profile of the impurity ion radiation, enabling calculation of the velocity shear and radial dependence of the emissivity. Finally, a combination CCD/PMT spectrometer enables the determination of the evolution of the amplitude of a single spectral line as well as the time integrated emission of a range of wavelengths. Results from these diagnostics indicate a strong correlation between periods of quiescence, elevated plasma velocities, and velocity shear greater than the theoretical threshold of $v_z/a > 0.1 kV_A$ required for stability. Furthermore, concurrent usage of these spectrometers has yielded consistent results from all three instruments. [Preview Abstract] |
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BP1.00022: Density Characteristics of a Sheared-Flow Z-Pinch S.L. Jackson, U. Shumlak, B.A. Nelson, R.P. Golingo, E.A. Crawford, T.L. Shreve, J.B. Pasko The ZaP Flow Z-Pinch experiment investigates the effects of sheared flow on gross plasma stability. The sheared-flow Z-pinches produced are characterized by a quiescent period, during which the Z-pinch exhibits low magnetic mode activity, high electron density on axis, and other characteristics of stability. Measurements made with a holographic interferometer are inverted using an Abel inversion and combined with measurements from a multi-chord He-Ne interferometer to track the time evolution of the radial electron density profile. A Z-pinch with a radius of 1 cm and an electron number density profile peaked at $10^{17}$ cm$^{-3}$ is observed during the quiescent period. The electron density drops as the quiescent period ends. These results are in agreement with the time evolution of the density profile from MACH2 simulations. The density profile is used to estimate temperature and current density profiles for the Z-pinch. Experimental parameters such as capacitor bank energy and neutral gas injection are adjusted to investigate their influence on the density and behavior of the Z-pinch. Results of these investigations show that lower capacitor bank energy and injection of more neutral gas into the experiment lead to a longer quiescent period. [Preview Abstract] |
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BP1.00023: Design of a Thomson scattering system for the ZaP experiment R.P. Golingo, U. Shumlak, B.A. Nelson, D.J. Den Hartog, D.T. Schmuland The ZaP Flow Z-Pinch Experiment is presently studying the effect of sheared flow on gross plasma stability. During a quiescent period in the magnetic mode activity, a dense Z-pinch with a sheared flow is observed on the axis of the machine. Presently the local plasma properties are found by deconvolving chord integrated measurements. A single point Thomson scattering system is being built to directly measure the local electron temperature in the Z-pinch. The cost of the system has been minimized by using available equipment: a Korad ruby laser, a Hibshman spectrometer, an ITT MCP and Lecroy 6880 digitizers. Scattered light can be collected up to 4 cm off the axis of the machine. The expected Thomson signal has been calculated to be 10 times the measured background radiation level. Initially the system will measure the electron temperature at a single point in the plasma. The design allows for the system to be upgraded to a multipoint Thomson scattering system which would measure the pressure profile of the Z-pinch. The design of the system and initial results will be presented. [Preview Abstract] |
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BP1.00024: Production and Study of High-Beta Plasma in LDX D.T. Garnier, A.K. Hansen, E.E. Ortiz, M.E. Mauel, A. Boxer, J.L. Ellsworth, I. Karim, J. Kesner, S. Mahar, E. Mimoun, A. Zhukovsky In this poster, the first experiments using the \urllink{Levitated Dipole Experiment (LDX)}{http://psfcwww2.psfc.mit.edu/ldx/} are summarized. Long-pulse, quasi-steady state microwave discharges lasting up to 12 seconds have been produced that are consistent with equilibria having peak beta values of 10\%. Detailed measurements have been made of discharge evolution, plasma dynamics and instability, and the roles of gas fueling, microwave power deposition profiles, and plasma boundary shape. In these initial experiments, the high-field superconducting floating coil was supported by three thin supports and later the coil will be magnetically levitated. The plasma was created by multi-frequency ECRH, and a population of energetic electrons, with mean energies above 50 keV, dominated the plasma pressure. Creation of high-pressure, high-beta plasma is possible only when intense hot electron interchange instabilities are stabilized by sufficiently high background plasma density. [Preview Abstract] |
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BP1.00025: Effect of Electron Cyclotron Resonance Heating Location upon Confinement in a Laboratory Dipole Plasma A.K. Hansen, D.T. Garnier, M.E. Mauel, E.E. Ortiz, J. Kesner, A.C. Boxer, J.L. Ellsworth, I. Karim, S. Mahar An important topic being investigated in the \urllink{Levitated Dipole Experiment}{http://psfcwww2.psfc.mit.edu/LDX/} is the effect on confinement of varying the deposition profile of the electron cyclotron resonance heating. We report the results of using different operational combinations of our RF sources, such as varying the power levels, sequencing of the onset time, and altering the active duration. In addition, we have employed external shaping coils to reduce the plasma volume, which in turn changes the locations of the resonances. Although in the levitated mode of operation the ability to alter the floating coil current, and thereby move the resonances but allow the plasma to occupy its full volume, is severely constrained, the current \emph{can} be varied in the supported mode, and these experiments have been performed. Results from these studies will be presented and discussed. [Preview Abstract] |
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BP1.00026: X-Ray Measurements of the Levitated Dipole J.L. Ellsworth, J. Kesner, D.T. Garnier, A.K. Hansen, M.E. Mauel, S. Zweben Initial plasma experiments in the Levitated Dipole Experiment focus on producing hot electron, high beta plasmas using a supported dipole configuration. Plasmas were created using multifrequency ECRH, and we find that most of the plasma energy is stored in the fast electrons, $T_e \sim$ 50 keV. The energy spectrum of the x-ray emission below 740 keV is measured by a four channel pulse height analyzer using CZT detectors. Temporal resolution is achieved by collecting multiple spectra during each shot. The electron temperature is inferred from the x-ray energy. A hard x-ray camera\footnote[2]{S. von Goeler et. al. Rev. Sci. Instrum. \begin{bf}65\end{bf}, 1621 (1994).} is used to view the spatial distribution of x-ray intensity in the plasma at a maximum of 60 fields per second. In addition, a single NaI detector (which views energies up to 3 MeV) measures the temporal fluctuations of x-ray emission from the plasma. X-ray measurements have shown the profile changes resulting from various ECRH configurations, and they are used with other diagnostics to determine plasma profiles and parameters. [Preview Abstract] |
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BP1.00027: Measurement of Equilibrium Current Profiles of LDX Plasma Ishtak Karim, Jay Kesner, Darren Garnier, Alexander Hansen, Mike Mauel We report measurement of the equilibrium plasma current profiles in the \urllink{Levitated Dipole Experiment (LDX)} {http://psfcwww2.psfc.mit.edu/ldx/} that exhibit a peak beta in excess of 10 percent. The beta of an LDX plasma is calculated by solving the Grad-Shafranov equation using the plasma current profile determined from magnetic measurements. The relevant magnetic sensors include nine pick-up coils normal to the vessel surface, nine coils parallel to the surface, and eight magnetic flux loops. Since the LDX dipole field is produced by a superconducting current ring, the dipole current decreases as the plasma current increases. Equilibrium profiles using different pressure models have been investigated. We find that the magnetic measurements primarily determine the plasma dipole moment, and additional constraints, including ECRH resonance zone locations and x-ray emission profiles, are needed to uniquely specify a pressure profile. The reconstruction results will be discussed along with the conditions that lead to the creation of high beta plasmas. [Preview Abstract] |
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BP1.00028: Microwave Interferometer for the Levitated Dipole Experiment A.C. Boxer, J. Kesner, M.E. Mauel, D.T. Garnier, A.K. Hansen Measuring and understanding the evolution of the plasma density is an important goal for the Levitated Dipole Experiment (LDX). Theoretical considerations suggest that the density profile may naturally evolve to a highly peaked profile with $\delta (n V) \sim 0$, or $n \sim 1/r^4$. Knowledge of the density profile is also necessary for the reconstruction of the overall equilibrium parameters of the confined plasma. In LDX we have built and tested the first channel of a multi-cord interferometer diagnostic using heterodyne receivers at 60 GHz. Using the single-cord interferometer, we have documented the rapid density rise that coincides with the transition from low-density to high beta operation. In the high-beta regime, the line-averaged density is approximately $3 \times 10^{10}$ cm$^{-3}$. Construction has begun on the additional channels that will allow measurement of the density profile and investigation of density profile evolution caused by plasma phenomena. [Preview Abstract] |
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BP1.00029: Probe Measurements of Electrostatic Fluctuations in LDX E.E. Ortiz, M.E. Mauel, D.T. Garnier, A.K. Hansen, J. Kesner, A. Boxer, J.L. Ellsworth, I. Karim High-frequency ($\sim 1$ MHz) and low-frequency ($\sim 5$ kHz) electrostatic fluctuations have been observed in high-beta plasmas created in the \urllink{LDX experiment}{http://psfcwww2.psfc.mit.edu/ldx/}. The high-frequency mode is characterized by frequency-sweeping at the drift-resonance of trapped energertic electrons and identifies the instability as the hot electron interchange (HEI) mode. The HEI mode limits plasma pressure, but it is stabilized when the rate of neutral fueling exceeds a threshold. The fluctuations often appear with coherent structures that have been detected on fast high-impedance electrostatic probes. Magnetic fluctuations of the HEI in the high-beta LDX have been measured, and the phase-relationship between the magnetic and electric fluctuations help to determine how the mode modulates the energetic electron distribution. Measurements that characterize these modes are compared to fast magnetic measurements in an attempt to put together a coherent picture of plasma behavior during these modes, including the consequences of these instabilities on plasma formation and pressure limits are presented. [Preview Abstract] |
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BP1.00030: Photodiode Array Measurements in LDX E. Mimoun, J. Kesner, D.T. Garnier, M.E. Mauel In a plasma with closed field lines such as LDX, large scale convective motions are possible due to a charging up of magnetic field lines. This convection can play an important role in transporting plasma, including local microturbulence, from the plasma core to the edge region. In LDX the plasma density is sufficiently low that neutral atoms can penetrate deeply into the plasma. When these atoms are excited by plasma they emit light in the visible range. Analyzing this light gives information about the convective motions within the plasma. A photodiode gives much faster information than a regular digital video camera and the observation of fast changes in the visible light intensity caused by plasma turbulence can be observed. To obtain spacial resolution we have installed an array of 16 photodiodes in LDX. Using correlation techniques we observe a circulation of plasma such as would be caused by convective cells. [Preview Abstract] |
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BP1.00031: Hot Electron Instability in a Dipole Confined Plasma J. Kesner, N. Krasheninnikova, P.J. Catto, M.E. Mauel In plasma containing energetic electrons, two interacting collective modes, an MHD-like mode and a hot electron interchange (HEI) mode\footnote{N. A. Krall, Phys. Fluids, {\bf 9}, 820 (1966).}, may be present. The linear stability of interchange modes in a z-pinch at arbitrary beta, including a bulk and hot electron species was recently studied\footnote{N. Krasheninnikova, P. J. Catto, Phys. Plasmas, {\bf 12}, 32101 (2005).}. Using the dispersion relation derived in this reference we show that when necessary conditions are satisfied the two modes may be present or absent in a closed-field line magnetic confinement geometry such as a hard core z-pinch or a dipole. The HEI instability and the MHD-like centrifugally-driven mode have been studied previously\footnote{B. Levitt, \textit{et al.}, Phys. Plasmas, {\bf 9}, 2507 (2002), and {\bf 12}, 055703 (2005).}, including a comparison between the measured mode structure and the predictions of a global low-beta simulation. The radial eigenmode is seen to effect the saturation level of the mode. In the \urllink{Levitated Dipole Experiment}{http://psfcwww2.psfc.mit.edu/ldx/} electron cyclotron resonance heating produces high beta plasmas containing hot electrons, and instability observations will be discussed and compared with theoretical predictions. [Preview Abstract] |
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BP1.00032: Effects of Hot Electrons on the Stability of a Closed Field Line Plasma. Natalia Krasheninnikova, Peter Catto Motivated by the electron cyclotron heating being employed on dipole experiments, the effects of a hot species on stability in closed magnetic field line geometry are investigated. The interchange stability of a plasma of background electrons and ions with a fraction of hot electrons is considered. The species diamagnetic drift and magnetic drift frequencies are assumed to be of the same order, and the wave frequency is assumed to be much larger than the background drift frequencies. The background plasma is treated as a single fluid, while a fully kinetic description is employed for the hot species. It is found that geometrical effects significantly complicate the analysis. In general dipolar geometry, poloidal variations of electric and magnetic fields cause the dispersion relation to become an integro-differential equation, which without approximations can only be solved numerically. To examine the possibility of at least a partially analytic solution as well as to obtain an intuitive understanding of instabilities we examine a point dipole and consider the effects of hot electrons to be small and introduce them pertubatively. The dispersion relation is analyzed for the frequency range much smaller as well as of the same order as the hot electron magnetic drift frequency. Two regimes of pressure balance are examined: one dominated by hot electrons and another with the background and hot pressures being comparable. [Preview Abstract] |
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BP1.00033: Neutral Beam Injection In MST B. Hudson, G. Fiksel, R.M. Magee, A.D. Beklemishev, Y. Tsidulko A high power (20 kV, 30A), short-pulse (1.5 ms) neutral beam injector is used to study fast ion confinement and the feasibility of neutral beam heating of an RFP. The confining magnetic field is believed to be stochastic based on both theoretical estimates as well as experimental data on electron heat transport. If fast ions from NBI were also stochastic no significant heating could result before they were lost. The injected fast deuterium ions undergo D-D fusion with the bulk ions and the product 2.45 MeV neutron flux is measured outside the vessel with a scintillator and a photomultiplier tube. The observed neutron signal indicates the presence of a well-confined fast ion population with a characteristic loss time in excess of 30 ms, which is not consistent with a stochastic loss scheme. Analytical and computational studies show that the guiding center drifts alter the safety factor of the guiding center motion and tend to bring it out of resonance with the background magnetic field. The `ion guiding center islands,' which are in direct analogy to magnetic islands, are typically smaller and more separated than the magnetic islands. As a result, the stochasticity of the fast ion population is reduced. The ions are predicted to become stochastic after having lost sufficient energy that their rotational transform approaches that of the background magnetic field. Work supported by D.O.E. and N.S.F. [Preview Abstract] |
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BP1.00034: EBW Heating Experiment in the Madison Symmetric Torus J.K. Anderson, M. Cengher, W.A. Cox, C.B. Forest, S.M. McMahon A heating and current drive system based on the electron Bernstein wave (EBW) is being constructed on the Madison Symmetric Torus. The EBW may facilitate auxiliary heating and current drive in the overdense RFP plasma where ECCD is inaccessible, and may improve stability to tearing modes by supplying the appropriate parallel current. The first step in the experiment is operational and supplies a 10 msec, 120 kW pulse of 3.6 GHz rf power to the plasma edge. Measurements of reflected power can be as low as 20\%, in agreement with coupling measurements made at low power for this antenna. Successful operation at this level validates the technical design for a higher power (300 kW) system based on the same type of antenna (S-band waveguide grill), vacuum windows, and rf sources (75 kW traveling wave tube amplifiers). It has been empirically determined that a dielectric antenna cover is necessary for high power operation. With no cover, plasma from the turbulent edge travels into the antenna resulting in poor coupling and transmission line failure. Experiments show that power handling of the antenna is sensitive to the antenna position, edge plasma conditions, and specific features of the dielectric antenna cover. Although not expected to have a significant effect at this power level ($P_{EBW} << P_\Omega$), data from x-ray and other distribution function diagnostics may show faint signs of heating and are compared to predictions from Fokker-Planck computations. [Preview Abstract] |
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BP1.00035: Localized measurements of ion flow fluctuations in MST D.A. Ennis, D. Craig, D.J. Den Hartog, G. Fiksel, S. Gangadhara Fluctuation induced transport, momentum relaxation and the MHD dynamo are important topics for laboratory plasmas as well as astrophysical plasmas. Measurements of ion flow are essential for furthering our understanding of these phenomena in the laboratory. In MST, C VI emission from neutral beam-induced charge exchange recombination at 343.4 nm is collected by a custom built, high-throughput spectrometer yielding measurements of carbon ion flow localized to +/- 1 cm with high time resolution ($\sim $100 kHz.) We have measured the correlation of poloidal velocity fluctuations with magnetic fluctuations across the plasma radius. Strong correlations are observed for modes with m=1, n=8-10 near the mid radius (r/a $\sim $ 0.6) and the observed phase implies a contribution to the MHD dynamo. Curiously, measurements at high plasma current show no flow correlation with the dominate m=1, n=6 mode perhaps indicative of undetected, very localized flows. Results of a first localized toroidal flow measurement will be presented, along with the effects of boronization on the charge exchange signal. Work supported by U.S.D.O.E. and N.S.F. [Preview Abstract] |
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BP1.00036: Multi-point multi-pulse Thomson scattering diagnostic on the MST Hillary Cummings, Joshua Reusch, Rob O'Connell, Daniel Den Hartog The new Thomson scattering diagnostic on MST is now in regular operation. The system is based on two 2.5 J Nd:YAG lasers, each of which can be fired once per plasma with an energy of 2.5 J or every 20 ms with an energy of 1 J. Thomson-scattered light is detected with 20 four-channel General Atomics polychromators with avalanche photodiode modules (APD). Position calibration of the detection system is performed with a miniature integrating sphere mounted on a stepper-actuated probe inserted into MST. This integrating sphere allows in situ spectral and transmission calibrations when fiber-optically coupled with a wavelength-tunable laser. The gain and noise levels of the individual APDs are found using an LED that can be run in either a 50 Hz pulsed mode to simulate a laser pulse or in a DC mode to simulate background radiation. The gain and noise of the system has been measured to be highly dependent on the temperature of the detectors. Because of this, the APDs are water-cooled, and their temperatures are individually monitored. Remaining sources of system noise are either being mitigated or taken into account in calculation of the temperature measurement uncertainty. [Preview Abstract] |
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BP1.00037: Poloidal Gap Field Error Control and its Effects on the MST plasma T.D. Tharp, A.F. Almagri, B.E. Chapman, D.J. Holly, K.J. McCollam, J.S. Sarff An active feedback control system has been installed to correct radial magnetic field errors at the poloidal gap of the Madison Symmetric Torus (MST). The field errors result from the axial asymmetry presented by the gap in MST's aluminum shell. The error causes increased plasma-wall interactions with adverse effects on several MST performance characteristics. Correcting the error increases the pulse duration and helps preserve plasma rotation. Previously the error correction was passive and incomplete; the new active system results in smaller errors for the entire discharge duration. Tests are planned to study the effects of active control on the duration of improved confinement, pulse-to-pulse reproducibility, fast particle confinement, and quasi-single helicity (QSH) mode rotation. [Preview Abstract] |
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BP1.00038: Effect of perpendicular plasma response on ion heating in RFPs Vladimir Svidzinski, Vladimir Mirnov, Stewart Prager During a sawtooth crash in the Madison Symmetric Torus RFP the ion temperature can spontaneously double in $\sim100\,\mu$s. It is also observed that high Z impurities are heated stronger than bulk ions. The heating may arise from tearing instabilities due to irreversible transfer of kinetic flow energy into heat caused by ion-ion collisions (viscosity). In our analysis we solve the kinetic equation with Landau collision operator in a given perpendicular electric field numerically. The heating rate is proportional to the square of the ion Larmor radius such that it is strong in low magnetic field configurations. An estimate of fluctuating electric field and velocity is based on the numerical solution of nonlinear resistive MHD equations. We found that velocity amplitude is comparable to the ion thermal velocity and that it is very localized near resonance surface. Results show that for bulk ions on ion-ion collision time scale viscous heating is effective only for strong localized sheared flows. High Z impurities are heated more effectively than the bulk because of their higher collision rates, slower thermal velocities and stronger response in given electric field. On shorter time scales ion-ion collisions are not important but the experimentally observed ion temperature rise can be explained by the modification of ion distribution function in strong perpendicular electric field of the tearing mode. [Preview Abstract] |
