Bulletin of the American Physical Society
56th Annual Meeting of the APS Division of Plasma Physics
Volume 59, Number 15
Monday–Friday, October 27–31, 2014; New Orleans, Louisiana
Session NP8: Poster Session V: DIII-D/Diagnostics; Measurement and Diagnostic Techniques; Low Temperature Plasmas; Production, Ionization Kinetics and Sheaths; Van Allen 100; LPI Short Pulse and Beams |
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Room: Preservation Hall |
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NP8.00001: DIII-D; DIAGNOSTICS |
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NP8.00002: DIII-D Upgrade to Prepare the Basis for Steady-State Burning Plasmas R.J. Buttery, H.Y. Guo, T.S. Taylor, M.R. Wade, D.N. Hill Future steady-state burning plasma facilities will access new physics regimes and modes of plasma behavior. It is vital to prepare for this both experimentally using existing facilities, and theoretically in order to develop the tools to project to and optimize these devices. An upgrade to DIII-D is proposed to address the three critical aspects where research must go beyond what we can do now: (i) torque free electron heating to address the energy, particle and momentum transport mechanisms of burning plasmas using electron cyclotron (EC) heating and full power balanced neutral beams; (ii) off-axis heating and current drive to develop the path to true fusion steady state by reorienting neutral beams and deploying EC and helicon current drive; (iii) a new divertor with hot walls and reactor relevant materials to develop the basis for benign detached divertor operation compatible with wall materials and a high performance fusion core. These elements with modest incremental cost and enacted as a user facility for the whole US program will enable the US to lead on ITER and take a decision to proceed with a Fusion Nuclear Science Facility [Preview Abstract] |
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NP8.00003: Achieving Steady-State Conditions in the High-Beta Hybrid Scenarios in DIII-D C.C. Petty, T.C. Luce, J.R. Ferron, A.M. Garofalo, A.W. Hyatt, G.L. Jackson, F. Turco, C.T. Holcomb, E.J. Doyle The natural attributes of the hybrid scenario, especially the anomalously broad current profile, with $q_{min} \agt 1$, allows steady-state conditions with zero surface loop voltage to be achieved at 1 MA plasma current in DIII-D. Using efficient central current drive, the surface loop voltage is driven down to zero for $>1\tau_R$, with $\sim$ 50\% bootstrap current fraction when $\beta_P$ is increased above 1.9. Interestingly, good alignment between the current drive and plasma current profiles is not necessary as the hybrid regime self-organizes the current density profile. Steady-state hybrid plasmas can achieve $\beta_N=3.6$ for the full duration of the NB pulse ($>1\tau_R$) without exciting the m/n=2/1 tearing mode, corresponding to $\beta_T$ up to 3.4\%. The thermal energy confinement time is excellent, with confinement factors up to $H_{98y2}=1.6$ even during strong EC heating. A 0-D physics model demonstrates that attractive scenarios with $Q_{fus}$=3.5-3.8 exist for steady-state operation in ITER and FNSF. [Preview Abstract] |
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NP8.00004: ITER Steady-State Demonstration on DIII-D J.M. Park, M. Murakami, A. Sontag, S.J. Diem, C.T. Holcomb, J.R. Ferron, T.C. Luce A systematic scan of $q_{95}$ (=4.5, 5.5, 6.5) at constant $\beta_N$ ($\sim$3) and high $q_{min}$ ($\sim$1.8-2.1) has been obtained in a lower single null ITER-like shape to study confinement, stability and edge pedestal characteristics using off-axis neutral beam current drive for the ITER steady-state mission ($f_{NI}=1$, $Q=5$). The edge pedestal height is found substantially lower than in similar 2008 experiments, resulting in lower $f_{NI}$ due to reduced edge pedestal bootstrap current. Toroidal Alfv\'en Eigenmode power fluctuation is well correlated with the estimated beam ion diffusion (D$_b$). Strong dependency of D$_b$ on $q_{95}$, $q_{min}$ and neutral beam power (PNB) has been found indicating that lower $q_{95}$ ($\leq$4.5) would have reasonably good beam ion confinement (D$_b \geq 0.3\,$m$^2$/s) even at $q_{min}>2$ and high PNB=12 MW. The calculated ideal $\beta_N$ stability limit increases with lower $q_{95}$ allowing access to high $\beta_N$ ($>$3.5) needed for $f_{NI}=1$ and $Q=5$. This study shows that optimum choice of $q_{95}$ ($\sim$5.5) and $q_{min}$ ($>$2) is crucial to achieving $Q=5$ steady-state mission for ITER. [Preview Abstract] |
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NP8.00005: Analysis of Ideal Stability Limits in DIII-D Discharges with High $\beta_N$ and $l_i$ J.R. Ferron, T.C. Luce, C.T. Holcomb, J.M. Park, W.M. Solomon Broad pressure profiles in DIII-D discharges with high $l_i$ enable stable access to high plasma pressure. As $\beta_N$ increases, the pressure peaking factor $f_p=P(0)/\langle P\rangle$ decreases, from $f_p\approx 3.7$ at $\beta_N\approx 2.9$ to $f_p\approx 2.4$ at $\beta_N>4.5$. Simultaneously, the ideal low-n stability limits calculated with a conducting wall increase from $\beta_N\approx 3.6$ to nearly 6, so that $\beta_N$ remains below the limit. In addition, $f_p$ decreases as $l_i$ is increased. Thus, the high $\beta_N$ stability limits result from both increased $l_i$ and decreased $f_p$. In a steady-state discharge, though, increased $\beta_N$ will limit the practical value of $l_i$ because of the increase in the bootstrap current density, particularly in the H-mode pedestal. Reducing the pedestal pressure with an n=3 magnetic perturbation increases $l_i$ but also increases $f_p$ so there is no net increase in the $\beta_N$ limit. A change in the discharge shape to reduce the pedestal pressure, to the single-null divertor ITER shape from a double-null, results in an $\approx$15\% drop in the $\beta_N$ limit. [Preview Abstract] |
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NP8.00006: Edge Modeling of DIII-D Steady-State Discharges A.C. Sontag, J.M. Canik, L.W. Owen, M. Murakami, J.M. Park A 25\% drop in the electron pressure at normalized minor radius of 0.8 corresponding to a 20\% drop in the total pressure has been observed when comparing steady-state ITER demonstration discharges on DIII-D performed in 2013 to those performed in 2008. This drop significantly degrades fusion gain in integrated modeling simulations of ITER that scale the experimental DIII-D pedestal profiles to use as a boundary condition. Several differences in these discharges are being examined to determine the cause of this drop in pedestal pressure. Disparities in plasma shape could affect peeling/ballooning stability, and moving the outer strike point closer to the divertor cryopump duct for better particle control leads to a change in fueling. The toroidal field direction was also reversed between the two cases, in order to achieve better off-axis neutral beam current drive in the 2013 discharges; this changes the $\nabla B$ drift direction away from the X-point and reverses the $E\times B$ drift direction. EPED modeling will be used to examine differences in the MHD stability that could affect the pedestal pressure, and SOLPS is used in interpretive mode to look at the effects of changing drift directions and particle fueling. [Preview Abstract] |
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NP8.00007: Core Turbulence and Transport Response to Increasing Toroidal Rotation and Shear in Advanced-Inductive Plasmas G. McKee, Z. Yan, C. Holland, T. Luce, C. Petty, T. Rhodes, L. Schmitz, W. Solomon Multi-scale turbulence properties are altered as core toroidal rotation and ExB shearing rates are systematically varied in relatively high-beta, advanced-inductive H-mode plasmas on DIII-D. The energy confinement time increases by 50\% as the toroidal rotation is increased by a factor of 2.5 (to Mo=0.5), while core turbulence, measured with BES, DBS and PCI, decreases in dimensionlessly matched plasmas ($\beta\approx2.7, q_{95}=5.1$). Low-wavenumber ($k_\perp\rho_<1$) density fluctuations obtained with BES near mid-radius exhibit significant amplitude reduction along with a slight reduction in radial correlation length at higher rotation, while fluctuations in the outer region of the plasma, $\rho>0.6$, exhibit, but little change in amplitude. Fluctuation measurements and transport behavior will be quantitatively compared with nonlinear simulations. The resulting reduction in confinement will need to be ascertained for low-rotating plasmas such as ITER and FNSF. [Preview Abstract] |
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NP8.00008: Effect of ECH on Turbulent Fluctuations During ITER Baseline Scenario-like Discharges on DIII-D A. Marinoni, J.C. Rost, M. Porkolab, E.M. Davis, R.I. Pinsker, K.H. Burrell Recent experiments on the DIII-D tokamak simulating ITER Baseline Scenario discharges have shown a strong increase in the intensity of low frequency fluctuations during intense electron cyclotron heating (ECH) phases [1]. The torque-free and spatially localized pure electron heating, compared to beam heating, is believed to modify flow shear and fluctuations, resulting in a slightly weaker dependence of stored energy on input power compared to the nominal ITER IPB 98(y,2) scaling. Within 30 ms after turning off ECH power, the phase contrast imaging (PCI) diagnostic detects an increase of the intensity of fluctuations at frequencies higher than 200 kHz, likely due to the prompt response of the electron temperature profile; fluctuations at lower frequency decrease in intensity on a longer time scale, after other equilibrium quantities evolve. Nonlinear gyro-kinetic simulations are in progress and will be compared to PCI measurements via a synthetic diagnostic.\par \vskip6pt \noindent [1] R.I.\ Pinsker et al., to be published in Euro.\ Phys.\ J.\ Conf.\ (2014). [Preview Abstract] |