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BP1.00039: Feedback Systems to Correct Magnetic Field Errors in MST D.J. Holly, A.F. Almagri, K.J. McCollam, J.S. Sarff, T.D. Tharp The MST device has a thick conducting shell with insulated gaps in both the poloidal and toroidal directions where localized magnetic field errors can occur. A feedback system to correct the magnetic field error at the poloidal gap has been in operation in preliminary form for about a year. The poloidal system uses 38 external drive coils and 32 in-vacuum sense coils coupled by a system that allows independent control of spatial Fourier harmonics. Design of a similar system to correct field errors at the toroidal gap is underway. We present the design of the poloidal gap correction system and measurements of its performance, as well as measurements of the field errors at the toroidal gap and initial design of the toroidal gap correction system. [Preview Abstract] |
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BP1.00040: Density Control and Limit(s) in MST Max Wyman, Brett Chapman, Steve Oliva, Stewart Prager, Stephen Combs, Charlie Foust, Dan Fehling, Bihe Deng, Weixing Ding, David Brower The MST RFP is fueled by gas puffing and pellet injection. Densities (n$_{e})$ approaching and exceeding the Greenwald limit (n$_{G})$ have been achieved. In some discharges, the plasma terminates early when n$_{e}$ exceeds n$_{G}$. In others, n$_{e}$ exceeds n$_{G}$ without premature termination. While not yet approaching n$_{G}$, we have increased n$_{e}$ by 70{\%} in improved confinement PPCD plasmas with pellet injection. Sawtooth suppression is observed in high density standard plasmas fueled by pellets and/or gas puffing. Measurements with a multichord far infrared interferometer/polarimeter show that pellet injection peaks the density profile, raising the central density to 5 x 10$^{13}$ cm$^{-3}$, while flattening the current profile. Presently, the density achievable with gas puffing is limited by the throughput ($\sim $10 Torr*L/s per valve) of MST's puff valve system. A limited number of valves are being modified for higher throughput ($\sim $300 Torr*L/s per valve) to achieve higher densities. Work supported by U.S.D.O.E.. [Preview Abstract] |
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BP1.00041: Development of fast helium beam emission spectroscopy on MST Joon-wook Ahn, Gennady Fiksel, Darren Craig, Daniel J. Den Hartog, Jay Anderson, Martin O'Mullane A fast helium beam emission spectroscopy (BES) diagnostic is being developed to measure electron temperature and density fluctuations on MST. The light signals for two HeI triplet wavelengths (587.6nm and 706.5nm) and two HeI singlet wavelengths (501.6nm and 667.8nm) have been observed and analyzed. The observed local emission fluctuations from a fast neutral helium beam injected into the plasma may be inverted to the local plasma parameter fluctuations by making use of a full collisional-radiative model, the Atomic Data and Analysis Structure (ADAS) code package. The existence of metastable fractions (2$^{1}$S and 2$^{3}$S) in the fast neutral helium beam affects the beam stopping and emission coefficients. The local metastable fractions must be known to calculate the plasma parameter fluctuations from the observed local emission fluctuations. A system of statistical balance equations is solved to estimate the local metastable fractions and thus the total line emission intensities at a given wavelength. This requires $T_{e}$, $n_{e}$, and $Z_{eff}$ profiles as an input and therefore implies that the calculated emission fluctuation reflects both the local plasma fluctuation and the global plasma parameters. This work was supported by the US Department of Energy. [Preview Abstract] |
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BP1.00042: Magnetic field inference from simulated images of a heavy ion beam in a plasma Chao Ling, Kenneth Connor, Diane Demers, Richard Radke, Paul Schoch, Jay Anderson Reconstructed ion beam trajectories and subsequently inferred confining magnetic fields are computed using simulated CCD images of an ion beam in a plasma. The simulation assumes that emission from the beam is sufficiently bright to be seen with cameras using narrow band optical filters, and models the effects of 1) beam current density profile, 2) camera position, 3) finite image resolution, and 4) imaging sensor noise. The ultimate goal is to measure the magnetic structure of the Madison Symmetric Torus using the existing Heavy Ion Beam Probe. The technique recovers a 3D ion beam trajectory from a pair of perspectively projected 2D trajectory images. Uncertainty in the reconstructed 3D trajectory is illustrated based on an error propagation analysis of the 2D image pair. The trajectory of an ion with known mass and energy is used to determine the component of a magnetic field that is perpendicular to the ion trajectory. The approach presented here uses a reconstructed simulated ion trajectory in conjunction with a simple shifted-circle flux surface model to reconstruct the full magnetic field. Future work will interface this technique with the equilibrium reconstruction code MSTFit. \textbf{Work supported by US-DOE}. [Preview Abstract] |
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BP1.00043: The role of magnetic fluctuations in ion heating on MST Sanjay Gangadhara, Darren Craig, David Ennis, Daniel Den Hartog, Gennady Fiksel, Stewart Prager Observations of ion heating during magnetic reconnection have been made in a number of laboratory and astrophysical plasmas. On the MST reversed field pinch, ion heating occurs during a sawtooth crash over a fast time scale ($\sim $ 100 $\mu $s) relative to the ion-ion collision time. The mechanism by which energy is deposited in the ions is unknown, but the amount of energy is similar to the drop in stored magnetic energy during the event. The role of magnetic fluctuations is investigated by correlating tearing fluctuation (mode) activity with localized measurements of the impurity ion temperature (T$_{i})$ obtained with fast time resolution using charge exchange recombination spectroscopy. Results indicate that in standard MST plasmas, where the dominant core tearing modes during reconnection are m=1 and the dominant edge modes are m=0, the ion heat source is broad. However, for similar plasmas in which the m=0 mode resonance is kept outside the plasma volume, no ion heating is seen. In addition, measurements suggest that the magnitude and structure of the heating is correlated with fluctuation behavior. Enhanced ion temperatures have also been observed in improved confinement plasmas when the hard x-ray flux is extraordinarily large. Correlations between T$_{i}$ and magnetic mode activity for these plasmas will also be presented. Work supported by U.S.D.O.E and N.S.F. [Preview Abstract] |
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BP1.00044: Experimental Investigations of Broadband Magnetic Turbulence in the Madison Symmetric Torus Reversed Field Pinch A.F. Almagri, D. Craig, J.S. Sarff, L. Marrelli, P. Martin, P. Piovesan Reversed field pinch plasmas are well documented to exhibit long wavelength magnetic fluctuations, a few percent of the mean field, due to MHD tearing instability. However, the origin of fluctuations observed at the smaller scales is not understood. The wave number and frequency spectra exhibit power decay suggestive of turbulent cascade physics, but the inertial range and dissipation range are not well separated in scale, if the spectrum is cascade in origin. The amplitude of fluctuations at all scales increases and decreases with the amplitude of the unstable tearing modes, controlled by varying plasma conditions, also suggestive of cascade physics at work. Other statistical properties also depend on the tearing amplitude, with increased intermittency observed when the tearing fluctuations are large. Studies to date have been limited to measurements at the plasma surface. A multi-coil movable probe with a 5mm coil separation is developed to investigate the radial dependence of the magnetic spectrum, important since the radial correlation lengths are very short at high frequency. The frequency bandwidth of the measurements is also extended to several MHz, which is at or above ion gyro frequencies. [Preview Abstract] |
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BP1.00045: Measurement of the drive of m=0 and m=1 modes in the RFP Seung Choi, Abdulgader Almagri, Brett Chapman, Darren Craig, Stewart Prager Magnetic fluctuations with poloidal mode numbers m = 0 and m=1 occur in MST as bursts in time.~ MHD computation predicts that m = 0 modes are linearly stable, but are driven through nonlinear coupling to unstable m = 1 modes.~ An experimental study is underway to determine the origin of the m = 0 mode.~ In the plasma edge we have measured the MHD linear drive term directly with magnetic and Langmuir probes (to measure the fluctuating E x B velocity). We find the m = 0 mode to be linearly damped during standard RFP plasma, consistent with theory. Extension of these measurements to the nonlinear drive terms involving coupled m=1 modes is underway. Measurements have also been made of m=0 (dominant mode in EC plasma) drive terms in EC plasma, which is self-generated enhanced confinement, and of m=1 (dominant mode in QSH plasma) drive terms in QSH plasma (Quasi Single Helicity). Work supported by U.S.D.O.E and N.S.F. [Preview Abstract] |
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BP1.00046: Hard-X-Ray Profile Measurements of Quasi-Single-Helicity Plasmas on MST D.J. Clayton, R. O'Connell, B.E. Chapman, J.A. Goetz, M.C. Kaufman, M.A. Thomas, R.W. Harvey Hard x rays (HXR) with energies up to 100 keV, indicating the presence of well-confined high-energy electrons, have been observed for the first time during quasi-single-helicity (QSH) plasmas in an RFP. The MST HXR diagnostic has been expanded to a radial array of 16 CdZnTe detectors, measuring an energy range from 10-300 keV. Measured HXR spectra can now be input into the CQL3D Fokker-Planck code to calculate the electron diffusion coefficient as a function of radius as well as velocity. HXR measurements are now being used to study QSH plasmas, where one mode dominates the core tearing mode spectrum. Soft-x-ray tomography measurements on RFX and MST and temperature measurements on RFX indicate that an island associated with the dominant mode is hotter than the surrounding plasma. Simulations predict closed flux surfaces, and hence improved confinement, within the island. Time-resolved HXR profile measurements during QSH in MST should determine if this is indeed the case. Work supported by the USDOE. [Preview Abstract] |
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BP1.00047: Transport Analysis on MST using Thomson Scattering and Equilibrium Reconstruction J.A. Reusch, J.K. Anderson, H.D. Cummings, D.J. Den Hartog, C.B. Forest, R. O'Connell Improvements in the Thomson Scattering system on MST coupled with the equilibrium reconstruction code MSTFit have enabled higher resolution transport analysis with improved accuracy in the plasma edge. Uncertainty in $\nabla$Te has been drastically reduced by both the higher spatial resolution of the new Thomson system (up to 20 points per laser pulse) and careful spatial calibration of the system using an insertable probe, which removes virtually all error in radial position. This greatly improves the accuracy of transport quantities such as the electron thermal conductivity, $\chi_e$. The new system can collect multiple Te profiles in a single discharge, helping to constrain the time rate of change of several quantities, most notably the stored thermal energy, $\dot{W}$, the pressure profile, and hence the magnetic equilibrium. We have developed a procedure to compute the ohmic input power, $P_\Omega$, based on the time rate of change of the equilibrium without a measurement of $Z_{eff}$. This has greatly improved the reliability of the measured energy confinement time, $\tau_E$. Sample transport analysis results are presented. [Preview Abstract] |
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BP1.00048: Progress in Oscillating Field Current Drive Experiments in the MST RFP A.P. Blair, F. Ebrahimi, K.J. McCollam, P.D. Nonn, S.C. Prager, J.S. Sarff To test Oscillating Field Current Drive, two 280Hz 1 MVA oscillators are installed in the toroidal and poloidal magnetic field circuits of the MST reversed field pinch. Current drive of about 10\% has been demonstrated, comparable to theoretical predictions. However, maximum current drive does not coincide with maximum helicity injection - possibly due to an observed dependence of both core and edge tearing modes on the relative phase of the oscillators. An entrainment of the natural sawtooth frequency to our applied oscillation was observed. Varying the natural sawtooth frequency via the nominal plasma current yields a controllable number of sawteeth per cycle while sustaining the entrainment effect - albeit less consistently at low current levels. A significant dependence of wall interactions on phase was observed, the largest interaction coinciding with negative current drive. A relaxed state model using measured gap voltages was used to study the effects of voltage amplitudes, frequencies, and waveforms on current drive and magnetic field profiles. Predicted current drive was comparable to experimental values. Phase dependence was not, due to the models sensitivity to noise and sawteeth. [Preview Abstract] |
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BP1.00049: Coupling to Electron Bernstein Waves with a Phased Twin Waveguide Antenna in the MST Reversed Field Pinch Mirela Cengher, Jay Anderson, Vladimir Svidzinski, Cary Forest Coupling to the electron Bernstein wave (EBW) via a phased array of waveguides is experimentally investigated in the MST reversed field pinch (RFP). EBWs may provide localized heating and current drive in overdense plasmas such as those in the RFP. Choice of antenna structure and launched wave polarization are important factors in optimizing coupling to EBW. The theory predicts the coupling efficiency will vary with launch angle and to depend sensitively upon the edge density profile. Amplitudes and phases of reflected power in each of the waveguides are measured experimentally, and compared to predictions. Reflection and phase were measured for different polarizations, launch angles and time varying density profiles. An asymmetry in reflection predicted by theory was found experimentally for X-mode with best reflection below 15{\%}. The dependence on density scale length predicted by theory was confirmed in the experiment. The phase of the reflected signal is shown to contain reflectometry-based information about the edge density profile. Work supported by USDOE. [Preview Abstract] |
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BP1.00050: Study of Current Profile Dynamics in MST with Fast Three-Wave Polarimetry-Interferometry B.H. Deng, D.L. Brower, W.X. Ding, J.K. Anderson, B.E. Chapman, K.J. McCollam, D. Craig, S.C. Prager, J.S. Sarff, M.D. Wyman A recently developed three-wave FIR laser interferometer- polarimeter system for MST is now routinely operational. The fast time response (~0.004 ms) for simultaneous density and current profile measurements allows one to follow profile evolution through (1) pellet injection, (2) oscillating field current drive, (3) the sawtooth cycle, and (4) enhanced confinement operation on MST. During pellet injection, the central current density first increases when the pellet ablates at the plasma edge and then collapses when the central electron density increases. When the pellet is fully ablated, sawtooth activity is suppressed. During oscillating field current drive, the driven edge poloidal magnetic field perturbation propagates to the plasma core on a characteristic reconnection time of about 0.2 ms. A functional fitting method has been developed to determine the toroidal magnetic field profile using the polarimetry measurements along with average and edge toroidal field measurements. The derived on-axis toroidal magnetic field agrees quantitatively with MSE measurements. [Preview Abstract] |
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BP1.00051: Improved-confinement plasmas with energetic electrons and increased ion temperature observed in the RFP. Rob O'Connell Recent improved-confinement plasmas in the Madison Symmetric Torus (MST) exhibit anomalously large hard x-ray (HXR) production and a two-fold increase in the ion temperature. Current profile modification using edge current drive (PPCD) is a well-established method for the reduction of magnetic fluctuations in the reversed-field-pinch (RFP)\footnote{MST (PRL 78 1997 62-65), RFX (PRL 82 1999 1462-1465), TPE-RX (PPCF 44 2002 335-349)}. In the MST, as the magnetic fluctuation level drops the electron temperature increases by up to a factor of three and hard x-rays, absent during standard RFP plasmas, are produced, indicating the presence of closed flux surfaces. Recent high current ($>500$ kA) PPCD plasmas exhibit a higher than predicted further increase in HXR flux. Thomson scattering measurements of the electron temperature show a large increase, however non-Maxwellian distortion of the measurements is suspected. Measurements of impurity ions also show, for the first time, an increase in the core ion temperature to approximately $800$ eV. CQL3D Fokker-Planck modeling suggests knock-on effects may explain the increase in x-rays. The large increase in the ion temperature is presently unexplained, but mechanisms such as wave-particle interation with energetic electrons are being explored. [Preview Abstract] |
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BP1.00052: Sawtooth-cycle variation of electron temperature in MST, and prospects for improvement of fast Thomson scattering measurements D.J. Den Hartog, H.D. Cummings, R. O'Connell, J.A. Reusch Initial measurements with the new Thomson scattering diagnostic on MST show a flattening of the Te profile during a sawtooth crash. These measurements were made in standard sawtoothing reversed-field pinch discharges, and show the core temperature dropping from 400 to approximately 150 eV, while the edge rises several-fold. Measurement of Te time dynamics in MST will be advanced by further development of the Thomson scattering diagnostic. In the near term, two independently triggerable lasers will be used to make two Te profile measurements separated by greater than or equal to 100 ns. By varying this separation time over the course of a data ensemble, an initial Te fluctuation spectrum will be produced. In the longer term, a third ``pulse-burst'' laser will be added to the diagnostic system. This laser will produce a burst of 10-30 approximately 1 J Q-switched pulses at repetition frequencies 5-250 kHz. The planned laser system will operate at 1064 nm and is based on existing Nd:YAG systems used to study fluid dynamics [Brian Thurow et al., Appl. Opt. 43, 5064 (2004)]. The burst train of laser pulses will enable the study of Te and ne dynamics in a single MST shot, and with ensembling, will enable correlation of Te and ne fluctuations with other fluctuating quantities. [Preview Abstract] |
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BP1.00053: Lower Hybrid Experiments on MST M.C. Kaufman, J.A. Goetz, M.A. Thomas, D.R. Burke, D.J. Clayton Current drive using rf waves has been proposed as a means to reduce the tearing fluctuations responsible for anomalous energy transport in the RFP. A traveling wave antenna that operates at 800 MHz and $n_{\|}\approx7.5$ is being used to launch lower hybrid waves into MST to assess the feasibility of this approach. The antenna routinely operates at 80 kW in a variety of plasma conditions including high confinement plasmas. Parameter studies show that edge density is a major factor in antenna/plasma coupling. The power damping length of the antenna, important for controlling the wave spectrum, will be compared to theory under different plasma parameters. Localized gas puffing near the antenna is shown to decrease the power damping length in plasmas that normally do not couple well to the antenna. Hard x-rays (HXR) in excess of 20 keV have been observed during lower hybrid injection and correlate well with rf input power. An HXR target probe will be constructed to perform a radial survey of x-ray producing fast electrons. The next generation antenna with a power handling capability of 300 kW will be installed and tested. [Preview Abstract] |
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BP1.00054: Characterization of Anomalous Ion Heating During Magnetic Reconnection Events using Neutron Flux Measurements Richard Magee, Gennady Fiksel, Ben Hudson, Brett Chapman Neutrons with energies of 2.45 MeV are products of the D-D fusion reaction whose cross section is sensitive to initial ion energy. An ion temperature measurement can therefore be inferred from a measured neutron flux. We apply this to sawtooth crashes, which occur quasi-periodically (T $\sim $ 2 -- 5 ms) on the MST under standard conditions. Rapid ion heating at the time of the crash has been inferred in the past from other diagnostics (CHERS, Rutherford Scattering), but neutron detection is novel in this application on the RFP. The sawtooth crash is thought to be a plasma relaxation associated with magnetic reconnection, although the precise ion heating mechanism is poorly understood. A question currently under investigation is whether the mechanism heats the bulk plasma or generates a runaway distribution. At a sawtooth crash, the neutron flux is seen to rapidly increase in a short time interval ($\tau \quad \sim $ 100 $\mu $s) from zero to a value on the order of 10$^{10 }$s$^{-1}$. The subsequent decay of the neutron flux indicates either the fast ion confinement time or the bulk ion energy confinement time. [Preview Abstract] |
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BP1.00055: Overview of MST Results and Plans S.C. Prager Recent results in the MST reversed field pinch include improved confinement with copious energetic electrons and elevated ion temperature; good confinement of fast ions with large gyro-radii in the standard RFP (inferred from neutron emission of neutral- beam-injected ions) despite the underlying stochasticity of the magnetic field; sustainment of 10 percent of the plasma current by oscillating field current drive (ac helicity injection), in agreement with theory for the applied power; determination that ion heating during reconnection events is global, as measured by charge exchange recombination spectroscopy; observation that global magnetic self-organization (characterized by sudden changes in magnetic energy, plasma momentum, and ion temperature) occurs only when spontaneous (m = 1) and driven (m = 0) reconnection are present simultaneously; and wave injection from electron Bernstein wave and lower hybrid wave antennas, satisfying theoretical expectations of plasma loading at power levels of about 100 kW. Development of many auxiliary and diagnostic systems is underway. [Preview Abstract] |
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BP1.00056: Uncertainty analysis of the HIBP equilibrium potential measurements in the MST-RFP X. Zhang, K.A. Connor, D.R. Demers, P.M. Schoch Measurements within the Madison Symmetric Torus at r/a $\sim $ 0.35 indicate that the average potential in standard discharges is on the order of 1.55 kV, but there are significant shot to shot variations up to $\pm $ 0.37 kV. To determine the sources of the variation, we consider characteristics of the Heavy Ion Beam Probe, and the interaction of the beam with the plasma and confining magnetic field. The plausible instrumental and plasma affects are investigated in a controlled fashion using simulations; these include beam scrape-off during a sawtooth cycle, non-ideal fields within the energy analyzer, noise on the detectors and loading of power supplies due to plasma and UV radiation, plasma density gradients, and beam attenuation. We present the relative uncertainty of each affect and conclude that the maximum uncertainty due to all examined sources (in the interior of standard discharges) is on the order of $\pm $ 0.14 kV. This is significantly smaller than the scatter of the data, implying other factors such as variations of plasma parameters including mode speed, electron density, and plasma current play a role. [Preview Abstract] |