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NP8.00009: Comparison of linear gyrokinetic and two-fluid stability analyses of DIII-D L-mode plasmas C. Holland, E.M. Bass We present results from a linear stability study of the edge and near-edge regions of well-studied DIII-D tokamak L-mode discharges, using both the gyrokinetic-Maxwell equations (as implemented in the GYRO code) and a range of two-fluid models implemented in the BOUT++ code. The goal is to identify instabilities that may help explain the well-known systematic under-prediction of near-edge DIII-D transport and fluctuation levels by some gyrokinetic codes, in particular those driven by edge physics not included within the gyrokinetic models. We first compare local and global gyrokinetic stability results spanning the region of $0.7 < \psi_N < 0.95$ to corresponding predictions from Braginskii-like models implemented in BOUT++, focusing on the influence of magnetic shaping and collisionality scalings for a range of low- to moderate-n modes, consistent with the observed discrepancies in fluctuation spectra. The closed-field line results are then compared against equivalent results that extend across the separatrix to the open field line region $0.7 < \psi_N < 1.05$, in order to assess whether inclusion of this region leads to any significant changes in linear stability. Progress on extending the linear analysis to inclusion of rotational and gyrofluid effects will also be reported. [Preview Abstract] |
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NP8.00010: Study of the L-Mode tokamak plasma ``shortfall'' with local and global nonlinear gyrokinetic $\delta f$ simulation Jugal Chowdhury, Weigang Wan, Yang Chen, Scott E. Parker, Richard J. Groebner, Christopher Holland, N.T. Howard L-Mode plasmas in DIII-D and Alcator C-Mod tokamaks have been analyzed using the nonlinear gyrokinetic simulation GEM based on particle-in-cell method. It is observed that the simulation results for ion heat flux are close to the experimental values at the core, but substantially lower than the experimental results at the outer radial location in the DIII-D case. On the contrary, the simulations show good agreement with the experimental values of heat flux for ions in Alcator C-Mod. Global simulations are carried out for DIII-D L-Mode plasmas to study the effect of turbulence spreading from the edge into the outer core where the ion heat transport shortfall is observed. It is found that edge turbulence enhances the outer core ion heat transport significantly through turbulence spreading. However, ion heat flux in the shortfall region even in the presence of the edge drive is still much lower than the experimentally observed value. [Preview Abstract] |
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NP8.00011: Comparison of Fluctuation Characteristics in High qmin and Low qmin Steady-State Scenario Plasmas on \mbox{DIII-D} Yan Zhao, Z. Yan, G.R. McKee, C.T. Holcomb, J.R. Ferron, W.W. Heidbrink Experiments investigating the impact of the safety factor ($q$) profile on transport and confinement have been carried out in steady-state scenario plasmas on DIII-D. The minimum safety factor was varied between $q_{min}\sim 1.4$ and $q_{min}\sim 2.3$ ($q_{95}$=6.5) using off-axis neutral beam current drive and early beam injection during moderately high beta plasmas ($\beta_N\sim\,$2.3.) The steady-state scenario plasmas with high$q_{min}$ have significantly lower global energy confinement. Long wavelength density fluctuations are measured with a 2D BES array located at $\rho\sim\,$ 0.35-0.85 (scanned during a set of three repeat discharges). The normalized ($\tilde{n}/n$) density fluctuation amplitude integrated over \mbox{50-2500 kHz} is found to be nearly double at higher $q_{min}$ in the region of 0.5 $<\rho <\,$0.85, which is consistent with the lower confinement at high $q_{min}$. In addition, a set of discrete coherent modes associated with energetic particle driven instabilities is observed in this frequency range. [Preview Abstract] |
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NP8.00012: Measurements of Fast Ion Transport Due to n=3 Magnetic Perturbations on DIII-D M.A. Van Zeeland, T.E. Evans, N.M. Ferraro, M.J. Lanctot, D.C. Pace, C. Collins, W.W. Heidbrink, M. Garcia-Munoz, J.M. Hanson, B.A. Grierson, G.J. Kramer, R. Nazikian, S.L. Allen, C.J. Lasnier, W.H. Meyer Measurements of fast ion (FI) transport due to applied n=3 magnetic perturbations on DIII-D have been made in both ELM suppressed H-mode as well as L-mode discharges. FIDA measurements probe the confined FI profile in the edge and losses to the wall are obtained with scintillator detectors as well as an infrared periscope. In ELM suppressed plasmas FIDA data show a significant depletion of the edge FI profile during application of n=3 fields. IR imaging of the beam ion prompt loss footprint shows a difference in wall heating depending on phase of the n=3 perturbation. Measurements of both the impact on the confined FI profile and prompt losses will be compared to full-orbit modeling which predicts up to 10\%-15\% of the injected beam ions are lost before thermalization. Orbit following simulations also predict an increase in losses due to resonance between the FI drift orbits and the applied n=3 fields. Measurements during L-mode current ramp plasmas used to scan for signatures of these resonances will be discussed. [Preview Abstract] |
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NP8.00013: Inference of Fast-Ion Density Profile from Tomographic Reconstructions of Fast-Ion D$_\alpha$ Measurements L. Stagner, W.W. Heidbrink, C. Collins, B.A. Grierson The fast-ion D$_\alpha$ (FIDA) diagnostic measures light that energetic particles emit in fusion plasmas. The diagnostic is sensitive to different velocity space regions depending on the viewing angle relative to the magnetic field. Consequently, viewing chords that share a radial location give different, yet still valid, results. Velocity space tomography allows for these viewing chords to be combined to infer the complete fast-ion distribution function from the different partial views. If this is done at many radial locations the fast-ion density profile is measured. We demonstrate this method for the case of a classically described, low power, MHD-quiescent plasma from actual FIDA measurements. FIDA measurements were taken at four radial positions, each with four different viewing angles. Simulation results are also shown. [Preview Abstract] |
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NP8.00014: Transport of Fusion Alpha Particles in ITER Scenarios E.M. Bass, R.E. Waltz We predict the fusion-born alpha particle density in steady-state and hybrid (reverse shear) ITER scenarios with an integrated 1D transport model [1]. The model combines ``stiff'' critical gradient alpha-driven Alfv\'en eigenmode (AE) transport with a quasilinear approximation of microturbulent transport [2]. In an ITER baseline case [3], AE transport is found to redistribute alphas within the core but not propagate to the loss boundary. The remaining microturbulence at the edge causes negligible alpha-channel energy flux there (neglecting ripple loss). We set the AE stiff transport critical gradient threshold at $g_{AE}=g_{ITG}$, below which microturbulence can nonlinearly suppress AE transport [4], and the more stringent condition $g_{AE}=0$.\par \vskip6pt \noindent [1] R.E. Waltz and E.M. Bass, ``Prediction of the fusion alpha density profile in ITER from local marginal stability to Alfven eigenmodes,'' accepted for Nucl. Fusion\par \noindent [2] C. Angioni et al., Nucl. Fusion {\bf49}, 055013 (2009)\par \noindent [3] J.E. Kinsey et al., Nucl. Fusion {\bf51} 083001 (2011)\par \noindent [4] E.M. Bass and R.E. Waltz, Phys. Plasmas {\bf17}, 112319 (2010) [Preview Abstract] |
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NP8.00015: Does a Critical Gradient Exist for Alfv\'en Eigenmode Induced Fast-Ion Transport? C.S. Collins, W.W. Heidbrink, M.A. Van Zeeland, C.C. Petty, D.C. Pace, B.A. Grierson In the critical gradient model, if local energetic particle (EP) drive exceeds the Alfv\'en eigenmode (AE) stability limit, particles diffuse to flatten the pressure profile until marginal stability is maintained. A key signature is a sudden increase in transport above the critical gradient. In DIII-D, the onset of AE-induced EP transport is examined by modulating the EP pressure profile using an off-axis neutral beam while AE activity gradually diminishes during the current ramp. The time evolution of the EP density profile is measured with fast-ion D$_\alpha$ (FIDA) spectroscopy. During quiescent periods, the FIDA intensity rises and decays approximately linearly during and after the beam pulse, whereas during strong AE activity, the modulated FIDA intensity amplitude and decay rate decrease, suggesting additional AE-induced radial diffusion. Hardware upgrades are underway to increase spatial resolution and accommodate the full D$_\alpha$ spectrum, providing better constraints when comparing to predictive models. [Preview Abstract] |
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NP8.00016: Plasma response and error field compensation with n=2 in DIII-D H-mode plasmas E.J. Strait, C. Paz-Soldan, R.J. La Haye, M.J. Lanctot, J.M. Hanson, F. Turco, S.R. Haskey, J.D. King, N.C. Logan, R.M. Nazikian, M. Okabayashi, J.-K. Park, B.J. Tobias, R.A. Moyer, M. Shafer Compensation of intrinsic error fields with toroidal mode number n=2 in DIII-D is found to reduce rotation braking. Some features of the response of H-mode plasmas to n=2 magnetic perturbations are similar to previous observations with both higher and lower mode numbers. As with n=1, the amplitude of the n=2 response rises with beta, consistent with excitation of a stable n=2 kink mode. As with n=3, n=2 perturbations above a threshold amplitude produce density pumpout. An n=2 field designed to minimize coupling to the stable kink creates little or no density pumpout, indicating that the kink mode coupling is important to particle transport. On the other hand, the part of the field that does not couple to the stable kink mode seems to play an important role in rotation braking, likely through neoclassical toroidal viscosity (NTV) effects.\par [Preview Abstract] |
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NP8.00017: Free-Boundary 3D Equilibria and Resistive Wall Instabilities with M3D-C1 N.M. Ferraro, L.L. Lao, S.C. Jardin, J.D. King, M.W. Shafer, F. Zhang A resistive wall model has been implemented in the two-fluid, 3D MHD code M3D-C1. This capability allows M3D-C1 to be applied to several new applications that were not possible with conducting wall boundary conditions, including vertical displacement events, resistive wall modes, and free-boundary equilibria. Examples of each for both DIII-D and NSTX are presented. This also permits direct comparison of M3D-C1 results with magnetic probe data. Generally good agreement with magnetics data and qualitative agreement with soft x ray imaging is found in simulations of the non-axisymmetric response to applied 3D fields. It is found that the response can be sensitive to the treatment of the open field-line region. The boundary conditions and treatment of the open field-line region are especially important for the n=1 response. In M3D-C1, the resistive wall and the external vacuum region are included within the computational domain, which allows good implicit scaling, walls of arbitrary thickness, and the evolution of wall conditions. [Preview Abstract] |
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NP8.00018: Computation of Low-$n$ Plasma Response to Resonant Magnetic Perturbations in DIII-D Experiments Ping Zhu, Carl R. Sovinec To understand the effects of resonant magnetic perturbations (RMPs) on tokamak plasmas, we have computed the plasma response to RMPs in DIII-D experiments \#126006 and \#142603 using the extended MHD models implemented in the NIMROD code. The I-coil vacuum field is imposed as the initial perturbations and as the boundary conditions at the tokamak wall location. Whereas the edge pedestals in these discharges are found unstable to the high-$n$ perturbations, low-$n$ plasma responses to RMP are obtained by following the evolution of these low-$n$ components into quasi-steady state subject to the RMP initial and boundary conditions. Here $n$ is the toroidal mode number. Computation results indicate that the existence and properties of the steady states for the low-$n$ plasma response strongly depend on the plasma dissipation regime, equilibrium rotation, and two-fluid effects. Progress on the benchmarking with other 3D codes will be discussed. [Preview Abstract] |
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NP8.00019: Statistical Analysis of Locked Modes and their Disruptivity at DIII-D R. Sweeney, W. Choi, K.E.J. Olofsson, F.A. Volpe A database has been developed to study locking and disruptivity of neoclassical tearing modes with poloidal and toroidal mode numbers m=2 and n=1. Approximately 30,000 DIII-D discharges are studied providing statistics on the fraction of disruptions containing locked modes (LMs) and the ratio of disruptive LMs to all LMs. Other quantities analyzed include the time-scales between mode-formation and locking, and between locking and disruption, the amplitude of the mode upon locking and disruption, the existence or lack of a rotating precursor, and the toroidal phase of locking. Correlations are examined between locking and disruptivity and parameters such as plasma beta and neutral beam torque. Simple interpretations are provided in terms of island size and torques acting on the island, and implications for an automatic locked mode controller are discussed. [Preview Abstract] |
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NP8.00020: Measurements of the Structure of the Plasma Rotation in Slowly Rotating Tearing Modes in DIII-D N.Z. Taylor, N.M. Ferraro, R.J. La Haye, C.C. Petty, C. Bowman A helically modified ion flow by an island can lead to helical ion polarization currents which can affect tearing mode stability. This issue is of particular importance to ITER where large inertia and relatively low torque will likely result in low rotation. In DIII-D cases either (1) a m/n=2/1 mode is slowed down to $\sim$1 kHz (faster than the inverse wall time) by near balanced neutral beams or (2) an island is entrained by applied rotating n=1 magnetic field at 10 Hz (slower than the inverse wall time). The n=1 island structure is measured with electron cyclotron emission radiometry. The ion rotation and temperature are measured by fast resolution (274$\,\mu$s) charge exchange recombination (CER) spectroscopy in the 1 kHz freely rotating case and by standard CER (5 ms) in the 10 Hz entrainment. Tangential and vertical CER arrays allow for the radial profile of the helically perturbed rotation to be determined. A comparison of the measured nonlinear island structures with that from the linear resistive stability code M3D-C1 will be presented. [Preview Abstract] |
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NP8.00021: External Kink Mode in Diverted Tokamaks A.D. Turnbull, N.M. Ferraro, L.L. Lao, J.M. Hanson, F. Turco, P. Piovesan In a straight tokamak model, the external kink mode with toroidal mode number n and poloidal mode number m is predicted to be unstable when the edge safety factor, $q_{edge}$, lies just below a rational value. In a torus, the picture is essentially unchanged and the 2/1 instability in particular is always encountered when $q_{edge}=2$. For a diverted plasma, the edge $q$ is infinite, but, the experimental limit is then $q_{95}= 2$, where $q_{95}$ is at the 95\% flux surface. However, no theoretical basis has been established for the importance of $q_{95}$ and ideal predictions indicate stability with $q_{edge}>2$ and $q_{95}<2$; instability is found only when the actual $q$ at the edge is below 2. Two possible solutions present themselves. The observed mode may be destabilized as a result of small 3D error fields. Alternatively, the observed mode may be destabilized by the rapidly increased resistivity at the plasma edge. Both possibilities are examined using ideal and resistive MHD tools in two and three dimensions. [Preview Abstract] |
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NP8.00022: Magnetic Control of Locked Modes in Present Devices and ITER F.A. Volpe, S. Sabbagh, R. Sweeney, T. Hender, A. Kirk, R.J. La Haye, E.J. Strait, Y.H. Ding, B. Rao, S. Fietz, M. Maraschek, L. Frassinetti, Y. In, Y. Jeon, S. Sakakihara The toroidal phase of non-rotating (``locked") neoclassical tearing modes was controlled in several devices by means of applied magnetic perturbations. Evidence is presented from various tokamaks (ASDEX Upgrade, DIII-D, JET, J-TEXT, KSTAR), spherical tori (MAST, NSTX) and a reversed field pinch (EXTRAP-T2R). Furthermore, the phase of interchange modes was controlled in the LHD helical device. These results share a common interpretation in terms of torques acting on the mode. Based on this interpretation, it is predicted that control-coil currents will be sufficient to control the phase of locking in ITER. This will be possible both with the internal coils and with the external error-field-correction coils, and might have promising consequences for disruption avoidance (by aiding the electron cyclotron current drive stabilization of locked modes), as well as for spatially distributing heat loads during disruptions. [Preview Abstract] |
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NP8.00023: Observing the Coupling of the Toroidal Plasma Rotation Due to m/n=2/1 and m/n=3/2 Neoclassical Tearing Modes by Uncorrected n=2 Error Field in DIII-D M. Okabayashi, B.J. Tobias, E.J. Strait, R.J. La Haye, C. Paz-Soldan, D. Shiraki, J.M. Hanson Injection of electromagnetic torque by tearing mode rotation control feedback can sustain rotation of the 2/1 NTM, avoiding mode locking for several seconds after the mode appearance. This feedback process optimizes the phasing of rotating applied n=1 field relative to the mode, hence preventing the locking and simultaneously compensating the n=1 error field (EF). In high beta discharges, the large amplitude sustained 2/1 NTM reduces the local toroidal rotation to near zero at the q=3/2 surface and =1, implying the angular momentum is coupled between the two rational surfaces. The mode at the q=3/2 surface is identified as a m/n=3/2. The mode is presumably affected by n=2 EF as well as remaining uncorrected n=1 EF. A possible process of sustained NTM with velocity shear due to the $E_r$ buildup by large size magnetic islands will also be discussed [Preview Abstract] |
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NP8.00024: Physics-model-based Modeling and Control of the Toroidal Rotation Profile for DIII-D W. Wehner, E. Schuster, M.L. Walker, D.A. Humphreys A model suitable for control purposes, a so-called ``control-oriented" model, requires only capturing the dominant underlying physics that is relevant for control design. A control-oriented model of the toroidal rotation profile evolution for DIII-D has been derived from a simplified version of the first-principles-based momentum diffusion equation combined with scenario-specific models of the momentum sources. For DIII-D, four momentum sources are available for consideration: the non-axisymmetric field coils; which provide rotation damping; the co-current on-axis neutral beam injectors (NBI); the co-current off-axis NBI; and the counter-current on-axis NBI. These four sources allow not only control of the bulk plasma rotation, but also control of the profile shape. Optimal state feedback with integral action has been designed from the model and demonstrated in simulation to regulate the rotation profile around a desired target shape. [Preview Abstract] |
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NP8.00025: TGLF Analysis of ITG-TEM Transitions in DIII-D Plasmas X. Wang, S. Mordijck, G.M. Staebler, O. Meneghini TGLF [1] calculations show that, when adding electron cyclotron heating, DIII-D H-mode plasmas can transit from the ion temperature gradient (ITG) to trapped electron mode (TEM) domain around mid-radius. With different turbulence scale-lengths as well as opposite drift direction, ITG/TEM plasmas have different transport features, and thus lead to different density and rotation profiles. Detailed TGLF sensitivity analysis shows that the trapped electron mode's growth-rate is more sensitive to electron temperature gradient than the density gradient. Furthermore, by varying the input torque from co-IP to counter-IP through neutral beams, the ITG/TEM reversing radial location changes. We compare three parameters: density fluctuations, linear instability growth-rates, and ExB shearing rates in these discharges. We show that, for co- and counter-beam discharges, the ExB shearing rate becomes comparable or even larger than the growth-rate in the plasma edge. However, under balanced beam injection, the growth-rate is always larger than the shearing rate.\par \vskip6pt \noindent [1] G.M.\ Staebler, et al.\ Phys.\ Plasmas {\bf 12}, 102508 (2005). [Preview Abstract] |