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BP1.00057: SXR tomographic imaging in MST P. Franz, F. Bonomo, M. Gobbin, L. Marrelli, P. Martin, P. Piovesan, G. Spizzo, B.E. Chapman, D. Craig, J.A. Goetz, S.C. Prager, J.S. Sarff We present SXR tomographic images of multiple magnetic islands in the core of the MST RFP plasma. The tomographic diagnostic is comprised of four multi-channel cameras viewing the plasma at different poloidal angles. The islands appear as a result of pulsed parallel current drive (PPCD). With the application of PPCD, all tearing mode amplitudes can be reduced, and the overlap of the associated islands decreases to such an extent that magnetic flux surfaces are at least partly restored, and multiple, discrete islands can form. In some PPCD plasmas, however, the mode resonant nearest the magnetic axis remains relatively large, resulting in a single island in the plasma core. The presence of multiple or single helical islands indicated by tomographic imaging is confirmed with numerical modeling. The correlation of these structures with transport and magnetic self-organization will be presented. In addition, preliminary measurements with “multicolor” SXR tomography (where the four cameras are used with filters of different thickness) will be shown. Work supported by U.S.D.O.E. [Preview Abstract] |
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BP1.00058: High Power Antenna Design for Lower Hybrid Current Drive in MST M.A. Thomas, D.R. Burke, J.A. Goetz, M.C. Kaufman, S.P. Oliva RF current drive has been proposed as a method for reducing the tearing fluctuations that are responsible for anomalous energy transport in the RFP. A system for launching lower hybrid slow waves at 800 MHz and n$_{||}\approx$7.5 is now in operation at 80 kW on MST. The antenna is an enclosed interdigital line using $\lambda$/4 resonators with an opening in the cavity through which the wave is coupled to the plasma. Although present operation is limited by available transmitter power, a new antenna has been developed for higher power capability. Design improvements include larger vacuum feedthroughs, better impedance matching, and RF instrumentation on all resonators. Full instrumentation will allow more detailed power deposition measurements. The goal is a modular design which can handle 300 kW per antenna and presents a VSWR of 1.4 or better without external tuning. [Preview Abstract] |
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BP1.00059: Fizeau Effect and Measurement Implications for High-Temperature Plasmas W.X. Ding, D.L. Brower, B.H. Deng, D. Craig, V. Mirnov, S.C. Prager, A.H. Mahdavi The Fizeau effect is a phase shift of an electromagnetic wave caused by the motion of a dielectric medium and has been measured in solids, liquids and gases. In MST, measurement of this effect is being pursued for the first time in a plasma. Fizeau interferometry provides a line-integrated measurement of electron current density and ultimately the electron velocity. The estimated phase shift associated with electron motion is estimated to be 2 degrees for typical MST plasmas. This value is well within the phase resolution of the existing FIR laser-based interferometer. The Fizeau interferometer being developed will use counter-propagating beams and measure equilibrium poloidal electron current in MST plasmas. This new diagnostic technique has direct application to burning plasma experiments. Initial results will be presented.\textit{ Work is supported by US DOE.} [Preview Abstract] |
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BP1.00060: Doppler spectroscopy and collective flows in RFXmod B. Zaniol, L. Carraro, E. Gazza, M.E. Puiatti, P. Scarin, M. Valisa, P. Zanca Doppler spectroscopy based toroidal and poloidal flow velocities measured on the modified RFX Reversed Field Pinch are compared with the results obtained in the previous machine [1]. Passive measurements with integrated lines of sight confirm the decaying dependence of the toroidal plasma velocity with electron density but show a higher dynamics with respect to the past, often featuring significant accelerations during the plasma discharge$. $Edge flow, opposite to the core co-current flow, in presence of induced bulges of localized MHD modes or externally induced radial magnetic perturbations reverses its direction: this corresponds to a reversal of the radial electric field. A new diagnostic neutral beam injector (50 kV, 2.5 A equivalent current) has been recently installed for spatially resolved measurements. Preliminary results are presented. Among the main objectives of the system is a detailed measurement of the radius at which the radial electric field reverses and whether such radius coincides with the magnetic field reversal. [1] \textit{L. Carraro et al. }Plasma Phys. Control. Fusion \textbf{40} (1998) 1021-1034. [Preview Abstract] |
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BP1.00061: Self-Similar Current Decay experiments in RFX-mod Paolo Zanca, Sergio Ortolani, Antonio De Lorenzi, Fulvio Auriemma, Rita Lorenzini, Alberto Alfier The Self-Similar Current Decay (SSCD) is a promising procedure to run a reversed field pinch (RFP) discharge. Proposed in a theoretical work by R. Nebel et al (Physics of Plasmas Vol. 9, n.12, (2002) pag 4968) it is a method to decrease the dynamo request and the associated level of magnetic fluctuations thereby improving the RFP global plasma parameters. The concept is that a suitable fast decaying (rate about 6*$\tau$R) of the magnetic field with fixed radial profile allows the reversed field pinch to exist in the cylindrical dynamo-free state. This decay can be induced by forcing the total plasma current and toroidal flux to decrease with the same temporal law. The decay is obtained by applying suitable poloidal and toroidal voltages at the plasma edge. In particular the toroidal voltage reverses its sign becoming negative. Here we present the results of the first SSCD experimental campaign operated in RFX-mod. Some interrelated beneficial effects have been clearly obtained: mode amplitudes reduction, steepening of the temperature profiles and unexpectedly long pulse duration. [Preview Abstract] |
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BP1.00062: Electron heat transport in RFX-mod A. Alfier, P. Nielsen, R. Pasqualotto, L. Marrelli, P. Martin, A. Canton, P. Innocente, R. Lorenzini, F. Auriemma Since the beginning of RFX-mod operations, an enhanced time and spatial resolution Thomson scattering diagnostic has been in operation. It measures the Te profile along a diameter in the equatorial plane with 84 spatial positions, with a spatial resolution of 7 mm from r/a=-.95 to r/a=.84, and a repetition rate of about 25ms, for 10 ND:YLF pulses. We present here a first characterization of Te profiles in low current discharges (Ip $<$ 700kA) and a comparison with lower resolution measurements taken in RFX at similar level of current and density. The on-axis value of the TS diagnostic is in agreement with the double filter Te. Scalings of the on-axis Te with Ip and I/N are presented. Further analysis is shown to compare standard discharge to other regimes such as Rotating Toroidal Field Modulation, Oscillating Poloidal Current Drive, Self Similar Current Decay and Quasi Single Helicity. Temperature and density profiles are used to estimate the confinement time $\tau_E$ and beta $\beta$. [Preview Abstract] |
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BP1.00063: QSH spectra in RFX-mod L. Marrelli, A. Alfier, F. Bonomo, A. Cravotta, P. Franz, M. Gobbin, P. Martin, R. Pasqualotto, P. Piovesan, G. Spizzo This paper is dedicated to an overview of recent experimental results on the quasi-single helicity (QSH) spectra in the RFX-mod device, that recently resumed operations. We report, in particular, on the occurrence of QSH states in low plasma current regimes. The results are compared with those obtained in the RFX device (with a thick shell) and in other RFP devices. Comparison of discharges obtained with a different start-up seems to indicate that a ramping start-up favors the onset of a dominant mode. The equilibrium is found to control the dominant helicity. Poloidally localized intermittent SXR structures are found during QSH regimes. These structures are detected by the SXR tomographic diagnostic. The intermittent behavior of the SXR structure can be correlated with the dynamics of the magnetic spectrum as detected by magnetic diagnostics. We will also describe the results of the first experiments on the active control of QSH state making use of the MHD active control system based on192 independently controlled saddle coils. [Preview Abstract] |
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BP1.00064: Single helicity and quasi-single helicity states in RFPs Charles Bathke, Gian Luca Delzanno, Luis Chacon, John Finn We present a systematic study of single helicity (SH) states and quasi-single helicity (QSH) states in RFPs. We begin with cylindrical paramagnetic pinch equilibria with uniform resistivity, characterized by a single dimensionless parameter proportional to the toroidal electric field, or the RFP toroidal current parameter $\Theta$. For suficiently high $\Theta$, there are several unstable $m=1$ ideal MHD instabilities, typically one of which is nonresonant, with 1/n just above $q(r=0)$. We evolve these modes nonlinearly to saturation for low Hartmann number H. We then obtain the $m=k=0$ quasilinear profiles, which typically have toroidal field reversal, and study their stability. For typical cases, these profiles may remain unstable to tearing modes, but only for sufficiently high $H$. For lower $H$ these states are stable. We show results indicating the proximity of these thresholds to the thresholds between SH and QSH behavior. [Preview Abstract] |
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BP1.00065: Computational Studies of Dynamo Suppression in the Reversed Field Pinch During PPCD Jim Reynolds, Carl Sovinec Laboratory experiments with Pulsed Poloidal Current Drive (PPCD) have shown reduction of the magnetic fluctuations that introduce stochasticity to magnetic field trajectories and lead to anomalous energy transport in the RFP[1,2]. We apply the NIMROD nonlinear MHD code[3] to study the interaction between the mean field evolution and the tearing fluctuations during PPCD. Post processing diagnostics show reduced power transfer from the dynamo fluctuations and the nonlinear growth rates of several modes sustained by nonlinear coupling decline. The linear growth rate calculations of dominant core resonant modes show similar trends to nonlinear results , justifying detailed linear analysis. We track the evolution of terms in Ohm's Law to assess the key features of the PPCD electric field transient that modify the parallel current. We use a linear stability analysis code to reveal how PPCD affects the source of free energy to the core modes. Eigenfunction calculations show early stabilization due to penetrating applied electric fields that reshape the parallel current profile near the mode resonance. [1] Anderson, et al. Phys. Plasmas. 11, L9 (2004). [2] Fiksel, et al. Phys. Rev. Letters. 72 (7) (1994). [3] Sovinec, et al. Phys. Plasmas. 10 (2003). [Preview Abstract] |
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BP1.00066: Hyper-resistivity due to viscous tearing mode turbulence John Finn The quasilinear hyper-resistivity coefficient D for flattening of a current profile (e.g. in RFPs) in resistive magnetohydrodynamics (MHD) is computed. It is found that D is independent of Delta-prime, the constant-psi matching parameter, for viscous tearing modes. This is in contrast with the situation for inertial tearing modes, for which D scales as Delta-prime to the -1/5 power. This situation for inertial tearing modes is problematic because D appears to go to infinity as quasilinear saturation is achieved[J. Krommes and C.-B. Kim, personal communication, 1989], and because D becomes complex for negative Delta-prime. It makes physical sense to compute D for viscous rather than for inertial tearing modes for because inertial tearing modes cross over to viscous modes near quasilinear saturation, when their growth rates are sufficiently small. [Preview Abstract] |
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BP1.00067: Pellet injection on TPE-RX, reversed-field pinch device Haruhisa Koguchi, David Terranova, Paolo Innocente, Rita Lorenzini, Hajime Sakakita, Tomohiko Asai, Yasuyuki Yagi, Yoichi Hirano, Kiyoyuki Yambe We have been operating a pellet injector to control the plasma density profile and to expand the operation region of TPE-RX (reversed field pinch device, minor radius, a=0.45 m, and major radius R=1.72 m). Density in the standard discharge of TPE-RX is restricted by recycling from the first wall. The first wall of TPE-RX is made by SUS and the limiter of TPE-RX is made by Molybdenum. The density operation region is relatively lower than other RFP devices that use graphite tiles. Gas puff experiment and magnetized plasma flow injection experiment also have been carried out in TPE-RX, and resulted in an extension of the density operation region. However, magnetic fluctuations increase due to these methods. Pellet injection also expands the density operation region, and a significant density increase is obtained both in standard and Pulsed Poloidal Current Drive discharges. The density increase in Pulsed Poloidal Current Drive is maintained for a long period, and this result predicts the good particle confinement. [Preview Abstract] |
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BP1.00068: PLASMA ACCELERATORS AND CHARGED PARTICLE BEAM PRODUCTION AND PROPAGATION |
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BP1.00069: TeV class afterburner and related issues Chengkun Huang, M.M. Sieth, D.K. Johnson, W. Lu, C.E. Clayton, M. Zhou, C. Joshi, K.A. Marsh, W.B. Mori, T. Katsouleas, P. Muggli, E. Oz, F.-J. Decker, P. Emma, M. Hogan, R. Iverson, P. Krejcik, R. Ischebeck, R. Siemann, D. Walz The plasma afterburner concept utilizes the output beams from a linac and double the energy of a trailing beam by riding on the plasma wakefield of a drive beam. A TeV class afterburner has been envisioned for the future linear collider, ILC. We investigate TeV class afterburners with realistic ILC parameters. Several important issues such as the optimized wakefield, the head erosion for the drive beam and the hosing instability for both the drive and trailing beam, the betatron radiation and the ion motion in such scenario are studied using the quasi-static PIC model. Results from the simulations will be presented. Work supported by DOE under DE-FGO3-92ER40727, DE-FC02-01ER41179, and DE-FG02-03ER54721 and by NSF under PHY-0321345. [Preview Abstract] |
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BP1.00070: Wake excitation and beam dynamics in self-ionized plasma wakefield acceleration for afterburner parameters Miaomiao Zhou, Chengkun Huang, Wei Lu, Frank Tsung, Viktor Decyk, Chan Joshi, Warren Mori For the parameters envisaged in possible afterburner stages of a plasma wakefield accelerator (PWFA), the self-fields of the particle beam can be intense enough to tunnel ionize some neutral gases. Wake excitation (including the optimal gas density) in self-ionized regimes are investigated by comparing with the pre-ionized wakes through simulation. Critical issues such as the hosing instability and the beam head erosion are also investigated with QuickPIC, a highly efficient 3-D quasi-static particle in cell (PIC) code which makes it possible to model these long term beam dynamics with hundreds of betatron oscillations. QuickPIC has been validated by benchmarking it against full PIC code OSIRIS and previous afterburner experiments. Comparisons with ongoing experiments at SLAC will also be shown. Work supported by DOE under DE-FGO3-92ER40727, DE-FC02-01ER41179, and DE-FG02-03ER54721. [Preview Abstract] |
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BP1.00071: Plasma based wakefield acceleration using a 46MeV multibunched electron beam Efthymios Kallos, Tom Katsouleas, Patric Muggli, Ilan Ben-Zvi, Igor Pogorelsky, Vitaly Yakimenko, Igor Pavlishin, Karl Kusche, Marcus Babzien, Feng Zhou, Wayne Kimura In the multibunch plasma wakefield acceleration scheme a series of electron microbunches are fed into a high density plasma and resonantly excite a wakefield that can accelerate the beam electrons. Here we present some recent experimental results conducted at Brookhaven's Accelerator test Facility (ATF) where $\sim $90 microbunches at 46MeV created through the IFEL effect with a 10.6$\mu $m CO$_{2}$ laser interact with a high density 10$^{19}$cm$^{-3}$ 12mm long plasma. Some further PIC simulations provide insight into the physics of the interaction. [Preview Abstract] |
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BP1.00072: Plasma Dark Current in Plasma Wake Field Accelerators (PWFA) Erdem Oz, Thomas C. Katsouleas, Patric Muggli, Christopher Barnes, Franz Josef Decker, Paul J. Emma, Mark J. Hogan, Rasmus Ischebeck, Devon K. Johnson, Chris Clayton, Richard H. Iverson, Wei Lu, Robert H. Siemann, Chan Joshi, Warren Mori, Dieter Walz, Ken Marsh, Chengkun Huang, Patrick Krejcik, Miaomiao Zhou, Caolionn O'Connell, Suzhi Deng Evidence of particle trapping has been observed in a beam driven Plasma Wake Field Accelerator (PWFA) experiment, E164X, conducted at the Stanford Linear Accelerator Center. Trapped electrons are observed when the wake field amplitude is above a certain threshold. Trapped electrons in plasma accelerators are the equivalent of the dark current in RF accelerators. Calculations show that the trapping threshold is lower in the case of a plasma field ionized by the electron bunch itself than in a pre-ionized plasma. Experimental and simulation results will be presented. [Preview Abstract] |
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BP1.00073: A 3D PIC simulation study of lwfa for $\sim $50fs lasers: 5-1000tw Michail Tzoufras, Wei Lu, Frank Tsung, Chandrashekhar Joshi, Warren Mori, Ricardo Fonseca, Luis Silva In light of the exciting recent progress in demonstrating the potential of laser plasma interaction to generate monoenergetic electron beams [1]-[4] and the development of a kinetic theory for the blowout regime [6] we have carried out a simulation study for current and near future lasers. Hence we present a series of 3D PIC simulations with the code OSIRIS for electron acceleration via LWFA. The simulations scan the SMLWFA regime to the ultra-relativistic blowout regime. We first compare the simulation results with the experiments [2]-[5] and argue that quantitative predictions using simulations are reliable. Our simulations indicate that the generation of GeV, nC monoenergetic electron beams with no external guiding requires laser power higher than 100TW. The differences between using a channel or a uniform plasma and additional phenomena that were observed in the simulations and affect the resulting electron beam quality are discussed. [1] F.S.Tsung et al, Phys. Rev. Lett., 93, 185002 (2004). [2] Mangles et al, Nature, 431, 535 (2004) [3] Geddes et al., Nature, 431, 538 (2004) [4] Faure et al., Nature, 431, 541 (2004) [5] V.Malka Phys. Plasmas 8, 2605 (2001) [6] W.Lu et al. Phys. Rev. Lett., submitted. Simulations performed at the Dawson cluster under support by nsf grant nsf phy-0321345. Work supported by de-fg02-er54721. [Preview Abstract] |
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BP1.00074: Optimal scaling for LWFA in the ultra-relativistic blowout regime: efficient generation of 1Gev+ mono-energetic beams Wei Lu, Michail Tzoufras, Frank S. Tsung, Chan. Joshi, Warren B. Mori, Luis O. Silva, Ricardo Fonseca Last year, both simulations [1] and experiments [2,3,4] showed that 100$\sim $200 MeV mono-energetic electron beams can be produced when 10$\sim $30 TW lasers were sent through mm's of plasma. PIC simulations show that all the experiments just reach the margin of a new regime of LWFA (Ultra-relativistic blowout regime or bubble regime) which has the following characteristics: spherical ion cannel, electron self-injection and self-guided laser propagation. In this poster, we will clarify the conditions for this regime to be reached and give scaling laws for the output beam energy, charge and energy conversion efficiency based on a nonlinear wakefield theory in the blowout regime. Optimal scaling laws for laser and plasma parameters are described which suggest that this regime can be scaled towards Gev or even Tev energies. Recently, we have verified this scaling law by simulating 0.1$\sim $1GeV stages using a 30$\sim $50fs 10$\sim $200TW lasers. Details of the theory and simulations will be presented. Work supported by DOE de-fg03-92er40727, de-fc02-01er41179, de-fg02-er54721and NSF nsf phy-0321345. Simulations are done on Dawson cluster. [1] F.S.Tsung et al., \textit{PRL}, \textbf{93}, 185002 (2004) [2] Mangles et al., \textit{Nature}, \textbf{431}, 535 (2004) [3] Geddes et al., \textit{Nature}, \textbf{431}, 538 (2004) [4] Fauve et al., \textit{Nature}, \textbf{431}, 541 (2004) [Preview Abstract] |
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BP1.00075: Simulation of Laser Wakefield Acceleration Experiments C.G.R. Geddes, E.H. Esarey, B.A. Shadwick, C.B. Schroeder, W.P. Leemans, E. Michel, D. Bruhwiler, J. Cary We present particle in cell simulations of recent experiments[1] which demonstrate the tuning of laser driven wakefield accelerators to produce high energy electron bunches with low emittance and energy spread, and of upcoming experiments designed to scale these accelerators towards GeV energies. Numerical effects in the simulations as well as comparison to experiments and fluid simulations are discussed. Data and simulations show that the high quality bunch in recent experiments was formed when beam loading turned off injection after initial self trapping, creating a bunch of electrons isolated in phase space. A narrow energy spread beam was then obtained by extracting the bunch as it dephased from the wake. These simulations are being used to understand and optimize trapping stability and accelerator performance. Numerical resolution and dimensional effects are important for application of the simulations to experiments. Using the simulations of recent experiments as a reference, simulations of upcoming 1 GeV acceleration experiments are presented. [1] Geddes et al., Nature, Sept 30 2004, p 538. Work supported by the US DE-AC02-05CH11231, NSF, and AFOSR [Preview Abstract] |
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BP1.00076: Effects of noise and fluctuations on self-trapping in the Laser J.R. Cary, C. Nieter, D. Bruhwiler, D. Dimitrov, R. Giacone, P. Messmer, W.P. Leemans, E.H. Esarey, C.G.R. Geddes Computations and experiments have shown electrons being spontaneously trapped in the wakes generated by intense laser pulses propagating in plasmas. To better understand the details of the mechanisms for trapping, we have carried out a series of convergence tests. These indicate that the amount of trapped charge is very sensitive to the numerical parameters, in part because of numerically induced fluctuations. In addition, we explore other possible trapping mechanisms, This includes collisions with background electrons and ions, which may cause scattering into the trapped region of phase space. Furthermore, the effects of density non-uniformities, perhaps generated by the plasma creation process or present in the pre-breakdown state of the neutral gas, will also be explored. Simulations will be compared to recent experiments at LBNL and elsewhere. [Preview Abstract] |