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NP8.00026: Gyrokinetics with Advanced Collision Operators E.A. Belli, J. Candy For gyrokinetic studies in the pedestal region, collisions are expected to play a more critical role than in the core and there is concern that more advanced collision operators, as well as numerical methods optimized for the strong collisionality regime, are needed. For this purpose, a new gyrokinetic solver CGYRO has been developed for precise studies of high collisionality regimes. Building on GYRO and NEO, CGYRO uses the NEO pitch angle and energy velocity-space coordinate system to optimize the accuracy of the collision dynamics, particularly for multi-species collisions and including energy diffusion. With implementation of the reduced Hirshman-Sigmar collision operator with full cross-species coupling, CGYRO recovers linear ITG growth rates and the collisional GAM test at moderate collision frequency. Methods to improve the behavior in the collisionless regime, particularly for the trapped/passing particle boundary physics for kinetic electrons, are studied. Extensions to advanced model operators with finite-$k_\perp$ corrections, e.g., the Sugama operator [1], and the impact of high collisionality on linear gyrokinetic stability in the edge are explored.\par \vskip6pt \noindent [1] H.~Sugama, et al., Phys.\ Plasmas {\bf 16}, 112503 (2009). [Preview Abstract] |
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NP8.00027: A Combined Phase Contrast Imaging-Interferometer System for the Detection of Multiscale Electron Density Fluctuations E.M. Davis, J.C. Rost, M. Porkolab, A. Marinoni ITER and next-step devices will have harsh neutron environment and limited port space, severely restricting many crucial plasma diagnostics. As such, it is essential that we develop robust diagnostics with minimal access restrictions, small port requirements, and high spatiotemporal bandwidths. \mbox{DIII-D's} Phase Contrast Imaging (PCI) system is a model of such a burning plasma diagnostic, using a 10.6$\,\mu$m laser to measure $\int\,\tilde{n}_e dl$ at 10~kHz $< f <\,$ 10 MHz and 1.5 cm$^{-1} < k <$~30 cm$^{-1}$. To eliminate PCI's low-$k$ cutoff, we have designed and are constructing a traditional interferometer along the existing PCI beam path, extending the minimum detectable $k$ to 0 cm$^{-1}$. The combined PCI-interferometer uses a single 10.6$\,\mu$m beam, two interference schemes, and two detectors to make the relevant measurements. In addition to diagnostic proof-of-principle, the combined PCI-interferometer's improved bandwidth will aid model validation and allow measurement of low and high $n$ MHD modes. Initial results will be discussed. [Preview Abstract] |
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NP8.00028: Modeling Tokamak Transport with Neural-Network Based Models O. Meneghini, C. Luna, J. Penna, S.P. Smith, L.L. Lao This work uses neural networks (NNs) as a means to extract information from the massive volume of aggregated data that are available either from experiments or from simulation databases, and distill an accurate transport model for the heat, particle, and momentum transport fluxes as a function of local dimensionless plasma parameters [1]. The resulting model has been benchmarked with over 4000 DIII-D plasmas in different regimes, and it is able to capture the experimental behavior inside of $\rho < 0.95$ with average error $<$20\% for all transport channels. The NN model was embedded into the ONETWO transport code and is now being used to develop time-dependent scenarios in support of \mbox{DIII-D} operations. The simulated temperature, density and rotation profiles closely match the experimental measurements, and a stiff response of the heat fluxes has been observed in the model for increasing source power. The numerical efficiency of the NN approach makes it ideal for real time plasma control and scenario preparation for current experiments and for ITER.\par \vskip6pt \noindent [1] O.~Meneghini, et al., Phys.\ Plasmas {\bf 21}, 060702 (2014). [Preview Abstract] |
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NP8.00029: Inclusion of Relativistic Broadening for ECE-$T_e$ Mapping in DIII-D M.E. Austin, M.W. Brookman Recent increased demands on electron cyclotron emission (ECE)-derived electron temperature ($T_e$) measurement accuracy in DIII-D have lead to a re-examination of calibration and analysis methods. In particular, a new technique using sawtooth oscillations to assess ECE-$T_e$ overlap in the core shows that the inclusion of relativistic broadening for the ECE channel positions is crucial to obtain accurate gradients in the central region of the plasma, even for $T_e$(0) values less than 4~keV. A calculation of ECE-$T_e$ position shifts $\delta R$ for the range of DIII-D operating parameters find that the $\delta R$ corrections range from 0.5 cm for $T_e\sim\,$2 keV to more than 2 cm for $T_e\sim\,$8 keV. Two methods for inclusion of the relativistic shifts in DIII-D $T_e$ profiles are compared, a slower full numerical calculation of absorption and emission profiles versus a faster analytical formula. [Preview Abstract] |
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NP8.00030: Modeling and Analysis for Tearing Mode Stability in DIII-D Hybrid Discharges Kyungjin Kim, J.M. Park, M. Murakami, R.J. La Haye, Yong-Su Na Plasma rotation in DIII-D hybrid scenario plasmas is found to change the stability of tearing modes (TMs) in a profound manner. It is important to understand the onset threshold and the evolution of TMs for developing a high-performance steady-state fusion reactor. The modified Rutherford equation (MRE) estimates the growth rate of an island and is used to analyze the TM stability. The change in TM stability was investigated in hybrid plasmas with various conditions including rotation, normalized beta, $q$ profile, and so on. The measured island width is larger in low $q_{95}$ cases and increased as plasma rotation was reduced. The island width calculated by MRE with TM stability index $\Delta^\prime$ assumed from its poloidal mode number, -m/r, showed a good agreement during high rotation, but could not be matched to the experimental island width at lower rotation. Simulations of TMs using resistive MHD codes such as NIMROD and PEST3 will also be presented and compared with experiments to determine the possibility for predicting TM onset by $\Delta^\prime$ calculation. [Preview Abstract] |
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NP8.00031: Classical m/n=2/1 Tearing Mode Stability Based on Initial Island Growth Rate of Neoclassical Tearing Modes in DIII-D ITER Baseline Scenario Discharges R.J. La Haye, G.L. Jackson, T.C. Luce, O. Meneghini, K.E.J. Olofsson, F. Turco, W.M. Solomon Deleterious m/n=2/1 tearing modes appear in some slowly evolving ITER baseline scenario DIII-D discharges. The destabilization is here interpreted as due to an initially positive (destabilizing) classical tearing index. Examples of 2/1 tearing occurring after at least 3 seconds into discharges are analyzed. Island width evolution is evaluated by the Mirnov magnetic probe arrays using the motional Stark effect EFIT equilibrium reconstructions and is calibrated by the electron cyclotron emission (ECE) diagnostic. The magnetics analysis code EIGSPEC is used to sort out multiple modes and determine the precise point at which the m/n=2/1 mode begins to grow. The classical stability index is determined from the modified Rutherford equation (MRE) by taking the helically perturbed bootstrap components (including both curvature and small island effects) and subtracting from the initial normalized island growth rate. The data is well described by the imbalance of the sum of the destabilizing classical tearing and the helically perturbed bootstrap current terms with the sum of the stabilizing curvature and ``ion polarization'' effects. Comparison of the empirically determined classical index will be made with that from the linear stability code PEST3 using kinetic EFITs from experiment. [Preview Abstract] |
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NP8.00032: Development and Testing of EFIT 3D Equilibrium Reconstruction Capability L.L. Lao, N.M. Ferraro, E.J. Strait, A.D. Turnbull, J.D. King Recent development and testing of EFIT capability to reconstruct tokamak 3D perturbed equilibrium are described. The 3D extension is based on an expansion of the MHD equations to account for the 3D effects. EFIT uses the cylindrical coordinate system and can include magnetic island and stochastic effects. Several linearization schemes are being explored to improve the EFIT 3D perturbed solutions. Algorithms are also being developed to allow EFIT to reconstruct 3D perturbed equilibria directly making use of plasma response to 3D perturbations from the MARS or M3D-C1 MHD codes. Other efforts include testing of the new EFIT 3D capability using simulated magnetic data based on response calculations from MARS and M3D-C1, and performing detailed benchmarking calculations against other 3D codes such as VMEC/V3FIT. Reconstruction examples using EFIT and the new DIII-D 3D magnetic measurements to reconstruct 3D perturbed experimental equilibria using well-diagnosed discharges from DIII-D error field, RWM, and RMP experiments will be presented. [Preview Abstract] |
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NP8.00033: Status and System Performance for the DIII-D ECH System M. Cengher, J. Lohr, Y.A. Gorelov, D. Ponce, R. Prater, C.P. Moeller The electron cyclotron heating (ECH) capabilities on DIII-D are being steadily updated, leading to increased experimental flexibility and high reliability of the system. A 110 GHz depressed collector gyrotron in the 1.0 MW class was installed and is being tested and conditioned to longer pulse length. A second depressed collector gyrotron is operational in addition to the four 110~GHz, 1 MW gyrotrons. A new design depressed collector gyrotron in the 1.5 MW class, operating at 117.5 GHz, is expected to be installed during 2015 following rework to address a high voltage standoff problem. This tube will operate in the TE20,9 mode and has achieved 1.8 MW for short pulses during factory testing. The individual power generated at the gyrotrons and the power injected into the tokamak are measured on a shot-to-shot basis for the present year, with calibration based on the measured linearity between the injected power and the gyrotron cavity loading. The individual average injected powers into the plasma are between 520 and 760 kW. The line transmission coefficient including the waveguide line and the MOU is between -1.04 dB and -1.43 dB. [Preview Abstract] |
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NP8.00034: High Speed Scanning of the ECH RF beams on \mbox{DIII-D} J. Lohr, M. Cengher, Y.A. Gorelov, S. Noraky, D. Ponce, A.S. Welander, R.A. Ellis, E. Kolemen The ECH launchers on the DIII-D tokamak have been modified for rapid poloidal sweeping of the rf beams using high speed dc motors and magnetic position encoders. This enables the system to perform such tasks as responding to the need to suppress growing tearing modes on different flux surfaces, modifing sawtooth oscillations and altering the current density profile. The mechanical capability is backed by real time motional Stark effect EFIT equilibrium calculations, magnetic spectra analysis by NEWSPEC, fast ray tracing using TORBEAM (checked by TORAY) and real time $n_e$ and $T_e$ profiles from Thomson scattering. The poloidal scan can cover the full 40$^\circ$ range across the plasma upper half plane in about 100 ms and provide position accuracy for the rf deposition of approximately 2~mm at the plasma center for a beam with 10 cm diameter at the -10 dB contour. The entire capability is orchestrated from the Plasma Control System, which can also modulate the gyrotron outputs as required. [Preview Abstract] |
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NP8.00035: Analysis and Alignment of a Gyrotron RF Beam on the DIII-D ECH System Y.A. Gorelov, J. Lohr, A. Torrezan, J.P. Anderson, D. Ponce, M. Cengher The DIII-D ECH transmission line installation comprises seven runs of up to 80 meters of 31.75 mm diameter waveguide, with transmission efficiencies from 69\%-79\%. The efficiency depends on the purity of the HE$_{1,1}$ mode in the waveguide. The Gaussian rf beam from a gyrotron is converted to the HE$_{1,1}$ waveguide mode with a single focusing mirror in the matching optics unit (MOU) that places a waist, $w_0=\,$19 mm, at the input of the waveguide. At tilt angles $\Theta$ of less than two degrees and small offsets $\Delta$ of less than 3 mm, the mode conversion from a pure HE$_{1,1}$ mode increases as $\Theta_2$ and $\Delta_2$ respectively. The rf beam from the newest gyrotron was recorded in free space propagation at 10 cm intervals from the gyrotron window using an infrared camera. These data were used for phase retrieval calculations to optimize the design of the focusing mirror in the MOU. The mirror position was then set to align the rf beam with the waveguide axis. The measurement was repeated for the rf beam radiated from a short length of waveguide and the HE$_{1,1}$ content was 87\%. This is slightly low compared to the measurements of the other waveguides [Preview Abstract] |
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NP8.00036: Finding Evidence for Scattering of ECRH Power by Density Blobs M.W. Brookman, M.E. Austin, K.W. Gentle, W.L. Rowan Thermal transport studies using electron cyclotron heating (ECH) have measured anomalous effects inconsistent with narrow deposition profiles expected for the DIII-D gyrotrons. Recent theoretical work suggests that density blobs in the edge are capable of scattering and frequency shifting ECH power and broadening the deposition [1]. Knowledge of the heating profile is necessary for understanding thermal transport and mode suppression, as well as searching for density blob. Deposition profiles of modulated ECH (MECH) pulses from DIII-D gyrotrons are derived from 2nd harmonic X-mode electron cyclotron emission (ECE) measurements using the University of Texas 40 channel heterodyne radiometer. The uncertainties and resolution limits of this measurement are explored in both time and frequency domain analysis. The ability of the ECE system to resolve broadening effects from density bubbles is examined and measured deposition profiles are compared to previous derivations.\par \vskip6pt \noindent [1] A.K.\ Ram et al., Phys.\ Plasmas {\bf 20}, 056110 (2013). [Preview Abstract] |
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NP8.00037: Near Real-Time Gyrotron Data Streaming and Data Acquisition with ns Resolution on the DIII-D ECH System A.C. Torrezan, D. Ponce, Y.A. Gorelov, M. Cengher, J. Lohr As part of the expansion and upgrade of the electron cyclotron heating (ECH) systen on DIII-D, a new data acquisition setup has been implemented to acquire and display waveform data from all gyrotrons in near real time with high time resolution. The data acquisition for each gyrotron system is based on a fast digitizer with 8 channels running at 2 MS/s/channel and a resolution of 14 bits. This enables the operator to monitor all gyrotron-relevant variables as well as fast diagnostic signals such as window arcs. The data are transferred to a local computer through a 132 MB/s PCI bus, and then are streamed to the ECH operator and to a local network attached storage using 1 GB Ethernet links. The data are displayed to the ECH operator by means of a graphical user interface developed in LabVIEW, replacing physical scopes. Acquired gyrotron data are accessible at DIII-D through a local database (PTDATA) connected to the ECH data acquisition system by an Ethernet line, a configuration that eliminates the need for legacy CAMAC hardware in the data link. [Preview Abstract] |
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NP8.00038: The ITER Plasma Control System Simulation Platform D.A. Humphreys, M.L. Walker, A.S. Welander, G. Ambrosino, G. de Tommasi, M. Mattei, G. Neu, C. Rapson, G.R. Raupp, W.M. Treutterer, A. Winter Design of the ITER plasma control system (PCS) will require extensive simulation to evaluate both candidate architecture and algorithms. The ITER Plasma Control System Simulation Platform (PCSSP) is a specialized simulation environment currently under development to satisfy the demanding requirements of this design process. The PCSSP supports control-level models of systems relevant to ITER plasma control, including key plasma responses, and provides very high flexibility in selection of these models for a given simulation purpose. This flexibility enables higher fidelity models to be used to focus on particular control loops, for example. The PCSSP also supports modeling and simulation of exceptions (off-normal and fault events), as well as PCS algorithms and policies for handling these exceptions. We describe the present implementation and status of PCSSP, including modules simulating axisymmetric MHD, basic kinetics, and tearing mode physics, along with corresponding controller and exception handling modules. [Preview Abstract] |
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NP8.00039: Using the TokSys Modeling and Simulation Environment to Design, Test and Implement Plasma Control Algorithms on DIII-D A.W. Hyatt, A.S. Welander, N.W. Eidietis, M.J. Lanctot, D.A. Humphreys The DIII-D tokamak has 18 independent poloidal field (PF) shaping coils and an independent Ohmic transformer coil system. This gives great plasma shaping flexibility and freedom but requires a complex control capability that imposes some form of constraint so that a given plasma shape and specification leads to uniquely determined PF shaping currents. One such constraint used is to connect most PF coils in parallel to a common bus, forcing the sum of those PF current to be zero. This constraint has many benefits, but also leads to instability where adjacent PF coils of opposite current can mutually increase, leading to local shape distortion when using the standard shape control algorithms. We will give examples of improved control algorithms that were extensively tested using the TokSys simulation suite available at DIII-D and then successfully implemented in practice on DIII-D. In one case using TokSys simulations to develop a control solution for a long sought plasma equilibrium saved several days of expensive tokamak operation time. [Preview Abstract] |
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NP8.00040: GPU-Based Optimal Control Techniques for Resistive Wall Mode Control on DIII-D M. Clement, G.A. Navratil, J.M. Hanson, E.J. Strait The DIII-D tokamak can excite strong, locked or nearly locked kink modes whose rotation frequencies do not evolve quickly and are slow compared to their growth rates. To control such modes, DIII-D plans to implement a Graphical Processing Unit (GPU) based feedback control system in a low-latency architecture based on system developed on the HBT-EP tokamak [1]. Up to 128 local magnetic sensors will be used to extrapolate the state of the rotating kink mode, which will be used by the feedback algorithm to calculate the required currents for the internal and/or external control coils. Offline techniques for resolving the mode structure of the resistive wall mode (RWM) will be presented and compared along with the proposed GPU implementation scheme and potential real-time estimation algorithms for RWM feedback.\par \vskip6pt \noindent [1] N.~Rath, Plasma Phys.\ Control.\ Fusion {\bf 55}, 084003 (2013). [Preview Abstract] |
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NP8.00041: Optimization of the Current Ramp-up Phase in \mbox{DIII-D} via Physics-model-based Control of Plasma Safety Factor Profile Dynamics J.E. Barton, W.P. Wehner, E. Schuster, T.C. Luce, G.L. Jackson, J.R. Ferron, D.A. Humphreys, A.W. Hyatt Simulations and experimental results in DIII-D are presented to demonstrate the potential of physics-model-based control of the $q$ profile to improve the reproducibility of plasma startup conditions by achieving a specified target $q$ profile at the end of the current ramp-up. Three different $q$ profiles ($q_{min}$ of 1.3, 1.65, 2.1 and $q_{95}$ of 4.4, 5.0, 6.2, respectively) were specified as targets. A feedforward + feedback scheme is utilized to control the $q$ profile and is constructed by embedding a nonlinear, physics-based model of the $q$ profile dynamics into the control design process. A unique characteristic of the feedforward trajectories obtained by solving the optimization problem is the regulation of the plasma current ramp-up rate to achieve the target $q$ profiles. The feedback controller is employed to add robustness to the control scheme and account for drifts due to external plasma disturbances. [Preview Abstract] |
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NP8.00042: Observed linkage between ELM occurrence times and the phase of full flux loop signals in JET plasmas R.O. Dendy, S.C. Chapman, T.N. Todd, N.W. Watkins, A.J. Webster, F. Calderon, J. Morris We have identified (S Chapman, R Dendy et al, POP \textbf{21} 062302 (2014)) a phase relation between the occurrence times of edge localized modes (ELMs) in H-mode plasmas in the Joint European Torus (JET), and the voltage in full flux toroidal loops in the divertor region. The ELMs are observed in Be II emission at the divertor, and arise from intrinsic ELMing, with no external control applied. We relate ELM occurrence times from the Be II signal to the time-evolving phase of full flux loop signals, which provide global measurements proportional to the voltage induced by changes in poloidal magnetic flux. Each ELM produces a rapid initial pulse in the full flux loop signals. The arrival time of the next ELM, relative to this pulse, is in two categories: (a)prompt ELMs, directly paced by the initial response in the flux loop signals; and (b)all other ELMs, which occur after the decay of the initial response of the full flux loop signals. The times at which ELMs in category (b) occur, relative to the first ELM of the pair, cluster at times when the instantaneous phase of the full flux loop signal is close to its value at the time of the first ELM. This may contribute to the distribution of inter-ELM time intervals reported by A Webster, R Dendy et al, PPCF \textbf{56} 075017 (2014). [Preview Abstract] |