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BP1.00077: The effect of laser focusing conditions on relativistic plasma wave formation and electron acceleration Karl Krushelnick, A.G.R. Thomas, S.P.D. Mangles, Z. Najmudin, A.E. Dangor, W. Rozmus, C.D. Murphy, P.A. Norreys, J.G. Gallagher, D.A. Jaroszynski, W.B. Mori The effect of laser focusing conditions on the evolution of relativistic plasma waves is studied both experimentally and with particle in cell simulations. For short focal length interactions (small focal spot sizes) broad energy spread electron beams are observed. Simulations show that beam breakup prevents stable propagation of the pulse. However for long focal length geometries (large spot sizes) simulations show a single optical filament capturing almost all of the laser energy. This is characterized by experimentally observed electron beams with multiple or even single electron beams of low energy spread. Preliminary results from collinear two beam all-optical injection experiments will also be shown. [Preview Abstract] |
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BP1.00078: Monoenergetic electron beam generation from a plasma produced by an intense laser pulse Eisuke Miura, Kazuyoshi Koyama, Susumu Kato, Naoaki Saito, Shin-ich Masuda, Masahiro Adachi, Takayuki Watanabe, Mitsumori Tanimoto We have so far reported the generation of a monoenergetic electron beam with an energy of 7 MeV by laser-driven plasma acceleration.[1] The monoenergetic beam generation was observed only in the narrow electron density range around $1.5 \times 10^ {20}~{\rm cm}^{-3}$. To obtain a monoenergetic beam with higher energy, it is necessary to make the acceleration length longer in the lower density plasma. To achieve the longer acceleration length, that is the longer interaction length, the experiment was conducted using an off-axis parabolic mirror with the longer focal length of 300 mm. A 3-TW, 50-fs laser pulse was focused on a He gas jet. At the electron density of $\sim 5 \times 10^ {19}~{\rm cm}^{-3}$, a monoenergetic electron beam with an energy of 25 MeV was observed. The number of electrons of the monoenergetic beam was estimated to be $10^{5}$. This work is supported by the Budget for Nuclear Research of the MEXT and the Advanced Compact Accelerator Project of the MEXT. \newline \newline [1] E. Miura et al., Appl. Phys. Lett. \underline{86} 251501(2005). [Preview Abstract] |
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BP1.00079: Laser Triggered Electron Injection into a Channel Guided Wakefield Accelerator K. Nakamura, C.G.R. Geddes, P. Michel, J. van Tilborg, Cs. Toth, G. Fubiani, C.B. Schroeder, E. Esarey, W.P. Leemans, C. Filip Laser-plasma accelerators have demonstrated the generation of narrow energy spread ($\simeq$ few $\%$) electron beams with considerable amount of charge ($>$100 pC). Stability of laser-plasma accelerators, as in the conventional accelerators, requires highly synchronized injection of electrons into the structured accelerating field. The Colliding Pulse Method[1] with pre-formed plasma channel guiding [2] can result in jitter-free injection in a dark-current-free accelerating structure. We report on experimental progress of laser triggered injection of electrons into a laser wakefield, where an intense laser pulse is guided by a pre-formed plasma channel. The experiments use the multi-beam, multi-terawatt Ti:Al$_2$O$_3$ laser at LOASIS facility of LBNL. The ignitor-heater method is used to first produce a pre-formed plasma channel in a hydrogen gas jet. Two counter propagating beams (wakefield driver:100-500mJ-50fs, injector:50-300mJ-50fs) then are focused onto the entrance of the channel. Preliminary results indicate that electron beam properties are affected by the second beam. Details of the experiment will be presented. [1]G.Fubiani, et al, Phys. Rev. E 70, 016402 (2004). [2]C.G.R. Geddes et al, Nature 431, 538 (2004). This work is supported by DoE under contract DE-AC02-05CH11231. [Preview Abstract] |
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BP1.00080: Injection Acceleration experiment in laser triggered capillary plasma channel. Dmitri Kaganovich, Antonio Ting, Daniel Gordon, Richard Hubbard, Theodore Jones, Phillip Sprangle, Arie Zigler Recent experiment on optically injected Laser Wakefield Acceleration (LWFA) demonstrated electron injection and acceleration in separate injection and acceleration stages. Accelerated electrons $>$20 MeV were observed, implying an acceleration gradient of 20 GeV/m for the 1 mm acceleration distance. Longer acceleration distance is required to achieve higher energies. Laser triggered capillary discharge is able to create long and stable plasma channel for guiding of the acceleration laser beam. Combination of the gas jet as injector and the capillary as the electron accelerator in a new experimental setup is discussed. Recent upgrade of the laser and the electron detection system are presented. [Preview Abstract] |
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BP1.00081: Highly Compressed Ion Beams for Warm Dense Matter Science Alex Friedman The Heavy Ion Fusion Virtual National Laboratory is developing the intense ion beams needed to drive matter to the High Energy Density regimes required for Inertial Fusion Energy and other applications. An interim goal is a facility for Warm Dense Matter studies, wherein a target is heated volumetrically without being shocked, so that well-defined states of matter at 1 to 10 eV are generated within a diagnosable region. In the approach we are pursuing, low to medium mass ions with energies just above the Bragg peak are directed onto thin target ``foils,'' which may in fact be foams with mean densities 1 to 10 percent of solid. This approach complements that being pursued at GSI Darmstadt, wherein high-energy ion beams deposit a small fraction of their energy in a cylindrical target. We present the beam requirements for Warm Dense Matter experiments. We discuss neutralized drift compression and final focus experiments and modeling. We describe suitable accelerator architectures based on Drift-Tube Linac, RF, single-gap, Ionization-Front Accelerator, and Pulse-Line Ion Accelerator concepts. The last of these is being pursued experimentally. Finally, we discuss plans toward a user facility for target experiments. [Preview Abstract] |
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BP1.00082: Pic Modeling of a Closed Drift Ion Source for Material Surface Processing Paul Schoessow, John Cary, Denis Shaw, Peter Stoltz The closed drift ion source is a technology that uses a DC discharge in a gas cell (Ar, O$_2$) to produce a uniform, linear 300-1500 eV ion beam for in-line surface treatment and cleaning of materials. Maximum ion beam currents are on the order of 1 A / meter of source length. A permanent dipole magnet forms a magnetic circuit with the ferromagnetic cathode enclosure, producing a focusing field in the ion extraction gap that confines electrons. The focusing field shape is controlled by the beveling of the gap. We have developed 2- and 3- dimensional PIC models of the closed drift ion source using the OOPIC Pro and Vorpal codes. We will present numerical results on the ion beam properties, including the ion energy spectrum on the target, and compare them to measurements. We will also investigate possible design modifications such as to the magnetic field shape and discharge electrode geometry that might lead to increased ion current. [Preview Abstract] |
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BP1.00083: Development of Fast Diagnostics for High intensity Ion beams S. Eylon, S.S. Yu, P.K. Roy, E. Henestroza, W.G. Greenway, F.M. Bieniosek, A.B. Safkow, E.P. Gilson, R.D. Davidson Ion beam neutralization and drift compression experiments are designed to study the compression of ion beams for high energy density physics (HEDP) and fusion energy research. In this experiment a 300-keV, 30mA K$^{+}$ ion beam was compressed to $<$5 nsec duration by a velocity tilt core in a one meter-long plasma column. We are developing several fast diagnostics, such as Faraday cups, wire current monitor (measured response in the range of 0.5 nsec) fast photo multiplier system combined with a fast aluminum-oxide scintillator and optical emission from a gas cloud to measure time-resolved beam distribution of short pulses. Simulation and experimental data will be presented. (This work was supported by U.S. Department of Energy under Contract No. DE-AC02-05CH11231) [Preview Abstract] |
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BP1.00084: Low Voltage Beam Experiments on the PLIA. J. Coleman, P. Roy, F. Bieniosek, W. Waldron, E. Henestroza, S. Yu, R. Briggs A new accelerator concept called the Pulse-Line Ion Accelerator (PLIA) has been developed. The PLIA operates as a transmission line with pulsed power sources generating a ramped traveling voltage wave on a helical coil. The oil dielectric helix has the ability to accelerate ion bunches to energies much greater than the peak applied voltage and over distances much larger than the ramp length. Low voltage beam experiments ranging from 10-80 kV have begun on the 1-m PLIA test section to verify the ability to accelerate an ion bunch and to investigate breakdown issues. Short-pulsed, pencil-like beams with energy ranges of 200-400 keV have been used for the acceleration experiments. Discharge issues have been addressed and evaluated for possible solutions. A semi-conductive chromium oxide film with a low secondary emission coefficient will be tested for elimination of the discharge. It is expected that the discharge issues will be resolved and beam experiments with high voltages will commence. Different acceleration scenarios will be examined relative to the axial focusing requirements. Experimental results and possible solutions for the elimination of the discharge will be described. (This work was supported by DOE under Contract No. DE-AC02-05CH11231). [Preview Abstract] |
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BP1.00085: Development of Barium Ion Source for the Paul Trap Simulator Experiment Moses Chung, Erik P. Gilson, Ronald C. Davidson, Philip C. Efthimion, Richard Majeski A barium ion source has been developed for the laser-induced fluorescence (LIF) measurement of the transverse ion density profile in the Paul Trap Simulator Experiment (PTSX). The PTSX device is a cylindrical Paul trap whose purpose is to simulate the nonlinear dynamics of intense charged particle beam propagation in alternating-gradient magnetic transport systems. Barium ions are produced at a hot platinum surface with a high work function by surface ionization. Although there are several transition lines for the laser excitation of barium ions, transition from the metastable state $\rm 5 ^2D_{3/2}$ to the excited state $\rm 6 ^2P_{1/2}$ is considered mainly because there exists a commercially available, stable, broadband, high-power laser system in this region of the red spectrum. The ion source is composed of a barium reservoir, platinum ionizer, and extracting electrode system. Initial bench-test results, final design, and installation of the barium ion source will be presented. [Preview Abstract] |
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BP1.00086: Low Inductance Z-discharge Metal Vapor (LIZ-MeV) source for high charge state ion production Jacob Sprunck, Eusebio Garate, Roger McWilliams, Alan van Drie, Ady Hershcovitch, Brant Johnson At the Brookhaven relativistic heavy ion collider, a LIZ-MeV ion-source-based preinjector may be an alternative to the currently-used tandem Van de Graaff. The goal is to generate highly-stripped heavy ions via a high current, high voltage discharge. Based on a previous work,\footnote{N. Debolt, A. Hershcovitch, B.M. Johnson, N. Rostoker, A. Van Drie F. Wessel, Rev. Sci Instrum. \textbf{73}, 741 (2002).} a source has been constructed with low overall impedance and high relative arc-gap inductance. Generated ions are accelerated through a set of extraction electrodes. Currently the electrode material is aluminum, eventually to be replaced with gold and perhaps uranium. To reduce charge exchange the system operates at better that 10$^{-7}$ Torr. Total inductance of the system, including transmission line, is 166 nH. Time-of-flight diagnosis using a Faraday cup measures the energy spectra of extracted ions. [Preview Abstract] |
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BP1.00087: Nonlinear Delta-f Particle Simulations of Collective Effects in High Intensity Bunched Beams Hong Qin, Ronald C. Davidson, Edward A. Startsev The nonlinear delta-f method, a particle simulation method for solving the nonlinear Vlasov-Maxwell equations, is being used to study the collective effects in high-intensity bunched beams. For high intensity bunched beams, the equilibrium and collective excitation properties are qualitatively different from those for long coasting beams. Due to the coupling between the transverse and longitudinal dynamics induced by the 2D nonlinear space-charge field, there exists no exact kinetic equilibrium which has anisotropic temperature in the transverse and longitudinal directions. Even in a thermal equilibrium with isotropic temperature, particle trajectories on constant-energy surfaces are non-integrable, which implies that it is impossible to perform an integration along unperturbed orbits to analytically calculate the linear eigenmodes. For the case of a thermal equilibrium beam with isotropic temperature, the self-consistent kinetic equilibrium for a bunched beam is first established numerically. Then, the collective excitations of the equilibrium are systematically investigated using the nonlinear delta-f method implemented in the Beam Equilibrium Stability and Transport (BEST) code. For the case of a beam with anisotropic temperature, an approximate equilibrium is adopted when the nonlinear coupling due to the space-charge field is weak. The validity and collective excitations about this approximate equilibrium are studied numerically. [Preview Abstract] |
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BP1.00088: Production, Acceleration, and Propagation of a Symmetric Neutralized Ion Beam Nathaniel Hicks, Alfred Wong The first experimental results demonstrating the production of a symmetric neutralized ion beam (SNIB) will be presented. SNIBs are composed of equal quantities of positive and negative ions of the same mass, in this case H$^{+}$ and H$^{-}$. The production of the SNIB is accomplished by merging together positive and negative ion beams from separate ion sources. An electrostatic transport system guides the beams to the merging point. A novel ``SNIB collector'' will be described that is used to diagnose the individual components of the beam. Once formed, the SNIB is injected into a radio frequency quadrupole (RFQ) accelerator operating at 200 MHz, with acceleration from 1 keV to 20 keV. This aims to verify the theoretical ability of an RFQ to simultaneously bunch and accelerate positive and negative particles, and also demonstrates the suitability of RFQs for energetic SNIB applications. SNIBs have the theoretical ability to propagate across a transverse magnetic field (the investigation of which is the ultimate scientific goal of this research program), and applications may therefore include injection into magnetic confinement devices for heating and fueling. This work is supported by DOE Contract DE-FG02-04ER54760. [Preview Abstract] |
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BP1.00089: Diagnostic Development for Heavy-Ion Based HEDP and HIF Experiments F. Bieniosek, D. Baca, P.K. Roy, P.A. Seidl, S.S. Yu, A.W. Molvik, M.K. Covo, D. Shiraki We discuss diagnostics used in the Heavy Ion Fusion Virtual National Laboratory (HIF-VNL). Time-resolved optical imaging diagnostics provide 4-D transverse beam phase space information on the experimental beams. Current work includes a high speed (sub-ns) optical system, a compact optical diagnostic suitable for insertion in transport lines, improved algorithms for data analysis, and a high-resolution electrostatic energy analyzer. A longitudinal diagnostic kicker/buncher generates longitudinal space-charge waves. Time of flight of the space charge wave and the electrostatic energy analyzer provide an absolute measure of the beam energy. Special diagnostics to detect secondary electrons and gases desorbed from the wall have been developed. Optical imaging of the gas cloud is used to study evolution of the gas cloud and as a beam current diagnostic. Experiment and diagnostics definition and layout for upcoming high energy density physics (HEDP) experiments are in the planning stages. [Preview Abstract] |
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BP1.00090: Gas and Electron Effects on Intense, Space Charge Dominated Ion Beams in Magnetic Quadrupoles: Comparison Of Experiments and Simulations P.A. Seidl, D. Baca, F.M. Bieniosek, J-L Vay, R. Cohen, A. Friedman, D. Grote, M. Kireeff Covo, S.M. Lund, A.W. Molvik, B.E. Rosenberg Accelerators for inertial fusion energy, high-energy density physics and other high intensity applications have an economic incentive to minimize the clearance between the beam edge and the aperture wall. This increases the risk from electron clouds and gas desorbed from walls. Using the High Current Experiment at LBNL, we have measured the beam (0.18 A, 1 MeV K$^{+})$ distribution upstream and downstream of a short lattice of magnetic quadrupoles where the 2rms beam size is $\ge $50{\%} of the quadrupole aperture, and the generalized perveance is $\approx $10$^{-3}$. Between magnets, the transverse beam distribution is also imaged. The beam potential is 1-2 kV, large enough to trap electrons produced by, for example, K$^{+}$ - gas collisions. The measurements are compared to WARP PIC simulations that include the self-consistent tracking of electrons and ions. [Preview Abstract] |
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BP1.00091: New self-consistent simulation tools for the modeling of particle beam/plasma interaction with its environment. J.-L. Vay, M.A. Furman, P.A. Seidl, R.H. Cohen, A. Friedman, D.P. Grote, M. Kireeff Covo, A.W. Molvik, P.H. Stoltz, S. Veitzer, J.P. Verboncoeur We have completed the first round of development of a new self-consistent 3-D simulation tool to study the interaction of intense charged particle beams with the environment in a particle accelerator; i.e. interactions with walls, electron clouds and background gas. The new capability is built around the 3-D PIC accelerator/plasma code WARP, with additional functionalities: (a) generation of secondary electrons and desorbed gas from ion and electron impact*, (b) tracking dynamics of neutrals and interactions with a beam through ionization of neutrals and/or beam particles, (c) bridging time scales between electron and ion motion in a space-and-time varying magnetic field with a novel particle mover***. We will present the new functionalities together with tests of the new mover on "textbook" cases and comparisons of the new capabilities with experiments**. * P. Stoltz, this conference, ** A.W. Molvik et al., P. Seidl et al., this conference, ***R. Cohen, POP, 12, 056708 (2005). [Preview Abstract] |
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BP1.00092: Experiment and simulation of non-linear structures in electron clouds A. Molvik, R. Cohen, A. Friedman, M. Kireeff Covo, J-L. Vay, F. Bieniosek, D. Baca, P. Seidl We discovered an unexpected effect while studying electron clouds in ion beams. (Our goal is to understand electron sources and accumulation in positively-charged beams, determine effects on the beam, and find mitigation mechanisms. We use simulations and the HCX experiment, with 1~MeV, 180~mA K$^+$ ion beams.) Experiments show oscillations in the current to a clearing electrode in a drift region between quad magnets when we do not suppress copious electron emission from the linac end wall. WARP PIC code simulations not only show the clearing electrode oscillations, but also show oscillations in the density and radial position of electrons drifting upstream through the last quadrupole magnet. The simulations show electrons increase in density as they stagnate at a quadrupole magnet. Non-linear structures are launched, growing to electron line charges exceeding the ion beam line charge. [Preview Abstract] |
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BP1.00093: Retarding Potential Analyzer: a versatile tool to measure effects of ion beam interactions with background gas Michel Kireeff Covo, Arthur W. Molvik, Ronald H. Cohen, Alex Friedman, Jean-Luc Vay, Frank Bieniosek, David Baca, Peter A. Seidl, Jasmina L. Vujic, Christian M. Leister, Beth Ellen Rosenberg The High Current experiment (HCX) is a 1 MeV linear DC accelerator that provides a K$^{+}$ ion beam current of 180 mA for 5 $\mu $s. It constitutes an excellent platform to measure beam interaction with walls and background gas. We placed a Retarding Potential Analyzer (RPA) inside a gap between quadrupole magnets, where the beam potential of $\approx $2200V traps electrons and expels ions (from ionized gas). The trapped electrons are then expelled at the end of the pulse, when the beam potential decreases. We will show that the RPA is a versatile tool to measure particle balance and study the effects of electron accumulation. [Preview Abstract] |
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BP1.00094: Cylindrical short-pulse Child-Langmuir law Wee Shing Koh, Lay Kee Ang Laser-driven short pulses have been prevalently used in photo-injectors to produce extremely high current densities. If the pulse length of the short-pulse current is less than the transit time across the gap, the space-charge-limiting (SCL) current density of the electron beam exceeds that of the classical long-pulse limit as given by the Child-Langmuir (CL) Law. The 1D short-pulse CL law for a planar electrode has been derived with verification from PIC simulation [1]. The extension to the 2D and 3D models of the short-pulse CL law has also been presented recently [2]. In the long pulse limit, the 2D and 3D CL laws for both planar and cylindrical diodes have also been developed [3]. In this paper, we will present the 1D and 2D short-pulse CL law in the coaxial cylinder configuration for both convergent and divergent flows. The analytical results will be compared with 2D PIC simulation results. [1] \'{A}g\'{u}st Valfells \textit{et. al.} , ``Effects of pulse-length and emitter area in virtual cathode formation in electron guns'', Phys. Plasmas 9, 2377 (2002). [2] W. S. Koh and L. K. Ang, "Two-dimensional Short-Pulse Chid-Langmuir Law", The 32nd International Conference on Plasma Science (ICOPS), N05CH37707, 3P38, pp. 298 (2005).[3] W. S. Koh, \textit{et. al.}, Three-dimensional Child-Langmuir law for hot electron emission, Phys. Plasmas 12, 053107 (2005). Email: elkang@ntu.edu.sg [Preview Abstract] |
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BP1.00095: Exact Analytical Solutions for Relativistic Parapotential Flow with Spatially Varying Total Energy R. Stowell, Chiping Chen The relativistic cold fluid and Maxwell equations are solved exactly in steady state (${\frac{\partial}{{\partial}{\:\!}t}}{\;}{=}{\;}0$) for flows which are perpendicular to the electric field (${\bf{u}}{\,}{\cdot}{\,}{\bf{E}}{\;}{=}{\;}0$). The usual assumption (Creedon '75) that total energy (${q}{\:\!}{\phi}{\,}{+}{\,}{\gamma}{\:\!}m{\:\!}{c^2}$) is independent of position is relaxed. The effect the energy profile on the density and velocity profiles is thus considered. Expressions are provided for the complete solution in terms of an arbitrary energy profile. Various geometries are treated. Applications include relativistic diodes and MILOs (magnetically insulated line oscillators). [Preview Abstract] |
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BP1.00096: Particle simulation of a virtual cathode in bipolar flow John Verboncoeur, Jean-Luc Vay, Peter Stoltz Virtual cathode formation is studied in a bipolar flow relevant to the High Current Experiment parameter regime. A 174 mA potassium ion beam with beam energy of 972 keV, confined in a conducting drift tube, is incident on a stainless steel plate. The ion impacts generate secondary electrons with an energy and angular dependent yield large compared to 1, resulting in a current sufficient for formation of a virtual cathode. Using enhanced ion-induced secondary models in the PIC codes XOOPIC and WARP, the formation of the virtual cathode and its effect on the bipolar flow is investigated, including time-dependent effects. Numerical issues including resolution of the virtual cathode scale length $l_{vc} \sim \lambda_{De}$ and time scale $t \sim 1 / \omega_{pe}$ are characterized. Results indicate an improvement in temporal and spatial fidelity leads to a lower and more stable virtual cathode potential as well as significant impact on neutrality in the bipolar flow. [Preview Abstract] |
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BP1.00097: Theory, Simulation and Design of High-Brightness, Space-Charge-Dominated Electron and Ion Beams Chiping Chen, Tom Bemis, Ronak Bhatt, Jing Zhou A method is presented for the generation, matching and transport of a high-brightness, space-charge-dominated circular electron or ion beam which maintains almost uniform density transversely. In particular, a circular diode theory is applied to design the electrodes for the diode. A theory is developed for the design of the ideal matching of the beam from the diode to a periodic permanent magnet (PPM) or periodic solenoidal magnetic focusing field. The magnetic focusing field is designed using OPERA3D. The beam formation and transport from the particle emitter to the periodic magnetic focusing channel is simulated using OmniTrak. Examples of high-brightness electron and ion beams are presented. Potential applications of such beams are discussed. [Preview Abstract] |
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BP1.00098: Numerical Simulations of a Pulse Line Ion Accelerator. Enrique Henestroza, Simon S. Yu The Neutralized Drift Compression Experiment (NDCX) is being constructed at the Lawrence Berkeley National Laboratory. NDCX will help develop novel, still unexplored beam manipulation techniques in order to establish the physics limits on compression of heavy ion beams for creating high energy density matter and fusion ignition conditions. A critical early component being developed in this series of experiments is the accelerating scheme for the high-current, short-bunch ion beams that are of interest for HEDP applications. The Pulse Line Ion Accelerator (PLIA) uses a slow-wave structure based on a helical winding, on which a voltage pulse is launched and propagated to generate the accelerating fields. An oil dielectric helix has the ability to accelerate ion bunches to energies much greater than the peak applied voltage and over distances much larger than the ramp length. Low peak voltage experiments ranging from 10-80 kV have begun on a 1-m PLIA test section to verify the ability to accelerate an ion bunch and to investigate breakdown issues. Short-pulsed, pencil-like beams with energy ranges of 200-400 keV have been used for the acceleration experiments. Numerical simulations of the PLIA beam dynamics will be presented. [Preview Abstract] |
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BP1.00099: Filling Factor for an Electrostatic Quadrupole Lattice C.M. Celata, F.M. Bieniosek, P.A. Seidl, L. Prost, D.P. Grote The cost of an accelerator is in part determined by the size of the beam pipe aperture. The effect on total cost is especially large for multiple-beam induction linac designs for IFE drivers, where extra clearance for each beam greatly enlarges the transverse scale of the machine. Limits to the amount of clearance between the beam and the vacuum pipe are set by nonlinear forces (image forces, focusing fringe fields, etc.), which can cause beam heating or beam loss. The filling factor possible (i.e., percent of aperture filled with beam), or ``dynamic aperture,'' is investigated in this work for the intense, space-charge-dominated ion beams of an IFE driver, using the 2-D transverse slice version of the 3-D particle-in-cell simulation code WARP. The focusing field is modeled using a 3-D solution of the Laplace equation for the biased quadrupole focusing elements, as opposed to previous calculations, which used a less-accurate multipole approximation. 80{\%} radial filling of the aperture is found to be possible. Results from the simulations, as well as corroborating data from the High Current Experiment at LBNL, will be presented. [Preview Abstract] |
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BP1.00100: Transverse compression of an intense ion beam, traveling through a periodic-focusing quadrupole lattice Mikhail Dorf, Ronald C. Davidson, Edward Startsev The transverse compression and dynamics of an intense beam, propagating through a periodic quadruple lattice, plays an important role for many accelerator physics applications. Typically, the compression can be achieved by means of increasing the focusing strength of the lattice along the beam propagation direction. However, beam propagation through the lattice transition region inevitably leads to a certain level of beam mismatch and halo formation. In this work we present a detailed analysis of these phenomena using the envelope equations in the smooth focusing approximation, which describe the average effects of a periodic lattice, and full particle-in-cell numerical simulations using the WARP code, taking into account the effects of the periodic-focusing quadrupole field. Simulations are presented for both space-charge-dominated beams, and beams with a moderate space-charge strength. [Preview Abstract] |
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BP1.00101: Ferroelectric Plasma Source for Heavy Ion Beam Charge Neutralization Philip C. Efthimion, Erik P. Gilson, Larry Grisham, Ronald C. Davidson, Simon Yu, William Waldron, B. Grant Logan Plasmas are a source of unbound electrons for charge neutralizing intense heavy ion beams to allow them to focus to a small spot size and compress their pulse length. Calculations suggest that plasma at a density of 1-100 times the ion beam density and at a length $\sim $ 0.1-1 m would be suitable. To produce one-meter-long plasma, sources based upon ferroelectric ceramics are being developed.$^{ }$ They can be scaled to large volumes and operate at low neutral pressures. The source utilizes the ferroelectric ceramic BaTiO$_{3}$ to form metal plasma. The drift tube inner surface of the Neutralized Drift Compression Experiment (NDCX) will be covered with ceramic, and high voltage ($\sim $ 5 kV) will be applied between the drift tube and the front surface of the ceramic. A prototype ferroelectric source 20 cm long has produced plasma densities of 5x10$^{11}$ cm$^{-3}$. It was integrated into the previous Neutralized Transport Experiment (NTX), and successfully charge neutralized the K$^{+}$ ion beam. Presently, the one-meter-long source is being fabricated. It will be characterized and integrated into NDCX for charge neutralization experiments. [Preview Abstract] |
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BP1.00102: Perturbative Particle Simulations of Eigenmodes in High Intensity Charged Particle Beams Weihua Zhou, Hong Qin, Ronald C. Davidson Two-dimensional body modes in high intensity charged particle beams are studied using the Beam Equilibrium Stability and Transport (BEST) code, which numerically solves the Vlasov-Maxwell equations using a low noise perturbative particle simulation method. The beam equilibrium is inhomogeneous in the radial direction and spatially uniform in the beam propagation direction. The beam equilibrium is determined self-consistently from the steady-state Vlasov-Maxwell equations for arbitrary space charge intensity, subject to transverse confinement by the applied smooth-focusing field. The two-dimensional body modes have an (r,z) mode structure localized inside the beam, with finite wavelength in the beam propagation direction. Wave-particle interactions, such as a strong Landau damping when the wave velocity in the beam frame matches the beam thermal velocity in the longitudinal direction, are numerically investigated for this family of eigenmodes. [Preview Abstract] |
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BP1.00103: A Fast Faraday Cup for Measuring Neutralized Drift Compression Adam Sefkow, Ronald Davidson, Phillip Efthimion, Erik Gilson, Simon Yu, Prabir Roy, Shmuel Eylon, Frank Bieniosek, Enrique Henestroza, Joshua Coleman, William Waldron, Wayne Greenway, David Vanecek, Dale Welch Heavy ion drivers for high energy density physics applications and inertial fusion energy use space-charge-dominated beams which require longitudinal bunch compression in order to achieve sufficiently high beam intensity at the target. The Neutralized Drift Compression Experiment-1A (NDCX-1A) at Lawrence Berkeley National Laboratory is used to determine the physics limits for neutralized drift compression. NDCX-1A investigates the physics of longitudinal focusing of an intense ion beam, achieved by imposing an initial velocity tilt on the drifting beam and neutralizing the beam's space-charge with background plasma. Accurately measuring the longitudinal compression of the beam pulse with high resolution is critical for NDCX-1A. The design and preliminary experimental results for a fast Faraday cup which measures the total beam current at the focal plane as a function of time are summarized. [Preview Abstract] |
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BP1.00104: Two-stream instability for a longitudinally-compressing charged particle beam Edward Startsev, Ronald Davidson The electrostatic two-stream instability for a cold, longitudinally-compressing charged particle beam propagating through a background plasma has been investigated both analytically and numerically. Small-signal coupled equations describing the evolution of the amplitudes of the perturbation are derived and the asymptotic solutions are obtained. The results are confirmed by direct numerical solution of the linearized fluid equations. In addition, the particle-in-cell delta-f code BEST has been used to carry out detailed numerical studies of the instability, and the results are in reasonably good agreement with the theory. It is found that the longitudinal beam compression strongly modifies the space-time development of the instability. In particular, the dynamic compression leads to a significant reduction in the growth rate of the two-stream instability compared to the case without an initial velocity tilt. [Preview Abstract] |
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BP1.00105: Nonlinear coupling between breathing and quadupole-like oscillations in magnetically focused beams Renato Pakter, Wilson Simeoni Jr., Felipe Rizzato A nonlinear stability analysis of breathing beams considering nonaxysymmetric perturbations is performed. It is shown that the breathing oscillations of an initially round beam may nonlinearly induce quadrupole-like oscillations, with a consequent increase of the beam size along one direction. The instability mechanism and its relevance to beam particle losses are discussed. Self-consistent simulations are performed to verify the findings. [Preview Abstract] |
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BP1.00106: Characterization of a Plasma Opening Switch Source by Interferometry and Probe Measurements A.G. Lynn, W. Clark, D.P. Jackson, M.A. Gilmore, M.E. Savage, R.A. Sharpe The Triggered Plasma Opening Switch (TPOS) at SNL is a unique device that exploits the high conductivity and low mass properties of plasma. The TPOS's objective is to take an initial $\sim $0.8MA ($\sim $250ns rise time) storage inductor current and deliver $\sim $0.5MA at $\sim $2.4MV ($\sim $10ns rise time) to a load of $\sim $5-10$\Omega $. Configuration advantages include low current jitter and resistive voltage drop, power gain, and minimization of trigger input power as the result of using two stages in series. This two-stage design is novel and is the first to demonstrate operation of magnetically triggered stages. Study of TPOS characteristics is in progress via an offline interferometer diagnostic; specifically, a CO$_{2 } $laser interferometer will be used to make density measurements of the source plasma. It is thought that the gross plasma source density is $\sim $10$^{14}$ cm$^{-3}$, but details of the spatial structure and temporal evolution have not previously been studied. In order to better understand switch operation, these details are essential. A double-tip Langmuir probe will also be used to provide an independent measure of plasma density and electron temperature. Current results from the interferometer using a single radial chord will be presented, along with multi-point radial measurements from the Langmuir probe. [Preview Abstract] |
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BP1.00107: Ion Current Collection Diagnostic for the Triggered Plasma Opening Switch Experiment D.P. Jackson, M.E. Savage, M.A. Gilmore, A.G. Lynn The novel Triggered Plasma Opening Switch (TPOS) is a unique device that exploits the high conductivity and low mass properties of plasma. The TPOS's objective is to take the initial $\sim $.8 MA ($\sim $250 ns rise time) storage inductor current and deliver $\sim $.5 MA at $\sim $2.5 MV ($\sim $10ns rise time) to a load of $\sim $5-10 $\Omega $. Study of the TPOS characteristics is in progress via an Ion Current Collection Diagnostic (ICCD). The ICCD has been designed, fabricated, tested, and is in use on the TPOS in order to explore the main switch opening profile. The ICCD utilizes 12 charge collectors (biased faraday cups) that are positioned perpendicularly to the main switch stage in order to collect radially traveling ions emitted from the plasma surface via the Child-Langmuir law. Magnetostatic simulations with self consistent space charge emitting surfaces of the main switch using the Trak static 2D finite element code have been conducted as well. Finally, ICCD experimental data have been recorded, and hopefully these data will provide evidence that support both theory and simulation. [Preview Abstract] |
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BP1.00108: Z-PINCHES/IC and IFE |
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BP1.00109: Energy deposition and expansion rates of single 25 $\mu$m copper wires with and without copper plated ends D.A. Chalenski, H.S. Banard, B.R. Kusse Previous experiments looking at the explosion of single, small diameter wires driven by 1-5 kA current pulses have shown that the energy deposited, and consequently the expansion rates, are strongly dependent on the electrical contacts at the ends of the wires. Continuing with this investigation we report here the results of a series of experiments using 25$\mu $m diameter, 2cm long copper wires driven by the Cornell LCP3 pulse generator which produced nominally 5 kA, 200 ns current pulses with 15 to 100 ns rise times. For these measurements some of the wires had additional copper electroplated on the ends to increase the diameter by approximately 35{\%}. The ends of the plated and un-plated wires made mechanical contact with the pulser by passing through small 0.35mm holes drilled in the electrodes. Comparisons were made of the explosions of plated and un-plated wires. Measurements were obtained for the energy deposited from current and voltage waveforms. Laser backlighting allowed observation of the radial expansion rates and axial uniformity of the explosions. This research was supported in part by Sandia National Laboratories, Albuquerque, contract AO258 and by the NNSA Stockpile Stewardship Academic Alliances program under DOE Cooperative Agreement DE-FC03-02NA00057. [Preview Abstract] |
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BP1.00110: X-Pinch Experiments Driven by a Compact Marx Generator Trevor Strickler, R.M. Gilgenbach, M. Gomez, J. Zier, W. Tang, Y.Y. Lau, T.A. Mehlhorn, M.E. Cuneo, M. Mazarakis Experiments are underway on an X-pinch driven by a 4-stage compact Marx generator with pulse-sharpening switch at the following design parameters: peak voltage $\sim $ 160-400 kV, current $\sim $ 20-50 kA, and 10-90{\%} risetime $\sim $170 ns. Diagnostics include ultraviolet emission spectroscopy, resonant laser shadowgraphy, soft x-ray PIN diodes, and Rogowski coils. Initial ultraviolet emission spectroscopy experiments have measured Al line emission during 30.3-micron-wire explosions at $\sim $ 20 kA peak current. \newline \newline * This work was supported by U. S. DoE through Sandia National Laboratories award number 240985 to the University of Michigan. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under Contract DE-AC04-94AL85000. T. S. Strickler was supported by the National Physical Sciences Consortium Graduate Fellowship in the Physical Sciences. [Preview Abstract] |
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BP1.00111: X-pinch Plasma Observations Bulmuo Maakuu, Richard Appartaim The parameters of an X-pinch plasma produced by the discharge of a microsecond capacitor bank through fine wires of tungsten, molybdenum, nichrome, aluminum etc, have been studied with a number of diagnostic techniques. We report on the results of measurements with PIN diodes, a Nd:YAG laser based interferometer, and preliminary results from a crystal spectrograph using a flat mica crystal. We will also demonstrate that for certain applications, the X-pinch produced by the cheaper capacitive discharge technique performs sufficiently well compared to more expensive pulsed power systems. [Preview Abstract] |
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BP1.00112: X-pinch Dynamics Observed with a Sub-200ps Laser Marc Mitchell, S.A. Pikuz, T.A. Shelkovenko, K.M. Chandler, D.A. Hammer The X pinch has proven itself as both an extraordinary x-ray backlighting source and high energy density plasma. However, much of the interesting physics of an X pinch happens on a time scale that is difficult to observe. The X pinch has been estimated to approach solid density and achieve temperatures greater than 1keV for a brief period of during which there is an intense x-ray burst. This intense x-ray burst, or ``bright spot,'' is on the order of a micron in size and about 0.1ns in duration. The final stages of implosion leading to the formation of the x-ray burst last only a few nanoseconds. Therefore, images of the X pinch taken with an apparatus that integrates over a few nanosecond is not sufficient to resolve the dynamics of the X pinch during the last critical stages of implosion. In turn, we have designed experiments using a sub-200ps laser pulse to observe several frames of each X-pinch pulse with both shadow and interference images. We hope to provide useful images both for direct observation of X-pinch dynamics and for comparison to codes used to model the X pinch as is currently pursued at Imperial College. This research was supported by DOE Grant DE-FG02-ER54496. [Preview Abstract] |
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BP1.00113: Time-resolved Cu K-shell spectra from four wire Manganin X pinches Katherine Chandler, T.A. Shelkovenko, S.A. Pikuz, M.D. Mitchell, D.A. Hammer Four wire 25 micron diameter Manganin (86 Cu, 14 Mn) X pinches were driven with a 450 kA current pulse in Cornell's XP Pulser and Cu K-shell spectra from the emitted plasma was analyzed. Time-resolved spectra were obtained using an x-ray streak camera together with a spherically bent mica crystal spectrometer. Time-integrated data was also obtained using an FSSR-1D with a spherically bend mica crystal and compared with the time-dependent data. A simple Gabriel model has been developed to model the experimental spectra and to extract the electron temperature as a function of time. An electron temperature of 1.7 - 2 keV has been inferred from Cu K-shell spectra from both the time-integrated and time-dependent experiments. The results of the experiments as well as the application of the model are discussed. [Preview Abstract] |
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BP1.00114: Reverse polarity x-pinch operation I.C. Blesener, P.U. Duselis, B.R. Kusse, S.A. Pikuz, T.A. Shelkovenko Conventionally an x-pinch is operated in negative polarity. That is to say the x-pinch wires are driven negative with respect to the ground of the pulsed power generator. This is largely due to the fact that breakdown in these machines is easier to prevent when they are operated in negative polarity. Low current experiments (1-5 kA) studying the explosions of single wires have shown that more energy can be deposited in the wires when they are driven in positive as opposed to negative polarity. These results have raised the question of what happens when pinch-type experiments are driven in positive polarity. Recently it has become possible to use the 450 kA XP pulser at Cornell to drive x-pinches in positive polarity. We present here the results of a series of experiments comparing the operation of an x-pinch in positive and negative polarity. Of particular interest are the x-ray yield, timing, radiating spot size and the number of radiating spots in a given x-pinch. This research was supported by DOE grant DE-FG03-98ER54496, by Sandia National Laboratories contract AO258, and by the NNSA Stockpile Stewardship Academic Alliances program under DOE Cooperative Agreement DE-FC03-02NA00057. [Preview Abstract] |
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BP1.00115: Energetic electron beam dynamics in 1 MA x-pinches and wire arrays by means of hard x-ray polarimetry Ishor Shrestha, Victor Kantsyrev, Alla Safronova, Vidya Nalajala, Shivaji Pokala, Steve Batie, Dan Macaulay, Bruno LeGalloudec The electron beams characteristics of x-pinches and wire arrays constructed from Al, Mo, W, combined Mo/W, Al/Mo, Al/W, Cu, stainless steel x-pinch were studied on the 1MA, 100 ns rise time Zebra generator. New time-resolved hard x-ray polarimeter (HXP) based on Compton scattering effect have been used together with fast x-ray detectors, time-gated pinhole camera, and spectrometers. The electron beams (energy more than 35 keV) have been investigated by the measurement of the polarization state of bremsstrahlung radiation emitted from plasma. An estimated degree of polarization varies from 15{\%} to 80{\%} for various types of plasma electron beams. It was assumed that in most cases the electron beams were well collimated in direction from cathode to anode. The hard x-ray polarization data were compared to slopes from other x-ray diagnostics on 1MA z-pinch generator ``Zebra.'' Work supported by the DOE/ NNSA under UNR grant DE-FC52-01NV14050 and by Sandia National Laboratories UNR Grant 111480. [Preview Abstract] |
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BP1.00116: Laser-based imaging of wire array z-pinches and X pinches on the COBRA pulsed power generator Ryan McBride, David Chalenski, Lloyd Maxson, Sergei Pikuz, Tatiana Shelkovenko, Jon Douglass, John Greenly, David Hammer, Bruce Kusse Initial laser-based imaging experiments of wire array z-pinches and X pinches on the 1 MA COBRA pulsed power generator are presented. The imaging system makes use of a frequency-doubled 532 nm green beam from a Nd:YAG laser source. This source also supplies the frequency-quadrupled 266 nm UV beam that is used to trigger the final output switches of the COBRA generator and initiate a current pulse. Thus, the desired timing of the imaging beam relative to the start of a current pulse is achieved by adjusting the optical path length of the imaging beam. Once the imaging beam has passed through the pinch region, the light is collected on a high resolution CCD camera that is linked to a PC for fast image processing. The laser-based images produced, along with images and measurements from other supporting diagnostics, are presented. [Preview Abstract] |