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NP8.00043: Gyrokinetic Analysis of ASDEX-Upgrade Inter-ELM Pedestal Profile Evolution David Hatch, M.G. Dunne, H. Doerk, F. Jenko, D. Told, E. Wolfrum, E. Viezzer The gyrokinetic GENE code is used to study Inter-ELM H-mode pedestal profile evolution on the ASDEX Upgrade Tokamak. Four main instabilities are observed during various inter-ELM phases---density gradient driven drift waves (DWs), microtearing modes (MTMs), kinetic ballooning modes (KBMs), and electron temperature gradient (ETG) modes. We focus in detail on three time points: 1) an early time point during which the radial electric field profile has not recovered and low ky DWs are the sole instability, 2) an intermediate phase during which the electron temperature gradient is fixed at a critical value, but the KBM limit has not been fully reached, and 3) the phase immediately preceding the ELM, during which the profiles are near or above the KBM limit. The properties of the dominant microinstabilities are generally consistent with the profile evolution. Complementary nonlinear simulations will also be presented. [Preview Abstract] |
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NP8.00044: Disruption avoidance through active magnetic feedback in tokamak plasmas Roberto Paccagnella, Paolo Zanca, Vadim Yanovskiy, Claudio Finotti, Gabriele Manduchi, Chiara Piron, Lorella Carraro, Paolo Franz Disruptions avoidance and mitigation is a fundamental need for a fusion relevant tokamak. In this paper a new experimental approach for disruption avoidance using active magnetic feedback is presented. This scheme has been implemented and tested on the RFX-mod device operating as a circular tokamak. RFX-mod has a very complete system designed for active mode control that has been proved successful for the stabilization of the Resistive Wall Modes (RWMs). In particular the current driven 2/1 mode, unstable when the edge safety factor, qa, is around (or even less than) 2, has been shown to be fully and robustly stabilized. However, at values of qa (qa \textgreater 3), the control of the tearing 2/1 mode has been proved difficult. These results suggested the idea to prevent disruptions by suddenly lowering qa to values around 2 where the tearing 2/1 is converted to a RWM. Contrary to the universally accepted idea that the tokamaks should disrupt at low qa, we demonstrate that in presence of a well designed active control system, tokamak plasmas can be driven to low qa actively stabilized states avoiding plasma disruption with practically no loss of the plasma internal energy. [Preview Abstract] |
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NP8.00045: HBT-EP Program: Active MHD Mode Dynamics \& Control G.A. Navratil, S. Angelini, J. Bialek, P.J. Byrne, A.J. Cole, B.A. DeBono, P.E. Hughes, J.P. Levesque, M.E. Mauel, Q. Peng, D.J. Rhodes, C.C. Stoafer, C.J. Hansen The HBT-EP active mode control research program aims to: (i) quantify external kink dynamics and multimode response to applied magnetic perturbations, (ii) understand the relationship between control coil configuration, conducting and ferritic wall effects, and active feedback control, and (iii) explore advanced feedback algorithms. Biorthogonal decomposition is used to observe multiple simultaneous resistive wall modes (RWM). Improved visualization of MHD kink mode structure is achieved using a tangential fast camera viewing visible light emission that augments magnetic probe data. A 512 core GPU-based low latency ($14 \mu s$) MIMO control system uses 96 inputs and 64 outputs for Adaptive Control of RWMs. An in-vessel adjustable ferritic wall was used to study ferritic RWMs with enhanced MHD response. A biased electrode in the plasma was used to control the rotation of external kinks. A Thomson scattering diagnostic measures $T_e$ and $n_e$ at 3 spatial points, soon to be extended to 10 points. A quasi-linear sharp-boundary model of the plasma's multimode response to error fields is developed to determine harmful error field structures and associated NTV and resonant torques. [Preview Abstract] |
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NP8.00046: Study of Kink Stability during Large Changes of Mode Rotation Induced by a Biased Probe Chris Stoafer, Q. Peng, J.P. Levesque, M.E. Mauel, G.A. Navratil A bias probe has been installed the High Beta Tokamak - Extended Pulse (HBT-EP) for studying MHD mode rotation and stability. By applying a voltage to the probe inserted into the edge of the plasma, the rotation of long-wavelength kink instabilities can be strongly modified. When the probe is biased to apply a torque in the direction of natural MHD mode rotation (7 - 10 kHz), the mode rotation can double. When the probe is biased in the opposite direction, wall-stabilized kinks can either stop rotating or be forced to counter-rotate in the ion drift direction. A time-varying bias can be applied to the probe with a 5 kW amplifier, which induces a time-varying mode rotation. An active controller can also be used to generate a bias voltage as a function of time. In this case, signals are generated through an active GPU-based digital feedback system, and this allows for MHD stability studies under the highly desirable condition of feedback controlled MHD mode rotation. Plasma rotation is measured with a Mach probe, and MHD mode rotation is measured by analyzing magnetic sensors on HBT-EP. Observations of plasma stability with HBT-EP's adjustable wall are reported for a wide range of mode rotation rates. [Preview Abstract] |
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NP8.00047: Initial Ferritic Wall Mode studies on HBT-EP Paul Hughes, J. Bialek, A. Boozer, M.E. Mauel, J.P. Levesque, G.A. Navratil Low-activation ferritic steels are leading material candidates for use in next-generation fusion development experiments such as a prospective US component test facility and DEMO [1]. Understanding the interaction of plasmas with a ferromagnetic wall will provide crucial physics for these experiments. Although the ferritic wall mode (FWM) was seen in a linear machine [2], the ferritic steel was observed to be compatible with high-performance operation in JFT-2M [3]. Using its high-resolution magnetic diagnostics and adjustable wall segments, HBT-EP now operates successfully with a high-permeability ($\mu \sim 8$) tiled ferritic first wall and is exploring the dynamics and stability of kink modes interacting with the ferritic tiles. In this poster, we report the first studies of the evolution of naturally rotating modes, increased plasma response to phase-flip resonant magnetic perturbations (RMPs) [4], and enhanced plasma disruptivity as wall configuration is adjusted from stainless wall to ferritic wall configuration.\\[4pt] [1] Kurtz, R.J., et. al. 2009 \textbf{J Nucl Mater} 386-388\\[0pt] [2] Bergerson, W., et. al. 2008 \textbf{Phys Rev Lett} 101\\[0pt] [3] Tsuzuki, K., et. al. 2006 \textbf{Nucl Fus} 46\\[0pt] [4] Shilov, M., et al. 2004 \textbf{Phys. Plasmas} 11, 2573 [Preview Abstract] |
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NP8.00048: Investigation of Multi-Mode MHD Behavior in Shaped HBT-EP Discharges Patrick Byrne, J.P. Levesque, M.E. Mauel, Q. Peng, D.J. Rhodes, P.E. Hughes, G.A. Navratil We report on investigations into the effect on multimode MHD of a newly installed poloidal field (PF) coil. The coil will allow the circular, limited HBT-EP to follow the main thrust of research towards a fusion reactor, which has been directed toward plasmas that are shaped and diverted. The coil shapes the high field side of the plasma up to and including imposing a PF null, while retaining compatibility with existing diagnostics and control systems. Multimode dynamics have been detected in naturally-rotating kink modes and during the response to 3D resonant magnetic perturbations.\footnote{Levesque, \textit{et al.}, \textit{Nucl Fusion} \textbf{53}, 073037 (2013).} Shaping changes both the resonant helical characteristics of MHD instabilities and the plasma response to external excitation and active control. Calculations using the TokaMac and DCON codes\footnote{Maurer, et al., \textit{Plasma Phys Contr F} \textbf{53}, 074016 (2011).} have predicted that the coupling between the two least stable MHD kink modes would be reduced when edge $q^*$ is near resonance. The RMP response, mode structure, and multi-mode content of diverted plasmas are compared to limited ones. Preliminary results show shaping changes the relative strengths of the $n=1$ and $n=2$ modes. [Preview Abstract] |
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NP8.00049: Advanced feedback control of wall modes using active rotation control on HBT-EP Qian Peng, Jeff Levesque, Chris Stoafer, Dov Rhodes, Paul Hudges, Patric Byrne, Micheal Mauel, Gerald Navratil The HBT-EP tokamak can excite strong, saturated kink modes whose growth rates and rotation frequencies evolve on a millisecond timescale. To control such modes, HBT-EP uses a GPU-based feedback system in a low latency architecture.\footnote{Rath, \textit{et al.}, \textit{Fusion Eng Des}, \textbf{87}, 1895 (2012).} When feedback is applied, the mode amplitude changes but the rotation frequency also changes quickly. The product of the latency, 20$\mu$s, and the mode frequency, around 8 kHz, is 0.16. This adds difficulty to robust feedback control even with a low latency controller.\footnote{Rath, \textit{et al.}, \textit{Nuc Fusion} \textbf{53}, 073052 (2013).} To overcome this challenge, we have included active control of the bias voltage applied on an edge probe into the feedback loop. The bias voltage applied on the edge probe enables us to influence the rotation of the modes in real-time. The variation in geometry of the system, where we observe that the detected mode amplitude has a phase dependency, is also taken into consideration in the algorithm design. In addition, HBT-EP can vary the wall configuration and includes ferritic wall effects. The feedback algorithm is tested on all cases and the performance will be reported. [Preview Abstract] |