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BP1.00117: Investigation of implosion dynamics and magnetic fields in 1-MA wire arrays by optical probing diagnostics P.J. Laca, V.V. Ivanov, V.I. Sotnikov, A.L. Astanovitskiy, B. Le Galloudec, T.E. Cowan, G.S. Sarkisov Multiframe optical probing diagnostics were applied for the investigation of implosion dynamics and magnetic fields in z-pinch plasma of wire arrays and x-pinches at the Nevada Terawatt Facility (NTF). Five shadow frames per shot, with a long 34-ns or short 9-ns pulse train, presents fine details of plasma evolution in the wire array. A Faraday rotation diagnostic consists of identical shadow and Faraday channels, shearing air-wedge interferometer, and schlieren channel. Evolution of the wire array z-pinch in different regimes of implosion was investigated. Fast dynamics of bubbles in plasma streams were studied in detail. A current in the plasma column of Al wire arrays and magnetic bubbles were found by the Faraday rotation diagnostic. [Preview Abstract] |
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BP1.00118: Theoretical study of the dynamics of highly-radiating plasmas, produced at 1 MA Zebra generator Andrey Esaulov, Victor Kantsyrev, Alla Safronova MHD modeling of highly radiating plasmas, produced by the x- pinch and wire array loads at 1 MA Zebra generator at the University of Nevada Reno is conducted to support the ongoing experimental campaigns and to facilitate the design and optimization of the future experiments with the increased radiation output. The MHD simulations and the predictions, given by the analytical models, show that the x-ray diagnostic complex, developed at the University of Nevada Reno is perfectly fit to the parameters of high energy density plasmas, produced at Zebra. Direct comparison of the theoretical data with experimental results demonstrates the consistency of the used MHD models. On the other hand, these models provide the new interpretation for the experiments, revealing the valuable information, which is hidden from traditional optical diagnostics of plasma. [Preview Abstract] |
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BP1.00119: Study of radiation yield dynamics and implosion characteristics of Al wire arrays in comparison with Al cylindrical wire arrays and X-pinches at 1MA Zebra generator Vidya Nalajala, Victor Kantsyrev, Alla Safronova, Dmitry Fedin, Ishor Shrestha, Shivaji Pokala, Steve Batie, Alexey Astanovitskiy, Bruno LeGalloudec, Josh Gradel Study of X-ray/EUV yields and timing characteristics of radiation bursts from Al planar wire arrays on 1MA, 100 ns Zebra generator have been performed. These results are compared with that of Al X-pinches and cylindrical wire arrays on the same generator. Planar arrays showed higher total energy yields (11kJ) and sub-keV yield (7 kJ) than x-pinches (8 and 3.5 kJ) and cylindrical arrays (4-6 and 1-1.7 kJ). Study of timing characteristics shown single shot x-ray burst from planar arrays (K-shell rise time 3-5 ns width), which is much less compared to cylindrical array (20-30 ns). In case of x-pinches, typically multiple bursts (more than 1-3, with shorter time 1-2 ns) occurred. Planar array power measured of 0.3-0.5 TW (sub-keV region) is greater than to x-pinches (0.1 TW) and much higher in comparison with cylindrical arrays (0.03-0.05 TW). Therefore, planar wire arrays proved to deliver higher powers than cylindrical wire arrays and x-pinches with time width of radiation bursts much closer to that of x-pinches. Work supported by the DOE/ NNSA under UNR grant DE-FC52-01NV14050 and by Sandia National Laboratories under DOE contract DE-AC04-94AL85000. [Preview Abstract] |
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BP1.00120: Excitation of Flute Mode Turbulence in High Beta Current-Carrying Z-Pinch Plasmas V.I. Sotnikov, V.V. Ivanov, T.E. Cowan, J.N. Leboeuf, B.V. Oliver, C. Coverdale, B. Jones, C. Deeney, T.A. Mehlhorn, G.S. Sarkisov, P.D. LePell Recent experimental data obtained with laser probing and Faraday rotation diagnostics have shown that a significant part of the current is flowing through the central region. Intense turbulence of short wavelength oscillations as well as formation of vortices have also been observed in that region. The electromagnetic interchange mode instability is the most likely candidate to explain the experimentally observed wave turbulence. To study its linear excitation and nonlinear evolution, a nonlinear set of equations for electrostatic potential, magnetic field, and density has been derived from two-fluid macroscopic equations. This set of equations has enabled a study of the coupled dynamics of large-scale structures and small-scale turbulence which have been observed in the experiments. Work supported by the US Department of Energy under Grant No. DE-FC52-01NV14050 at UNR, Grant No. DE-FG02-04ER54740 at UCLA and Contract DE-AC04-94AL85000 at Sandia National Laboratories [Preview Abstract] |
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BP1.00121: Investigation of plasma turbulence, cells, and magnetic bubbles in the precursor and stream bubbles in wire array z-pinches V.V. Ivanov, V.I. Sotnikov, T.E. Cowan, P.J. Laca, A.L. Astanovitskiy, B. Le Galloudec, G.S. Sarkisov, B. Jones, C. Deeney Turbulence in the plasma column of Al wire arrays was investigated in 1-MA z-pinch by laser probing diagnostics. Plasma streams from wires imprint the surface of the precursor and seed plasma turbulence in the early stage. Small-scale turbulence and large-scale cells arise in the plasma column in the nonlinear stage before implosion. Magnetic bubbles and current in the precursor were observed by Faraday rotation diagnostics. Scales of exited waves are in agreement with the theoretical model associated with flute mode turbulence. Bubbles in streams indicate breaks in the wire cores and the start of material movement. Bubbles bring the main mass contained in the wires to the array axis. A regime of implosion without a precursor was found in thin-wire arrays. [Preview Abstract] |
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BP1.00122: Modeling of L-shell radiation from Ni and Invar wire array Z-pinch experiments on the 1MA COBRA generator Nicholas Ouart, Alla Safronova, Tatiana Shelkovenko, Sergei Pikuz, Jonathan Douglass, Ryan McBride, David Hammer Wire array experiments using eight 8$\mu $m pure Ni wires or eight 10$\mu $m Invar (36{\%} of Ni and 64{\%} of Fe) wires were carried out at the 1MA COBRA facility at Cornell. The spatially resolved, time integrated L-shell X-ray line spectra have been recorded through a 100$\mu $m slit using a flat spectrometer with a KAP crystal. A non-LTE collisional-radiative atomic kinetic model of Ni developed earlier [1] was applied to determine plasma parameters from the Ni-containing wire array experiments. The results of this modeling are presented and the interpretation of Ni radiation from Ni and Invar wire array experiments are compared and discussed. This research sponsored by the NNSA under DOE Cooperative Agreement DE-F03-02NA00057 and in part by fellowship support from the National Physical Science Consortium and Sandia National Laboratories, Albuquerque. \newline [1] K. M. Chandler, A.S. Shlyaptseva, N.D. Ouart, S.B. Hansen, M.D. Mitchell, S. A. Pikuz, T. A. Shelkovenko, D.A. Hammer, V. Kantsyrev and D. Fedin, ``Spectroscopic Analysis of X-ray Bursts from Nichrome and Conichrome X-Pinch Plasmas,'' Rev. Sci. Instr. 75, 3702 (2004). [Preview Abstract] |
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BP1.00123: Implosion and stagnation dynamics of wire array z-pinches S.N. Bland, S.V. Lebedev, D.J. Ampleford, S.C. Bott, J.P. Chittenden, G.N. Hall, C.A. Jennings, J.B.A. Palmer, J. Rapley, D.A. Hammer, S.A. Pikuz, T.A. Shelkovenko, I.H. Mitchell, J.A. G\'{o}mez We present detailed measurements of the implosion and stagnation phases of wire array z-pinch experiments on the MAGPIE generator (1MA, 240ns). The implosion of the array, which consists of an accelerating snowplough of current traveling towards the axis, critically depends upon the redistribution of mass ablated from the wires prior to implosion. Array configurations that alter the ablation of plasma from the wires of the array (e.g. by varying B$\theta$, introducing Bz and Br and/or by reversing the direction of Er) are used to explore the dependence of implosion on this process. The initiation of the implosion phase and formation of the snowplough sheath is investigated. The width of the sheath colliding with a precursor plasma column on axis is consistent with the rise time of the X-ray pulse observed in the experiments; and whilst the level of emission increases, the stagnated body of plasma on axis compresses, and axial electron beams are observed. After peak emission, the stagnated plasma shows large scale instabilities that then cause discontinuities. This research was sponsored by Sandia National Laboratories Albuquerque, the SSAA program of NNSA under DOE Cooperative Agreement DE-FC03-02NA00057 [Preview Abstract] |
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BP1.00124: Wire-Array Precursor Plasma Interactions With On-Axis Foam Targets J.B.A. Palmer, S.N. Bland, S.V. Lebedev, S.C. Bott, J.P. Chittenden, D.J. Ampleford, G. Hall, M. Sherlock, J. Rapley The Dynamic Hohlraum (DH) Z-pinch on Z at Sandia National Laboratory (SNL) has been used to drive Inertial Confinement Fusion (ICF) and High Energy Density Physics (HEDP) relevant experiments. The power pulse from the DH cannot yet be reproduced using codes that can reproduce the performance of a Vacuum Hohlraum (VH) configuration on Z. Unlike the VH the DH has a low-density CH foam cylinder placed on the array axis. Production of precursor plasma, prior to the main implosion, is not included in the codes. This plasma is accelerated towards the array axis by the global J x B force and impacts onto the on-axis target. This bombardment alters the foam in various ways. Experiments have been performed on the 1 MA MAGPIE generator at Imperial College, London, to investigate the effect of this precursor bombardment. Diagnostics used were point-projection radiography with x-pinches, x-ray emission framing cameras, shadowgraphy and photoconduction diodes. Results show ablation of low-density plasma from the foam surface and compression of the foam by precursor pressure. Research sponsored by AWE, SNL, the SSAA program of NNSA under DOE Cooperative Agreement DE-FC03-02NA00057. [Preview Abstract] |
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BP1.00125: Spherical Wire-Array Implosion Experiments G.N. Hall, S.V. Lebedev, S.N. Bland, S.C. Bott, J.P. Chittenden, D.J. Ampleford, C.A. Jennings, A. Ciardi, J.B.A. Palmer, J. Rapley The results of the first spherical wire array z-pinch implosion experiments are presented. Arrays were driven by the MAGPIE generator (1MA, 240ns) and consisted of 8x25$\mu $m aluminium wires forming the lines of longitude of a sphere with an equatorial diameter of 21mm. Plasma dynamics around each pole of the spherical array are comparable to those observed in radial wire array z-pinch experiments. The smaller radius at the poles produces a higher magnetic field than at the equator, leading to a higher rate of ablation of the wires into plasma. The ablated plasma from each pole forms a jet; and the jets from the poles collide near the centre of the sphere to produce a high-density precursor plasma. Later in time the array implodes, with a pair of plasma bubbles being launched from the poles towards the centre of the array, driving a pair of shock waves onto the precursor plasma. This research was sponsored by Sandia National Laboratories Albuquerque, the SSAA program of NNSA under DOE Cooperative Agreement DE-FC03-02NA00057. [Preview Abstract] |
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BP1.00126: Tungsten wire-array power variations on the Z facility versus implosion time D.B. Sinars, M.E. Cuneo, E.P. Yu, D.F. Wenger, D.E. Bliss, M.G. Mazarakis, E.M. Waisman, J.L. Porter, S.V. Lebedev We report on a set of tungsten wire array experiments where the array diameter (20 mm), height (10 mm), and wire number (300) were kept constant, but the mass of the array was varied (by altering the initial wire diameters) to vary the implosion time. Multiple tests using total masses of 1.14, 2.5, and 6.0 mg (65, 81, and 100 ns implosion times, respectively) were done, showing average peak powers of about 100 TW, 120 TW, and 95 TW, respectively. The high powers obtained using lower masses are notable because the short implosion times result in lower peak currents (12.7 MA, 16.3 MA, and 17.3 MA, respectively). Radiography, shadowgraphy, radiated power, and other data from these arrays will also be discussed. [Preview Abstract] |
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BP1.00127: Azimuthal Clumping Instabilities in a Z-pinch Wire Array W. Tang, T.S. Strickler, Y.Y. Lau, R.M. Gilgenbach, J. Zier, M. Gomez, C. Garasi, Edmund Yu, M.E. Cuneo, T.A. Mehlhorn Recent simulations of a high wire-number array Z-pinch reveal a strong azimuthal clumping instability [1]. This instability is found to be entirely analogous to the Jeans instability in a self-gravitating disk, where the mutual attraction of gravity is replaced by the mutual attraction of neighboring wires that carry currents in the same direction. The unstable modes are heavily crowded. We have studied the temporal evolution of initial perturbations which are randomly and uniformly distributed among all modes, i.e., the spectral equivalent of white noise. An analytic scaling law is derived, which shows that randomly seeded perturbations evolve at the rate of the fastest unstable mode, almost from the start. Extension to a coronal plasma, and the coupling of this clumping instability to the magnetic Rayleigh-Taylor instability, will be reported. [1] T. Strickler \textit{et al}., Phys. Plasmas \textbf{12}, 052701 (2005). * This work was supported by U. S. DoE through Sandia National Laboratories award number 240985 to the University of Michigan. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under Contract DE-AC04-94AL85000. [Preview Abstract] |
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BP1.00128: Current Scaling Studies with the Z Accelerator Michael G. Mazarakis, Mike E. Cuneo, Henry C. Harjes, William A. Stygar, Daniel B. Sinars, Christopher Deeney, Eduardo Waisman, Thomas J. Nash, Kenneth W. Struve, Dillon H. McDaniel The dependence of the peak radiated x-ray power and total radiated energy on the load current is presently under investigation with the Z accelerator in a new experimental campaign. Two load currents, peaking at 16-MA and 13-MA, are utilized. The load masses are lighter than usual and equal approximately to 2.5mg for the higher current and 1.1mg for the lower current. The arrays are of 20mm diameter and 10mm height with 300 tungsten wires. The 16-MA shots give peak powers of up to 190 TW, the highest ever obtained with single 20-mm tungsten arrays. The implosion times are shorter, 80 ns versus 95 ns for the heavier loads; however the peak powers are much higher. The improved performance of the lighter arrays could be attributed to shorter ablation times and less mass left behind. Our up-to-date results demonstrate that faster implosions are better and most attractive for ICF applications where high peak powers are of paramount importance. Hence future high current Z-pinch drivers with shorter rise times may be preferable if the cost remains reasonable. Results will be presented and compared with analytical theory predictions. *Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the USDOE's National Nuclear Security Administration under Contract DE AC04-94AL85000. [Preview Abstract] |
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BP1.00129: Scaling of radiation yield from wire array Z-pinch L.I. Rudakov, A.S. Chuvatin, A.L. Velikovich Wire-array Z-pinch plasmas, being inhomogeneous at small scales, generate extremely reproducible radiation pulses at implosion. Non-linear resistivity of the strongly inhomogeneous plasma may exceed the classical Spitzer's value due to the Hall effect that drives fast penetration of magnetic field along the low dense plasma structures inside the array. This resistivity enhancement explains high experimental efficiency of magnetic energy conversion into radiation, the characteristic peak-on-pedestal shape of the x-ray pulse that starts before the stagnation of the main mass, and a substantial trailing mass left behind the implosion front. We suggest the scaling of the wire array Z-pinch radiated power, where the enhanced resistivity generates the pedestal $\sim $ I$^{4}$/M$^{2}$ and kinetic energy of imploding plasma gives the peak $\sim $ I$^{2}$ on the pedestal (I and M are the current and imploding mass). Fitting the scaling to the existing experimental data and recommendations for the future experiments will be presented. [Preview Abstract] |
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BP1.00130: A Comparison of Calculated and Experimental X-ray Power Outputs Using a Phenomenological Formula for Enhanced Energy Coupling K.G. Whitney, J.W. Thornhill, L.I. Rudakov, A.L. Velikovich, J. Davis, C. Deeney, C.A. Coverdale Wire array Z-pinch implosions exhibit a large amount of 3-dimensional substructure in addition to their average 1-dimensional fluid behavior. It has recently been proposed that, due to Hall terms, the diffusion of magnetic field in such an inherently chaotic substructure produces an enhanced resistivity to the flow of current that is proportional to the square of the magnetic field. It was also recently shown that such resistivities are needed to explain the large late-in-time energy couplings that were observed in a set of Saturn, aluminum wire array experiments that were conducted at the Sandia National Laboratories. In this set of experiments, the array mass, radius, and length were varied, and a significant amount of late-time x-ray emission was observed. In this talk, we investigate the ability of the proposed enhanced resistivity formula, when employed in 1-D MHD calculations, to replicate the total and K-shell power data that was recorded in these experiments. [Preview Abstract] |
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BP1.00131: Dynamics and Radiation Characteristics of Copper Wire Arrays at the Z Accelerator C.A. Coverdale, B. Jones, C. Deeney, A.S. Safronova, N. Ouart, J. Chittenden, P.D. LePell Experiments to study the dynamics and radiation characteristics of single and nested copper z-pinches have been fielded at the 20 MA Z facility. Wire initiation and early implosion phases have been studied through monochromatic backlighting, shadowgraphy, and self-emission imaging and show instability and trailing mass. Pinhole images of the imploding plasma and stagnated pinch also show structure; the single arrays show multiple hotspots, while the nested arrays show more uniform emission. The K-shell output varies less for nested than for single arrays, suggesting hot spots dominate the K-shell radiation pulse for single arrays. Plasma parameters extracted from modeling of time-integrated K-shell and L-shell spectra indicate that more than one plasma source contributes to the radiation, which could result from hot spots or temporal gradients in the spectrum. The alignment of outer to inner wires of nested arrays has also been studied, though modeling suggests that outer/inner array interaction is dominated by modulations in the magnetic field caused by the return current configuration. *Sandia is a multi-program laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under Contract DE-AC04-94AL85000. [Preview Abstract] |
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BP1.00132: Opacity of the K Shell as a Function of Atomic Number in Z-Pinch Plasmas J.P. Apruzese, J.W. Thornhill, J. Davis, R.W. Clark, C.A. Coverdale, C. Deeney Many Z-pinch experiments performed on a variety of generators have succeeded in producing copious yields of K-shell x rays from elements ranging from neon (Z=10) to copper (Z=29). As the trend toward Z-pinch drivers of higher current and deliverable energy continues, the achievable masses, densities, and therefore opacities of pinches will increase. The effects of opacity on ionization, emissivity, and pinch dynamics, already important issues, are likely to become even more significant. In this work, we examine the variation of K-shell opacity with atomic number. Detailed calculations of photon escape probabilities, and their effects on emitted power and ionization within the pinch are presented. The relationship of single-flight to ultimate escape probability is derived in terms of basic atomic parameters. The range of pinch conditions where opacity is significant is seen to exhibit pronounced changes with atomic number, and is compared with conditions experimentally achieved to date. [Preview Abstract] |
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BP1.00133: Monochromatic backlight imaging of dynamic hohlraum driven capsule experiments G.A. Chandler, D.B. Sinars, K. Peterson, D.E. Bliss, J.E. Bailey, G. Cooper, R.W. Lemke, G.A. Rochau, C.L. Ruiz, S.A. Slutz, W.S. Varnum, R.J. Leeper, T.A. Mehlhorn, D.G. Schroen Dynamic hohlraum driven ICF is appealing because of the efficient generation and coupling of x-rays to capsules. This efficiency is achieved by closely coupling the dynamic hohlraum system to the ICF capsule. It is therefore extremely important to interrogate the hydrodynamics in this ICF scheme. In these experiments on the Z-accelerator we have for the first time observed backlight images of a dynamic hohlraum system where we have coupled $\sim $60 kJ of x-ray energy into a NIF size capsule with a diameter of 2.5 mm and with a radiation drive peaking at $\sim $220 eV. These capsules have a 65 $\mu $m Ge-doped CH wall and are filled with $\sim $20 atm of Deuterium and 0.06 atm of Argon. The dynamic hohlraum hydrodynamics were also explored using a fast optical framing camera. Comparisons of the experimental data with simulations will be presented. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the U.S. Dept. of Energy under contract No. DE-AC04-94AL85000. [Preview Abstract] |
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BP1.00134: Modeling of Dopant Spectral Emission in Z-Pinch Dynamic Hohlraum Experiments Joseph MacFarlane, I.E. Golovkin, P.R. Woodruff, P. Wang, G.A. Rochau, K. Peterson, J.E. Bailey, T.A. Mehlhorn X-ray spectra have been obtained from Si-doped low-density foams in dynamic hohlraum z-pinch experiments at Sandia National Laboratories. The purpose of the dopants is to provide spectroscopy signatures for constraining the time-dependent conditions within the hohlraum. In these experiments, $\sim $ 16 - 18 MA of current is delivered to a load comprised of a tungsten wire array which surrounds a low density cylindrical CH$_{2}$ foam. The z-pinch magnetic field accelerates the W plasma radially inward, reaching velocities $\sim $ a few x 10$^{7}$ cm/s. As the W plasma strikes the foam, a strong shock propagates through the foam, with temperatures behind the shock reaching $\sim $ a few x 10$^{2}$ eV. Time- and space-resolved x-ray spectra from Si K-shell lines are recorded, providing spectra from regions both within the shock and ahead of the shock. To model these spectra, we use the SPECT3D multi-dimensional collisional-radiative spectral analysis code. In this study, we investigate the influence of photopumping of Si transitions in the unshocked foam due to radiation emitted by the shocked foam region. We will present results from recent simulations, and discuss the sensitivity of the spectra to the conditions in the shocked and unshocked foam regions. [Preview Abstract] |
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BP1.00135: Z-Pinch Implosion Dynamics Studies by Using Laser Induced Fluorescence and Shearing Interferometer Niansheng Qi, Bruce Failor, Jeff Banister, Jerry Levine, Henry Sze, David Lojewski The energy coupling and the x-ray output of z-pinches are determined by the implosion history. Measurements from the initial gas, the MHD implosion, to the pinch phase are needed. Many diagnostics have been developed for the pinch phase. We have developed a Planar Laser Induced Fluorescence (PLIF) technique to characterize the gas puff in the initial phase and demonstrated a Laser Shearing Interferometer (LSI) approach for the implosion and pinch phases on a high current accelerator. The instruments are user-friendly and give the data quickly. Using the PLIF, the gas density and flow velocity profiles were obtained with sub-mm spatial resolutions. From the LSI, the shape of the imploding plasma was determined and the wavelengths of the R-T instability were measured. Combination of the PLIF and (LSI) measurements will enable us to better understand the implosion dynamics. [Preview Abstract] |