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NP8.00050: Modeling Error-Field Response of the Resistive Wall Mode Dov Rhodes, A.J. Cole, G.A. Navratil, J. Bialek, A.H. Boozer, R. Fitzpatrick, J.P. Levesque, M.E. Mauel, Q. Peng Stability of the resistive wall mode (RWM) is sensitive to error-fields as small as 10$^{-4}$ [1]. Error-fields introduced by slight coil misalignments can induce localized resonant torques generated by shielding currents in a resistive plasma, as well as a global non-resonant torque created by symmetry-breaking deformations of the magnetic surfaces. The latter effect, termed neoclassical toroidal viscosity (NTV), is of particular interest as tokamak plasmas become hotter and less collisional. Both torques contribute to rotation damping which tends to non-linearly destabilize the RWM. Fitzpatrick's 2010 [1] quasi-linear sharp-boundary model of the multi-mode plasma response includes the non-ideal effect of resonant torques. We present an extended model including NTV to study the effect of error-field driven torques and plasma shaping on the evolution of the RWM. The model will be formulated to provide input for Bialek's VALEN code [2], which calculates RWM stability in the presence of a realistic conducting wall geometry. A better understanding of the plasma response to error-fields will facilitate more effective design of feedback control systems in a tokamak.\\[4pt] [1] R. Fitzpatrick, Phys. Plasmas 17, 112502 (2010).\\[0pt] [2] J. Bialek, Phys. Plasmas 8, 2170 (2001). [Preview Abstract] |
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NP8.00051: MEASUREMENT AND DIAGNOSTIC TECHNIQUES |
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NP8.00052: Fast, Deep-Record-Length, Fiber-Coupled Photodiode Imaging Array for Plasma Diagnostics Samuel Brockington, Andrew Case, F. Douglas Witherspoon HyperV Technologies has been developing an imaging diagnostic comprised of an array of fast, low-cost, long-record-length, fiber-optically-coupled photodiode channels to investigate plasma dynamics and other fast, bright events. By coupling an imaging fiber bundle to a bank of amplified photodiode channels, imagers and streak imagers of 100 to 1000 pixels can be constructed. By interfacing analog photodiode systems directly to commercial analog-to-digital converters and modern memory chips, a prototype 100 pixel array with an extremely deep record length (128k points at 20 Msamples/s) and 10 bit pixel resolution has already been achieved. HyperV now seeks to extend these techniques to construct a prototype 1000 Pixel framing camera with up to 100 Msamples/sec rate and 10 to 12 bit depth. Preliminary experimental results as well as Phase 2 plans will be discussed. Work supported by USDOE Phase 2 SBIR Grant DE-SC0009492 [Preview Abstract] |
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NP8.00053: In-situ Testing of the EHT High Gain and Frequency Ultra-Stable Integrators Kenneth Miller, Timothy Ziemba, James Prager, Ilia Slobodov, Dan Lotz Eagle Harbor Technologies (EHT) has developed a long-pulse integrator that exceeds the ITER specification for integration error and pulse duration. During the Phase I program, EHT improved the RPPL short-pulse integrators, added a fast digital reset, and demonstrated that the new integrators exceed the ITER integration error and pulse duration requirements. In Phase II, EHT developed Field Programmable Gate Array (FPGA) software that allows for integrator control and real-time signal digitization and processing. In the second year of Phase II, the EHT integrator will be tested at a validation platform experiment (HIT-SI) and tokamak (DIII-D). In the Phase IIB program, EHT will continue development of the EHT integrator to reduce overall cost per channel. EHT will test lower cost components, move to surface mount components, and add an onboard Field Programmable Gate Array and data acquisition to produce a stand-alone system with lower cost per channel and increased the channel density. EHT will test the Phase IIB integrator at a validation platform experiment (HIT-SI) and tokamak (DIII-D). [Preview Abstract] |
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NP8.00054: Real-time EFIT data reconstruction based on neural network in KSTAR Sehyun Kwak, YoungMu Jeon, Young-chul Ghim Real-time EFIT data can be obtained using a neural network method. A non-linear mapping between diagnostic signals and shaping parameters of plasma equilibrium can be established by the neural network, particularly with the multilayer perceptron. The neural network is utilized to attain real-time EFIT data for Korea Superconducting Tokamak for Advanced Research (KSTAR). We collect and process existing datasets of measured data and EFIT data to train and test the neural network. Parameter scans such as the numbers of hidden layers and hidden units were performed in order to find the optimal condition. EFIT data from the neural network was compared with both offline EFIT and real-time EFIT data. Finally, we discuss advantages of using neutral network reconstructed EFIT data for real time plasma control. [Preview Abstract] |
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NP8.00055: Generating synthetic 3D density fluctuation data to verify two-point measurement of parallel correlation length Jaewook Kim, Young-chul Ghim A BES (beam emission spectroscopy) system and an MIR (Microwave Imaging Reflectometer) system installed in KSTAR measure 2D (radial and poloidal) density fluctuations at two different toroidal locations. This gives a possibility of measuring the parallel correlation length of ion-scale turbulence in KSTAR. Due to lack of measurement points in toroidal direction and shorter separation distance between the diagnostics compared to an expected parallel correlation length, it is necessary to confirm whether a conventional statistical method, i.e., using a cross-correlation function, is valid for measuring the parallel correlation length. For this reason, we generated synthetic 3D density fluctuation data following Gaussian random field in a toroidal coordinate system that mimic real density fluctuation data. We measure the correlation length of the synthetic data by fitting a Gaussian function to the cross-correlation function. We observe that there is disagreement between the measured and actual correlation lengths, and the degree of disagreement is a function of at least, correlation length, correlation time and advection velocity of synthetic data. We identify the cause of disagreement and propose an appropriate method to measure correct correlation length. [Preview Abstract] |
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NP8.00056: A parallelized Python based Multi-Point Thomson Scattering analysis in NSTX-U Jared Miller, Ahmed Diallo, Benoit LeBlanc Multi-Point Thomson Scattering (MPTS) is a reliable and accurate method of finding the temperature, density, and pressure of a magnetically confined plasma. Nd:YAG (1064 nm) lasers are fired into the plasma with a frequency of 60 Hz, and the light is Doppler shifted by Thomson scattering. Polychromators on the midplane of the tokamak pick up the light at various radii/scattering angles, and the avalanche photodiode's voltages are added to an MDSplus tree for later analysis. This project ports and optimizes the prior serial IDL MPTS code into a well-documented Python package that runs in parallel. Since there are 30 polychromators in the current NSTX setup (12 more will be added when NSTX-U is completed), using parallelism offers vast savings in performance. NumPy and SciPy further accelerate numerical calculations and matrix operations, Matplotlib and PyQt make an intuitive GUI with plots of the output, and Multiprocessing parallelizes the computationally intensive calculations. The Python package was designed with portability and flexibility in mind so it can be adapted for use in any polychromator-based MPTS system. [Preview Abstract] |
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NP8.00057: An Alternative Optical Concept for Electron Cyclotron Emission Imaging (ECEI) Jason Liu, June-eok Leem, Manfred Bitter, Burkhard Plaum, Woochang Lee, Hyeon Park, Gunsu Yun, Walter Kasparek The implementation of advanced ECEI systems on tokamak experiments has revolutionized the diagnosis of MHD activities and improved our understanding of instabilities, which lead to disruptions. It is therefore desirable to have an ECEI system on ITER. However, present ECEI systems require large access ports and employ cumbersome optical components that have, up to now, precluded the implementation of such an ECEI system on ITER. This poster describes an alternative optical ECEI concept that utilizes a single spherical mirror as the primary optical component and exploits the astigmatism of such a mirror to produce an image of the radiating plasma with one-dimensional spatial resolution on the detector. Since this alternative approach would only require a thin slit as the viewing port to the plasma, it would make the implementation of an ECEI system on ITER feasible. Experimental results from laboratory characterization of this optical system are presented and compared to numerically simulated results. Possible approaches to implementing this ECEI system on ITER are also discussed. [Preview Abstract] |
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NP8.00058: Probing Runaway Electrons with Nanoparticle Plasma Jet I.N. Bogatu, J.R. Thompson, S.A. Galkin, J.S. Kim The injection of C$_{60}$/C nanoparticle plasma jet (NPPJ) into tokamak plasma during a major disruption has the potential to probe the runaway electrons (REs) during different phases of their dynamics and diagnose them through spectroscopy of C ions visible/UV lines. A C$_{60}$/C NPPJ of $\sim75$ mg, high-density ($>$10$^{23}$ m$^{-3}$), hyper-velocity ($>$4 km/s), and uniquely fast response-to-delivery time ($\sim1$ ms) has been demonstrated on a test bed. It can rapidly and deeply deliver enough mass to increase electron density to $\sim2.4\times10^{21}$ m$^{-3}$, $\sim60$ times larger than typical DIII-D pre-disruption value. We will present the results of our investigations on: 1) C$_{60}$ fragmentation and gradual release of C atoms along C$_{60}$ NPPJ penetration path through the RE carrying residual cold plasma, 2) estimation of photon emissivity coefficient for the lines of the C ions, and 3) simulation of C$_{60}$/C PJ penetration to the RE beam location in equivalent conditions to the characteristic $\sim1$ T B-field of DIII-D. The capabilities of this injection technique provide a unique possibility in understanding and controlling the RE beam, which is a critical problem for ITER. [Preview Abstract] |