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BP1.00136: Soft x-ray laser interferometry of colliding plasmas Mike Purvis, Jonathan Grava, Jorge Filevich, Mario C. Marconi, Jorge Rocca, James Dunn, Stephen J. Moon, Raymond F. Smith, Joe Nilsen, V.N. Shlyaptsev We report results of an experiment designed to study the evolution of dense colliding plasmas created by irradiating a semi-cylindrical target geometry. The measurements were conducted using a 46.9 nm wavelength capillary discharge laser probe and a robust high throughput Mach-Zehnder interferometer based on diffraction gratings. The colliding plasmas were created irradiating a Cu target with a 800 nm wavelength laser pulse of 120 ps duration and $\sim $ 1J energy. The plasmas are seen to expand off the target surface and collide in a focal region creating a concentrated plasma with densities reaching 1 x 10$^{20}$ cm$^{-3}$. Plasmas with various degrees of collisionality can be studied by tailoring the irradiation conditions and selecting the target material. Results obtained using an Al target are compared with those of the Cu plasmas and model simulations. Work sponsored by the NNSA-SSAA program through DOE Grant {\#} DE-FG03-02NA00062 and U.S. DOE by the U. of California LLNL through the ILSA, contract No. W-7405-Eng-48. [Preview Abstract] |
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BP1.00137: Role of the two-stream instability in IEC devices Alberto Marocchino, Giovanni Lapenta, Evstati G. Evstatiev, Richard A. Nebel, Jaeyoung Park Recent experimental observations conducted on the LANL IEC device [1] suggest that the electron-electron two-steam instability can play a significant role in the physics of spherical virtual cathodes. Previous work based on fluid models suggested that the electron-electron two-steam instability would become unstable when the well depth of the virtual cathode was 14\% of the applied voltage [2]. We are reconsidering this phenomenon using a 2D PIC code CELESTE2D properly modified for this particular problem. The experiments have demonstrated that a crucial parameter in the evolution of the instability is the angular momentum present in the system. We analyze this effect with a simulation campaign aimed at investigating the stability boundary for the system. We verify for each case the stability of the virtual cathode using a variety of diagnostics: wavelet transform to analyze the temporal evolution of the instability and phase space diagrams to identify its effects on the particle distribution function.\newline [1] J. Park et al. Phys. Plasma 12, 56315 (2005)\newline [2] R.A. Nebel et al. Phys. Plasma 12, 12701 (2005). [Preview Abstract] |
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BP1.00138: Two-dimensional numerical simulations of the inertial electrostatic confinement device (IEC) Evstati G. Evstatiev, Richard A. Nebel, Jaeyoung Park, Giovanni Lapenta, Alberto Marocchino The theoretical works of Barnes and Nebel [1] have shown that a small ion cloud immersed in a uniform background electron density may undergo self-similar oscillations while being in local thermal equilibrium at all times. During the collapse phase of this oscillation the density and temperature of the ion cloud my reach extremely large values thus making this scheme particularly attractive for fusion application. One main purpose of the present experimental and theoretical work is to understand the stability properties of the electron background. Recent experimental results [2] indicate that the formation of a uniform electron background and a parabolic potential of up to 60\% of the grid potential is possible. However increasing of the grid potential past a certain value, while keeping all other parameters unchanged, leads to the destruction of the potential well. This instability is studied with the help of the two-dimensional particle-in-cell code CELESTE2D [3]. Results from different injection schemes will be shown and their influence on the stability of the background are presented. \newline [1] R.A. Nebel, D. C. Barnes, Fusion Technology 34, 28 (1998); Physics of Plasmas 5, 2498 (1998). \newline [2] R.A. Nebel et al. - Phys. Plasma 12, 12701 (2005). \newline [3] G. Lapenta, Phys. Plsmas, 6, 1442 (1999); J.Computat. Phys., 181, 317 (2002). [Preview Abstract] |
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BP1.00139: Effect of Dipole Mode Oscillation During High-current Heavy Ion Beam Transport and Longitudinal Compression Takashi Kikuchi, Tetsuo Someya, Masahide Seino, Kentarou Miyazawa, Shigeo Kawata, Mitsuo Nakajima, Kazuhiko Horioka High-current beam dynamics with a dipole oscillation is investigated during final beam bunching for a heavy ion fusion driver. The space-charge-dominated beam can be transported using a magnetic confinement system of a lattice structure. Misalignments of the transport system may cause the dipole oscillation of the beam during the transport with the longitudinal bunch compression. By using particle-in- cell simulations, we study the emittance growth due to the dipole oscillation during the final beam bunching. The results show that the transverse emittance is increased by the longitudinal compression with the dipole oscillation, and the growth rate is larger than that in the case without the oscillation. [Preview Abstract] |
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BP1.00140: Sub-megajoule high performance KrF direct-drive target designs Denis Colombant, S.P. Obenschain, A.J. Schmitt, S.T. Zalesak, J. Bates, A. Velikovich, J.H. Gardner, B.B. Afeyan, W. Manheimer In direct-drive ICF, the intensity on target has historically been kept lower than about $10^{15}/cm^{2}$ to avoid potential laser plasma instabilities. However, because of the $I\lambda^{2}$ scaling of most of the laser plasma instabilities, the KrF laser at 248 nm has a factor of 2 advantage over its closest competitor, the third harmonic of Nd glass laser light (351 nm). In addition, the smaller wavelength of KrF gives higher collisional absorption and the $>$ 1 THz bandwidth is advantageous for both beam smoothing and instability suppression. The laser architecture makes it easy to zoom the focal spot to follow an imploding pellet and thereby increase absorption efficiency. The purpose of this paper is to investigate what can be gained in terms of performance and hydrodynamic stability by making use of the factor of 2 in laser intensity, as well as zooming. Our preliminary results indicate that KrF allows substantial reductions in the laser energy required for ignition while maintaining moderate gains. Target designs will be presented showing how the trade-off between gain and hydrodynamic stability is altered by using the advantages of KrF. [Preview Abstract] |
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BP1.00141: Experiments to study radiation transport in inhomogeneous plasmas M.J. Taylor, J.M. Foster, P.A. Rosen, C.D. Bentley, C.C. Smith, S.J. Davidson, P.A. Keiter, J.R. Fincke, M. Gunderson, T.S. Perry Calculations of radiation transport in heated materials are greatly complicated by the presence of regions in which two or more materials are inhomogeneously mixed; approximate, statistical treatments of this problem have been developed by Pomraning and others, but their application has not been tested in detail. We describe laboratory experiments to test modelling of radiation transport through inhomogeneous plasmas. An Omega laser-heated hohlraum is used as a thermal source to drive radiation through a polymer foam containing randomly-distributed gold particles. We discuss the design of these experiments, and the relation between the choice of hohlraum driver and the energetics and hydrodynamics of heat transport through the hot, gold mixture. Experimental data are compared with full post-shot simulations using different models for the opacity of the inhomogeneously-mixed material. Enhancements to the post-shot modelling will be outlined, and potential areas of improvement highlighted to increase the quantitative nature of the experiment. [Preview Abstract] |
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BP1.00142: NLTE Radiative Cooling in ICF Capsule Implosions Kyle J. Peterson, G.A. Rochau, J.E. Bailey, T.A. Mehlhorn, I. Golovkin, J.J. MacFarlane, R.C. Mancini NLTE radiation cooling of ICF capsule implosions was simulated with HELIOS-CR, a 1D hydrodynamic code that includes inline collisional-radiative physics for modeling non-LTE time- dependent kinetics. The effects of Krypton and Argon line emission on implosion dynamics and characteristic target signatures will be shown for Z-pinch dynamic hohlraum, NIF and Omega ICF targets. Z-pinch dynamic hohlraum simulations will also be compared with two series of experiments that have been conducted on Sandia National Laboratories Z facility to study the effect of non-LTE radiative cooling in Argon and Krypton doped capsule implosions. In these experiments, spectroscopic analysis was used to determine peak fuel conditions from capsules containing 24 atm of DD Fuel and 0.085 atm of Argon while varying Krypton dopant concentration at pressures of 0.01 atm, 0.04 atm, or 0.085 atm. Capsules studied were 2.0 mm in diameter with either a 50 or 70 $\mu $m thick CH shell. Experimental data analysis and comparison with post processed simulation results will be also presented. [Preview Abstract] |
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BP1.00143: Non-LTE coronal Au plasma simulation in the configuration average approximation Jiankui Yuan, Gregory Moses We present the non-LTE calculation for a highly ionized Au plasma using our YAC atomic physics code. We compare our results with experiments recently conducted at the Livermore electron beam ion trap EBIT-II. The configuration average model explicitly includes all possible open M-shell configurations from a given configuration group. The collisional and radiative rates connecting the included configurations are calculated in the configuration average approximation for population kinetics. Parallel iterative solvers in the PETSc software library are used for the solution of the rate equations. Performance of the parallel iterative solver will also be discussed. [Preview Abstract] |
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BP1.00144: Implicit Monte Carlo radiative transfer in DRACO Gregory Moses, Jiankui Yuan An Implicit Monte Carlo (IMC) transport module for two- dimensional non-orthogonal mesh, multi-frequency radiative transfer is presented. This module has been implemented in the DRACO radiation hydrodynamics laser fusion target simulation code. In the IMC method a forward plasma temperature is estimated at the beginning of the time step in the transport equation using the so-called Fleck factor to provide for an implicit solution for long time steps. A simple equilibrium solution in an infinite medium is used as an analytical benchmark of this implicit solution. In other dynamic test cases, analytical macroscopic cross sections are applied to compare with previously reported one-dimensional results by Fleck and Cummings. Finally, our IMC results are compared to the flux-limited multi-frequency diffusion model for uniform and non-uniform meshes in two dimensions. [Preview Abstract] |
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BP1.00145: Kinetic Modifications to the Ion Threat Spectra on IFE Reactor First Walls John F. Santarius, Gregory A. Moses Most ion threat spectra calculations for inertial fusion energy reactor first walls use a radiation hydrodynamics model based on the fluid approximation. During the post-burn expansion, however, ion collisional mean free paths can become significantly larger than the shock thickness, limiting the maximum energy transfer. We focus on the high average power laser (HAPL) fusion reactor [J.D. Sethian, et al., \textit{Nuclear Fusion} \textbf{43}, 1693 (2003)]. The University of Wisconsin's 1-D radiation hydrodynamics code, BUCKY, predicts that, at 34.592~ns, the primary shock wave occurs in the zones at the CH{\-}DT interface just outside of the pure DT zones, and another shock occurs at the interface where the plastic impacts the gold. The dense core inside r$\sim $10~$\mu $m remains well described by hydrodynamics. The mean free path in the primary shock's frame for slowing down of CH ions on the shock DT ions and electrons approximately equals the shock thickness. The shock thickness where the CH ions impact the Au ions is nearly 1000 times smaller than the mean free path. Results of addressing the problem by implementing energy deposition for moderate-to-large mean free paths using zone-by-zone differential masses and velocities in the BUCKY code will be reported. [Preview Abstract] |
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BP1.00146: Effects due to beryllium microstructure in cryogenic NIF ignition capsules* M.M. Marinak, N.R. Barton, R. Becker, J.D. Salmonson, S.W. Haan The polycrystalline structure of a beryllium ablator can produce asymmetries that seed hydrodynamic instabilities. Our NIF ignition capsules are designed to minimize beryllium grain effects by promptly melting the ablator before or during the passage of the first shock. The release of residual stresses during shock melting still induces small perturbations ($\delta$v/v $\sim 10^{\neg 4}$) in the flow field. We simulate their effect on a capsule having a beryllium ablator uniformly doped with copper. A 3D polycrystalline model is employed in ALE3D, a multiphysics arbitrary Lagrange Eulerian code, to calculate the response to the first shock. It resolves the anisotropic elastic and plastic response, down to individual grains. Perturbations are then linked to a HYDRA simulation of the full implosion. Simulations resolving modes $\ell \sim$ 11- 600 show minor perturbation growth in a fully melted ablator. We also consider a recent capsule design with graded dopant concentrations in a silicon-lined hohlraum. High levels of x-ray preheat from the silicon cause the beryllium grains to expand into the ice. Effects of the perturbations induced at the ice-ablator interface are simulated.\\ [2] *This work 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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BP1.00147: A Hydrodynamic Investigation of Beryllium Defects Paul Keiter, James Fincke, George Kyrala, Stephane Laffite, Douglas Wilson One of the proposed methods for filling ignition capsules for the NIF is to drill a hole in the beryllium layer and use a fill tube. One must evaluate this design to determine the effect it may have on capsule performance. Simulations of an experimental design for an OMEGA experiment have been performed. In the experiment a hohlraum temperature drive is incident on a Be disk with two different aspect ratio holes. Two jets are created when the temperature drive interacts with these holes. The hydrodynamic motion of the jets as a function of time is studied via x-ray radiography. We present simulations of our experimental design and, if available, preliminary experimental results from an experiment performed on the OMEGA laser to study the hydrodynamic evolution of a ``fill hole'' in Be. \textit{This was performed by the Los Alamos National Laboratory under the auspices of the United States Department of Energy under contract no. W-7405-ENG-36.} [Preview Abstract] |
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BP1.00148: Rollup of specifications for NIF ignition targets into an error budget Steven Haan, Mark Herrmann, Peter Amendt, Debra Callahan, Thomas Dittrich, John Edwards, Abraham Fetterman, Ogden Jones, John Lindl, Marty Marinak, David Munro, Stephen Pollaine, Jay Salmonson, Brian Spears, Larry Suter Targets 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)] are being simulated and the simulations are used to set and update the specifications for target fabrication, the laser, and the experimental programs leading up to ignition. Recent design work has focused on designs that assume only 1.0 MJ of laser energy. Three capsule designs and several hohlraum designs are being considered. Complete tables of specifications have been prepared for all targets, specifying the contribution of each uncertainty to an error budget that allows adequate margin beyond the uncertainties. The specifications are being formalized and documented. Specifications are based on several kinds of simulations: linear analysis, using growth factors from 2D simulations; 2D simulations that include combinations of 1D errors and 2D perturbations; and 3D simulations with random combinations of all expected uncertainties. \newline [2] *This work performed under the auspices of the U. S. Department of Energy by the University of California Lawrence Livermore National Laboratory under contract W-7405-eng-48 [Preview Abstract] |
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BP1.00149: Modeling Small-Amplitude Perturbations in Inertial Confinement Fusion Pellets Steven Zalesak, N. Metzler, A.L. Velikovich, J. Schmitt, D.G. Colombant, J.H. Gardner, W. Manheimer Recent advances in inertial confinement fusion (ICF) technology serve to ensure that imploding laser-driven ICF pellets will spend a significantly larger portion of their time in what is regarded as the ``linear'' portion of their perturbation evolution, i.e., in the presence of small-amplitude but nonetheless evolving perturbations. Since the evolution of these linear perturbations collectively form the initial conditions for the subsequent nonlinear evolution of the pellet, which in turn determines the energy yield of the pellet, the accurate numerical modeling of these small-amplitude perturbations has taken on an increased importance. This modeling is difficult despite the expected linear evolution of the perturbations themselves, because these perturbations are embedded in a highly nonlinear, strongly-shocked, and highly complex flow field which in and of itself stresses numerical computation capabilities, and whose simulation often employs numerical techniques which were not designed with the proper treatment of small-amplitude perturbations in mind. In this paper we will review some of the techniques that we have recently found to be of use toward this end. [Preview Abstract] |
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BP1.00150: Nonlinear Model for Start-Up Phase Pressure Perturbation Spectra from Time-Dependent Optically Smoothed ISI Laser Imprint Michael Keskinen, A. Schmitt, A.L. Velikovich The spectrum of early time pressure perturbations, due to optically smoothed induced spatial incoherence (ISI) laser imprint, is computed for a planar target using a forced, dissipative model. The ISI laser deposition is computed using a time-dependent electromagnetic full wave Maxwell code. We find that the pressure spectrum evolves into a power law in which spectral power is transferred from large to smaller scales through a nonlinear cascade process. A nonlinear analytical model is also developed and compared with simulations using the forced, dissipative model. We also compare the model predictions with experimental observations. [Preview Abstract] |
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BP1.00151: High Sensitivity Imprint Measurements on Nike Laser Max Karasik, Yefim Aglitskiy, V. Serlin, J.W. Bates Hydrodynamic instability seeded by laser non-uniformity (laser imprint) is an important factor in performance of direct-drive ICF targets. Most of the imprint occurs during the initial low-intensity (``foot'') part of the pulse, necessary to compress the target to achieve high gain. Experiments are carried out on Nike KrF laser with induced spatial incoherence (ISI) smoothing. The amount of imprint is varied by changing the uniformity the foot of the pulse. The resulting Raleigh-Taylor (RT) amplified areal mass non-uniformity is measured by face-on x-ray radiography using Bragg reflection from a curved crystal coupled to an x-ray streak camera. The streak camera was recently retrofitted with a new high sensitivity CCD camera. The sensitivity of the CCD has enabled it to be fiberoptically coupled directly to the streak camera output, without an image intensifier and lens coupling. This gave an increased overall spatial resolution as well as lower noise. Because of the strong short wavelength component of RT amplified imprint, the increased resolution and lower noise resulted in much lower noise floor in the measurement. Experimental results are compared with 2D simulations using FAST hydrocode for a range of foot uniformities and intensities. Work supported by the U. S. DOE/NNSA. [Preview Abstract] |
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BP1.00152: Stability studies of sub-megajoule high performance KrF direct-drive target designs. Andrew J. Schmitt, D.G. Colombant, S.P. Obenschain, S.T. Zalesak, J.W. Bates, A.L. Velikovich, J.H. Gardner, D.E. Fyfe Sub-megajoule direct-drive targets that ignite and produce gain have been designed\footnote{D.Colombant \emph{et al.}, this conference}. These designs utilize higher drive intensity and implosion velocities than are typical of high-gain targets. The higher intensity is possible because of the low-wavelength, large-bandwidth advantages of KrF lasers, and the relative ease with which one can ``zoom'' the focal spot with the KrF laser architecture allows maximum utilization of the laser energy. However, the crucial question is whether these targets can be made to be stable enough to survive the implosion and still produce appreciable gain. We present here the stability studies we have done with our FAST hydrocode (utilizing recently developed low-noise algorithms\footnote{S.T. Zalesak \emph{et al.}, Phys. Plasmas {\bf 12}, 056311 (2005).}) and compare the results to expectations from simple models. [Preview Abstract] |
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BP1.00153: Acceleration and Deceleration Phase Nonlinear Rayleigh-Taylor Growth at Spherical Interfaces Daniel Clark, Max Tabak The Layzer model for the nonlinear evolution of bubbles in the Rayleigh-Taylor instability has recently been generalized to the case of spherically imploding interfaces [D. S. Clark and M. Tabak, Phys. Rev. E 71, 055302(R) (2005).]. The spherical case is more relevant to, e.g., Inertial Confinement Fusion (ICF) or various astrophysical phenomena when the convergence is strong or the perturbation wavelength is comparable to the interface curvature. Here, the model is further extended to the case of bubble growth during the deceleration (stagnation) phase of a spherical implosion and to the growth of spikes during both the acceleration and deceleration phases. Differences in the nonlinear growth rates for both bubbles and spikes are found when compared with planar results, and the model predictions are verified by comparison with numerical hydrodynamics simulations. The new nonlinear growth rates are also incorporated into a Haan-type saturation model to give improved predictions of multi-mode saturated growth for ICF capsules. [Preview Abstract] |
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BP1.00154: Machinability of Carbon Resorcinol Formaldehyde Aerogel Nicole Petta, Sue Carter, Joe Florio, Josh Gregory, Ed Hsieh, Derrick Mathews, Brian Motta, Katharine Nelson, Keith Shillito, Diana Schroen Resorcinol Formaldehyde (RF) foam is optically transparent due to its small pore size. It is understood that slight variations in chemistry alter this pore size. This is evident in the foam's altered opacity at lower resorcinol to catalyst (R/C) ratios. This study examines the parameters necessary to machine the carbonized material while decreasing ``pull-outs'' and improving surface finish. [Preview Abstract] |
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BP1.00155: 400 Micron Thick CRF with a 15 Micron Step Katharine Nelson, Sue Carter, Joe Florio, Josh Gregory, Jim Heuer, Ed Hsieh, Derrick Mathews, Brian Motta, Nicole Petta, Keith Shillito, Diana Schroen A Resorcinol-Formaldehyde (RF) aerogel of 100 mg/cc was synthesized then carbonized by being brought up to high temperatures under an inert environment. This carbon aerogel was then machined 400 micron thick with a 15 micron step. Some of the issues encountered in the attempts to machine the CRF down to the very thin ranges with relatively fine surface finishes were: The material tended to ``flake'' around the perimeter. This would sometimes lead to catastrophic failure of the work piece. The material also tended to become flexible when thin. This allowed the material to distort into the vacuum holes. These phenomena were minimized by tuning vacuum hole diameter and location, and controlling total vacuum being drawn. Likewise progressively shallower cuts were required in order to obtain the relatively fine surface finishes exhibited. This was necessary in order to minimize ``pull out'' of the material which is porous and brittle. The machined CRF steps at less than 400 microns total thickness, with a 15 micron step represent precision diamond turning thin section CRF. The CRF was characterized using 3-D confocal microscopy and SEM for dimensions, foam pore size and surface finish. This work is supported under DOE DE-AC03-01F22260. [Preview Abstract] |
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BP1.00156: Michigan Fabrication and Assembly Techniques for Thomson Scattering Targets T.L. Donajkowski, K.L. Killebrew, R.P. Drake, A.B. Reighard, C.M. Krauland, M.R. Taylor, D.C. Marion, D.J. Kremer Thomson scattering is an interesting physical phenomena. We show the design and build of experiments created to detect Thomson scattering off of a directly driven shock. The experiments are similar to past radiative, shock tube experiments with the addition of a secondary arm to allow a 4$\omega $ probe beam to enter and scattered light to exit the main shock tube. We will discuss the difficulties associated with the design, fabrication and assembly of these targets. This research was sponsored by the National Nuclear Security Administration uner the Stewardship Science Academic Alliances program through DOE Research Grant DE-FG52-03NA00064, and through DE FG53 2005 NA26014 and other grants and contracts. [Preview Abstract] |
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BP1.00157: Study of manufacturing induced defects in beryllium capsules for the National Ignition Facility (NIF) James Cooley, Stephane Laffite, Doug Wilson Copper doped beryllium (BeCu) has several advantages over plastic (CH) for capsule ablator material. Unfortunately, the manufacturing process for the capsules may introduce defects that may reduce or impede capsule ignition. In this paper, we present current results from an ongoing study at the Los Alamos National Laboratory to address some of these manufacturing related issues. In particular we present results of numerical simulations that examines the effect of a thin, $<$1 $\mu $m, aluminum (Al) or CH joint that would be present between two machined hemispheres of BeCu. In particular, we examine the jet resulting from the heavily mixed layer$^{3}$ after the shock front emerges from the BeCu into the DT ice and DT gas and we speculate on the effect of this jet on ignition. Then, we examine the effect of a fill-hole or fill-tube on jet formation for the BeCu capsule. First, we compare the results of simulations with recent experiments carried out on Omega and discuss implications for future studies. Then, we discuss simulations results for fill-holes and fill-tube and speculate on the effect of these defects on perturbations of the final ignition hotspot. $^{1}$ to whom correspondence should be addressed jhcooley@lanl.gov $^{2}$ CEA/DIF, BP12, 91680 Bruy\`{e}res le Ch\^{a}tel, France $^{3}$ S. A. Bel'kov et al., \textit{Physics of Plasmas} \textbf{6}, 4728 (1999) [Preview Abstract] |
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BP1.00158: Foil Cooling for the Rep-Rated Electron Beam Pumped Electra Laser J.L. Giuliani, M.C. Myers, J.D. Sethian, F. Hegeler, T. Albert, M.F. Wolford, S. Abdel-Khalik The Electra program at the Naval Research Laboratory is developing the science and technologies for implementation of krypton-fluoride (KrF) lasers in inertial fusion energy. Large aperture KrF lasers are pumped by electron beams which transit a foil separating the gas target at $\ge $1 atm pressure from the vacuum diode. A fraction of the beam energy is deposited in the foil and thus long term ($\ge $10$^{8}$ shots), rep-rated (5 Hz) operation requires active cooling of the foil to prevent thermal yield relaxation and cycling fatigue. This paper will report on experimental data and theoretical analysis of two diverse approaches to foil thermal management: convective and conductive cooling. Convective turbulent cooling has been operational on the Electra main amp through the use of oscillating louvers within a gas recirculator containing the pumped lasing region. At 5 Hz the foil temperature ($T_f )$ can be maintained at $\sim $400 $^{o}$C for a 1 mil SS foil. Conduction cooling provides the simplest configuration with only the need for water channels in the ribs of the hibachi. For a 1 mil Al foil, $T_f $ is predicted to be $\sim $140 $^{o}$C at 5 Hz. Comparison of experimental and theoretical results and advanced foil materials will be discussed. [Preview Abstract] |
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BP1.00159: MINI-CONFERENCE: ASTROPHYSICAL EXPLOSIONS: FROM ENGINES TO REMNANTS |
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BP1.00160: Could the Cas A Explosion Have Produced a Gamma Ray Burst? Martin Laming, Una Hwang We analyze the polar regions of the recently acquired very deep 1 Ms Chandra X-ray observation. We infer that the so-called ``jet'' regions are indeed due to jets emanating from the explosion center, and not due to polar cavities in the circumstellar medium at the time of explosion. We place limits on the equivalent isotropic explosion energy in the polar regions (around $1.5 \times 10^{52}$ ergs), and the opening angle of the x-ray emitting ejecta (around 7-10 degrees), which give a total energy in the NE jet in the range $0.5 - 1 \times 10^{50}$ ergs; about an order of magnitude lower than inferred for "typical" GRBs. While the Cas A progenitor and explosion exhibit many of the features associated with GRB hosts, e.g. extensive presupernova mass loss, rotation and jets associated with the explosion, we speculate that the recoil of the compact central object, with velocity 330 km/s, may have rendered the jet unstable. In such cases the jet rapidly becomes baryon loaded, if not truncated altogether. Although unlikely to have produced a gamma ray burst, the jets in Cas A suggest that such outflows may be common features of core-collapse SNe. Work supported by Chandra GO Program and Basic Research Funds of the Office of Naval Research. [Preview Abstract] |
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BP1.00161: A New Mechanism of Magnetic Field Generation in Supernova Shock Waves and its Implication for Cosmic Ray Acceleration Patrick Diamond, Mikhail Malkov SNR shocks are the most probable source of galactic cosmic rays. We discuss the diffusive acceleration mechanism in terms of its potential to accelerate CRs to 10$^{18}$ eV, as observations imply. One possibility, currently discussed in the literature, is to resonantly generate a turbulent magnetic field via accelerated particles in excess of the background field. We indicate some difficulties of this scenario and suggest a different possibility, which is based on the generation of Alfven waves at the gyroradius scale at the background field level, with a subsequent transfer to longer scales via interaction with strong acoustic turbulence in the shock precursor. The acoustic turbulence in turn, may be generated by Drury instability or by parametric instability of the Alfven (A) waves. The essential idea is an A-->A+S decay instability process, where one of the interacting scatterers (i.e. the sound, or S-waves) are driven by the Drury instability process. This rapidly generates longer wavelength Alfven waves, which in turn resonate with high energy CRs thus binding them to the shock and enabling their further acceleration. [Preview Abstract] |
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BP1.00162: Effects of Magnetic Turbulence on Radiation Spectra in GRB Shocks Sarah Reynolds, Sriharsha Pothapragada, Mikhail Medvedev Relativistic collisionless shocks in space are driven by strong explosions, such as supernovae and gamma-ray bursts (GRBs). These shocks produce small-scale Weibel magnetic fields and accelerate electrons, which emit jitter/synchrotron radiation. We consider the dependence of the jitter radiation spectrum on the properties of the magnetic turbulence at the shock and its temporal evolution. For a viewing angle $0<\Theta<\pi/2 $ between the observer's position and the direction of the shock propagation $\bf n$, the jitter radiation spectrum depends upon both the transverse and longitudinal (along $\bf n$) magnetic field spectra, $f_{xy}$ and $f_z$, respectively. However, the shapes of $f_{xy}$ and $f_z$ are not, in general, known. Therefore, we use heuristic arguments and the results of recent numerical 3D PIC simulations. We calculate the jitter radiation spectra for different viewing angles, $\Theta$, and several key parameters of the magnetic field spectra. We speculate that the coupling of relativistic aberration and the radiation spectrum anisotropy naturally explains the spectral evolution of GRB prompt emission. [Preview Abstract] |
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BP1.00163: Correlation Analysis of Temporal Variation of Spectral Parameters of Prompt GRB Emission Sriharsha Pothapragada, Sarah Reynolds, Mikhail Medvedev Prompt emission from a gamma-ray burst (GRB) exhibits very rapid and complicated temporal and spectral evolution, which likely contains a lot of information about the GRB origin. Using the BATSE time- resolved spectral fit data, we performed a number of correlation tests for individual bursts and for the entire sample. We report on the strong correlations between the low-energy power law index, $\alpha$, and the observed photon flux, $F$, in some GRBs. Since, however, the observed flux is a poor measure of the intrinsic flux (because of dust extinction, unknown distances, etc.), we calculate the normalized flux $F_*$ assuming that the correlation of $\alpha$ and $F$ is unique for all GRBs. We carefully examine statistical biases this technique may produce. We speculate that the observed $F-\alpha$ and $F_*-\alpha$ correlations support the predictions of an anisotropic jitter radiation theory from Weibel-generated magnetic fields. [Preview Abstract] |
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BP1.00164: Hydrodynamical instabilities in core collapse supernovae and differentially rotating proto-neutron stars Shangli Ou, Joel Tohline We present results from three-dimensional hydrodynamic simulations of the development of nonaxisymmetric structure in two related scenarios: the post-bounce core of a massive star based on the work of Ott et al. (2005); and differentially rotating, proto-neutron stars (NS). A one-armed, m=1 spiral instability, which was found by Centrella et al. (2001) in n=3.33 polytropes, arises in both cases. Further investigations strongly suggest that this instability is directly triggered by corotation points residing inside cores with strong differential rotation. This instability is also capable of transporting angular momentum from the inner regions to the outer regions of the core, which may help to explain why proto-NSs have smaller spin rates compared to earlier predicted values. The instability is not limited to the m=1 mode, but may also arise in higher order (e.g., m=2 and 3) modes, as long as their corotation points exist inside the newly formed core. This superposition/mixture of multiple unstable modes within a single core or proto-NS is consistent with the linear analysis presented by Watts et al.(2004). Gravitational waves from such scenarios are in general not monochromatic. The possible influence of magnetorotational instability (MRI) on the development of these modes is discussed. This work has been supported, in part, by NSF grants AST-0407070 and PHY-0326311, and by the Center for Computation and Technology at LSU. [Preview Abstract] |
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BP1.00165: Transient large-scale dynamos in supernova progenitors Jason Nordhaus, Eric Blackman A large-scale, transient interface dynamo model is presented in the context of a supernova progenitor. Torodial and polodial field strengths are calculated at the boundary between the strong shear layer and the surrounding convective envelope. Dynamical shear is included and drained through field amplification and turbulent diffusion. For a 15 solar mass progenitor with varying outer shear layer rotation rates, we present a range of models which may provide enough energy to help power a supernova through poynting flux, viscous dissipation or a combination of both. [Preview Abstract] |
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BP1.00166: Disks and Ring Structures Around Collapsed Objects Suitable for the Emergence of Jets B. Coppi The emergence of jets from plasma accretion disks surrounding for instance a black hole requires that the magnetic energy densities within the disk be significant relative to the plasma pressure. Thus the axisymmetric equilibrium configurations, that can form in the strong gravitational field of a central object and where the currents within differentially rotating disks can produce a ``crystal'' magnetic structure\footnote{B. Coppi, \textit{Phys. Plasmas} \textbf{12}, 057302 (2005)}, are shown to be characterized by strong modulations of the plasma density and pressure when the magnetic energy densities are comparable to the thermal energy densities. Moreover, when the external magnetic field in which the plasma is immersed is relatively weak, the internal currents can produce a configuration consisting of a sequence of plasma ring pairs\footnote{B. Coppi, F. Rousseau, M.I.T. (LNS) Report HEP05/01 (2005), \textit{submitted to Ap.J}}. The processes which can sustain these configurations are discussed. [Preview Abstract] |
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BP1.00167: Three-dimensional Relativistic Particle Acceleration by Magnetic Cylinder and Torus Koichi Noguchi, Edison Liang We study the time-development of magnetic cylinder and torus in 3D PIC simulation of electron-positron plasma. The cylinder case may represents the uniform region of hoop-stress-supported jets, and the torus represents the case when a jet head emerges from the surface of a collapsar, or when a new born magnetar wind blows out the progenetor envelope. Initial magnetic field distribution is purely azimuthal, and it expands with accelerating plasma. In the cylinder case, plasma forms a radially expanding tube, and electrons and positrons get accelerated in the radial and axial directions without charge separation. In the torus case, charge separation occurs because of the finite axial length of the torus, and electrons and positrons are distributed over radially expanding hemispheres with relativistic acceleration. We will discuss the momentum distribution and radiation. [Preview Abstract] |
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BP1.00168: Particle Acceleration in Electromagnetic Dominated Outflows with Background Plasma and Clumps Koichi Noguchi, Edison Liang The effect of background plasma on particle acceleration via Poynting fluxes is studied in 3D PIC simulation of electron-positron plasmas. When strongly magnetized plasma at the center expands to background low-temperature electron-positron plasma, EM wave front accelerates background plasma, and captures particles in the Ponderomotive potential well. When a low-density clump is embedded in the background plasma, particles in the clump are also accelerated and captured in the potential well. In both cases, we do not observe any instability, and the momentum distribution of background and clump forms a power law of slope close to -1 with a sharp peak in the middle. We also discuss the ion-electron background and clump case. [Preview Abstract] |
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BP1.00169: Explosive plasma expansion in strong magnetic field R. Presura, A. Esaulov, V.V. Ivanov, Y. Sentoku, V.I. Sotnikov, L.F. Wanex, T.E. Cowan Experiments to investigate the interaction of a laser-produced-plasma with a strong magnetic field showed the formation of a dynamic boundary layer with high density gradient at the plasma-field interface.[1,2] The shape of the expansion is explained by 3D-ideal-MHD modeling, but kinetic effects are found to be important in understanding the shock penetration in the strong field region. Follow-up experiments using a higher intensity laser and stronger magnetic field will allow control of the plasma collisionality and magnetization in the plasma-field interaction region. These laboratory experiments will achieve conditions relevant to plasma astrophysics interactions. Experimental results and several new experimental concepts will be presented. \newline \newline [1] R. Presura et al., ApSS 299, 299-303 (2005) \newline [2] W. Horton et al., Advances in Space Research, In Press, 2005 [Preview Abstract] |
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BP1.00170: Simulations of Jetted Relativistic Blastwaves in Astrophysics Jay Salmonson, Chris Fragile, Peter Anninos We present new 2D relativistic hydrodynamic simulations of jetted blastwaves using the Cosmos++ astrophysics code. In particular, we simulate the asymmetric outflow resulting from the giant flare of December 27, 2004 from SGR 1806-20. We find that the asymmetric radio nebula observed to expand over the months following the flare cannot be explained by a simple ballistic ejection of material during the flare, but requires angular dependence of the energy injection with respect to the jet axis. In addition, we present simulations of jetted blastwaves of the relativistic afterglows resulting from gamma-ray bursts. Evolving these jetted blastwaves from Lorentz factors of order 10, we explore the dependence of observed lightcurves on initial jet opening angle, energy distribution, and observer angle with respect to the jet axis. 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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BP1.00171: Three-dimensional Modeling of Laboratory Jets that Scale to Astrophysical Jets B.H. Wilde, R.F. Coker, P.A. Rosen, J.M. Foster, R.J.R. Williams, B.E. Blue, P. Hartigan, A.M. Khokhlov, R.P. Drake, J.P. Knauer, A. Frank We have fielded hohlraum-driven jet experiments on the Omega laser at the University of Rochester that attempt to scale to astrophysical jets. High-resolution point-projection images show a central collimated jet with complex structure in the shedding of the mushroom that indicates potential turbulence. These jets have Euler and Reynolds numbers similar to those present in models of jet-driven supernova explosions. The jet is created by the acceleration of a plug of titanium through a free-run vacuum region in a titanium washer. The supersonic jet enters a low density RF foam creating a bow shock in front of it. We will present 2- and 3-dimensional calculations of these jets with the continuous-adaptive-mesh-refinement radiation-hydrodynamics code RAGE. 3-dimensional calculations are required since the jet breaks up in the mushroom before going turbulent and since there are typically 3-dimensional driving asymmetries. We will also present results from experiments that have jets interacting with a spherical object that simulate astrophysical jets interacting with stellar clouds. [Preview Abstract] |
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BP1.00172: Mitigation of Earth-asteroid collisions via explosive, intense radiation sources Aaron Miles, James Sanders The Universe is continually producing astrophysical explosions that generate intense bursts of electromagnetic and particle radiation. Interaction of this radiation with nearby objects can effect significant changes to their dynamics through a variety of processes including ionization, ablation, and shock generation. The next time a large asteroid or comet is found to be approaching the Earth on an impact trajectory, humans may find it prudent to mimic nature by using the most intense radiation sources available to alter the incoming object's trajectory and avert a catastrophic collision. With this in mind, we consider the effect of nuclear explosives on nearby would-be Earth impactors. Neutrons and x-rays produced in the explosion are deposited in a thin layer of the asteroid's surface, resulting in ablation and shock and thereby imparting a deflection velocity. A Monte Carlo code is used for radiation transport and energy deposition, while the subsequent dynamic evolution of the asteroid is followed with the hydrodynamics code CALE. We consider the dependence of the deflection velocity on the source energy and spectrum, the asteroid or comet composition, and the standoff distance between the target and the source. 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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BP1.00173: Collapsing Radiative Shocks in Xenon Gas on the Omega Laser A.B. Reighard, R.P. Drake, K.L. Killebrew, D.J. Kremer, T. Donajkowski, M. Grosskopf, M.R. Taylor, D.C. Marion, C. Krauland, S.G. Glendinning, B.A. Remington, R.J. Wallace, D.D. Ryutov, J. Greenough, J. Knauer, S. Bouquet, L. Boireau, M. Koenig, T. Vinci A number of astrophysical systems involve radiative shocks that collapse spatially in response to energy lost through radiation, producing thin shells believed to be Vishniac unstable. We report experiments intended to study such collapsing shocks. The Omega laser drives a thin slab of material at $>$100 km/s through Xe gas. Simulations predict a collapsed layer in which the density reaches 45 times initial density. X-ray backlighting techniques have yielded images of a collapsed shock compressed to $<$1/25 its initial thickness (45 $\mu $m) at a speed of $\sim $100 km/s when the shock has traveled 1.3 mm. Optical depth before and behind the shock is important for comparison to astrophysical systems. This research was sponsored by the National Nuclear Security Administration under the Stewardship Science Academic Alliances program through DOE Research Grants DE-FG52-03NA00064, DE-FG53-2005-NA26014,~and other grants and contracts. [Preview Abstract] |
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BP1.00174: Michigan Fabrication and Assembly Techniques for Rayleigh-Taylor Modal Supernova-Relevant Experiments K.L. Killebrew, R.P. Drake, C.C. Kuranz, D.J. Kremer, M. Grosskopf, T.L. Donajkowski, M.R. Taylor, C.M. Krauland, D.C. Marion, J.L. Kaae, J. Smith, H.F. Robey, B. Blue, J.F. Hansen, A.R. Miles, J.P. Knauer, D. Arnett We show the design and build of experiments created to model aspects of the Rayleigh-Taylor instability during supernova explosions. The experiments utilize an advanced micro-machined polyimide/CHBr surface. The surface has been machined in an egg crate sinusoidal pattern and is flush against a low-density, carbonized resorcinol formaldehyde foam. The complexity of these targets is increased with the use of a dual axis backlit pinhole radiographic diagnostic. We will discuss the difficulties associated with the design, fabrication and assembly of these targets. This research was sponsored by the National Nuclear Security Administration under the Stewardship Science Academic Alliances program through DOE Research Grant DE-FG52-03NA00064, and through DE FG53 2005 NA26014 and other grants and contracts. [Preview Abstract] |
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