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NP8.00059: A method of diagnosing magnetized plasmas using Raman scattering MinSup Hur, MyungHoon Cho, YoungKuk Kim We propose a method to diagnose a magnetized plasma using Raman scattering. It is found from the X-mode dispersion relation that the frequency of the backward scattered wave is downshifted by an amount of upper hybrid frequency, while that of the forward scattered wave merely depends on the magnetic field. Such a spectral difference is used to measure simultaneously the plasma density and magnetic field of magnetized plasmas. The idea was verified by a series of 1D PIC simulations, where we used the directional field splitting method to obtain accurate peak position of the scattered waves' frequencies. Theoretical expectation of the frequency shift and the growth rate gives a possibility to applying diagnostics of Tokamak. [Preview Abstract] |
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NP8.00060: Analysis of Tangential Camera Views of Tokamak Plasmas W.H. Meyer, M.E. Fenstermacher, M. Groth Commercial digital video cameras are increasingly being employed as sensor and digitizer for fusion plasma diagnostics. Here we describe some of our recent work to significantly improve tomographic analysis of tangential viewing camera data in toroidal geometry. Fiducial images obtained during vents are used to produce direct linear transformation matrices for each diagnostic camera and determine an accurate camera position and viewport. A tokamak solid model for rendering is used between vents to check camera alignment or when hardware fiducial images are unavailable. The rendering provides camera pixel distance of integration, vessel impact position, and angle of incidence. Reflection characterization is also performed using this solid model. After determining the camera geometry parameters, a response matrix is calculated for toroidally symmetric solution grids. We apply standard sparse linear solvers to the back projection problem for filtered visible light cameras. Forward projections are utilized for theory code validation and visualization. Poloidal IR profiles are extracted from tangential viewing IR camera data using surface parameters determined from solid model rendering. We present examples of each of these applications. [Preview Abstract] |
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NP8.00061: In-situ measurement of low Z coating thickness on metal substrates A.L. Roquemore, M. Jaworski, C.H. Skinner, J. Miller The surface composition of plasma facing components in magnetic fusion devices critically affects plasma performance but is difficult to measure in situ. We have used a compact assembly of an annular solid state detector and alpha particle source to employ Rutherford backscattering (RBS) to measure the thickness of a coating of low Z material on a heavier substrate in as little as 2 hours per location. RBS of energetic particles is typically used as a technique to measure near surface concentrations of high Z atoms in a substrate comprised mainly of lighter atoms. We have demonstrated its utility to determine the thickness of a coating of a low Z material on a heavier substrate in a short time. With a moveable probe, this technique could be used to provide an in situ thickness measurement of the Be, Li, B, and C coatings on metal tiles in 2 hours per location. A test stand was used to determine the range of low-Z thicknesses that can be measured on Mo tiles. The detector and electronics were calibrated by detecting the 5.423 MeV alpha particles emitted from a thin, 1 nCi, $^{\mathrm{241}}$Am source. This enabled the energy spectrum emitted from the thicker 0.1 mCi source to be measured (4.24 MeV centroid and 0.62 MeV FWHM) and the energy loss of the alpha particles in the low Z coating to be determined. A Monte Carlo program was used to investigate effects of the large energy spread of the thick 0.1 mCi source and of the detailed geometrical setup. [Preview Abstract] |
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NP8.00062: Determination of the Relative Two-photon Absorption Cross-section Between Xenon and Hydrogen Drew Elliott, Earl Scime, Dustin Mccarren, Robert Vandervort, Mark Soderholm Two-photon Absorption Laser Induced Fluorescence (TALIF) is a non-perturbative method for measuring the density and temperature of neutral hydrogen in a fusion plasma. Calibration of a TALIF system, for absolute density measurements, requires a measurement of a known density of particles under controlled conditions. Since hydrogen is diatomic, hydrogen TALIF system calibration requires measurements of target cold monatomic gas with a two-photon transition from the ground state and fluorescence decay at accessible energies. Here we present single-sided TALIF (angular momentum change of 2) measurements of a new transition in xenon with absorption and emission wavelengths nearly identical to those of the hydrogen TALIF sequence (the n $=$ 3 to n $=$ 2 emission in hydrogen is at 656.27 nm whereas it is at 655.99 nm in xenon). The xenon calibration approach provides the first opportunity for absolute calibration of Doppler-free (angular momentum change of 0) hydrogen TALIF. We first measure the relative TALIF absorption cross section between xenon and krypton and then use the known cross section ratio between the krypton and hydrogen transitions to calculate the relative xenon-hydrogen cross section. Single isotope xenon samples are used to remove the confounding factors of isotopic and hyperfine splitting. [Preview Abstract] |
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NP8.00063: Development of Low Pressure High Density Plasmas on the Helicon Plasma Experiment (HPX) Royce James, Phillip Azzari, Paul Crilly, Omar Duke-Tinson, Jackson Karama, Richard Paolino, Carter Schlank, Justin Sherman The small Helicon Plasma Experiment (HPX) at the Coast Guard Academy Plasma Lab (CGAPL), continues to progress toward utilizing the reputed high densities (10cm$^{-3}$ and higher) at low pressure (.01 T) [1] of helicons, for eventual high temperature and density diagnostic development in future laboratory investigations. HPX is designed to create repeatedly stable plasmas induced by an RF frequency in the 10 to 70 MHz range. We employ a 400 to 1000 Gauss electromagnet that promotes energy conservation in the plasma via external energy production in the magnetic field facilitated by decreased inertial effects, in order to reach the Helicon Mode. HPX is completing construction of triple and mach particle probes, magnetic probes, and is designing a single point 300 W Thompson Scattering system backed by a 32-channel Data Acquisition (DAQ) system capable 12 bits of sampling precision at 2 MS/s for HPX plasma property investigations. Progress on the development of the RF coupling system, Helicon Mode development, magnetic coils, and observations from the optical, particle, and electromagnetic scattering diagnostics will be reported. \\[4pt] [1] K. Toki, et al., Thin Solid Films 506-507 (2005) [Preview Abstract] |
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NP8.00064: Status of motional Stark effect and Zeeman effect diagnostics for KSTAR Jinseok Ko, Jinil Chung, Maarten de Bock The motional Stark effect (MSE) diagnostic system is under development aiming at commissioning in 2015. The design and fabrication of the polarization preserving front optics has been complete, including the multi-layer dielectric coated mirror and beam splitter, the latter being required to split the incident light into that above 600 nm for MSE and that below 600 nm for the Charge Exchange Spectroscopy (CES) that shares the front optics with MSE. The bandpass filters with a sharp transmission function and a minimum distortion against tilting have been procured. Both the analog lock-in and the post-processing numerical Fourier transform will be exploited. The Li-beam based Zeeman effect (ZE) diagnostic system is under conceptual design. Its details on the design are introduced in this work including the radial resolution and sensitivity to the change of the magnetic field pitch near the pedestal region. [Preview Abstract] |
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NP8.00065: LOW TEMPERATURE PLASMAS; PRODUCTION, IONIZATION KINETICS AND SHEATHS |
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NP8.00066: Temporally and Spatially Resolved Electron Density Measurements of an Air Breakdown Plasma Using a 1.4 MW, 110 GHz Gyrotron Samuel Schaub, Jason Hummelt, William Guss, Michael Shapiro, Richard Temkin A megawatt-class, 110 GHz gyrotron was used to produce a linearly polarized, quasioptical beam in 3 $\mu $s pulses. Using a lens, the beam was focused to a 3.2 mm spot size, producing a peak electric field of 57 kV/cm, after transmission losses. This electric field was great enough to produce a breakdown plasma in air at pressures ranging from a few Torr up to atmospheric pressure. The resulting breakdown plasma spontaneously forms a two-dimensional array of filaments, oriented parallel to the polarization of the beam, that propagate toward the microwave source. In our latest experiments, a needlepoint initiator has been introduced at the focal point of the beam, creating highly reproducible plasma arrays. Taking advantage of this reproducibility, the dynamics of the array formation and propagation were captured using a 2 ns fast gating intensified CCD camera (ICCD). The ICCD was combined with a two-wavelength laser interferometer, operating at 532 and 635 nm, to make spatially and temporally resolved electron density measurements of the plasma array. Abel inversion techniques were applied to the resulting line integrated data resulting in local measurements of electron density. [Preview Abstract] |
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NP8.00067: Time-Resolved High-Spatial-Resolution Measurements of Underwater Laser Ionization and Filamentation Ted Jones, Mike Helle, Dmitri Kaganovich, Antonio Ting, Joe Penano, Bahman Hafizi, Yu-Hsin Chen Intense underwater laser propagation, filamentation, and ionization are being investigated at NRL for applications including laser-guided discharges, advanced micromachining, and low-frequency laser acoustic generation. Time-resolved spectroscopy of intense underwater propagation and filamentation reveal strong Stimulated molecular Raman Scattering with ps temporal structure and frequency chirp. In addition, fs-time-resolution perpendicular shadowgraph images of ns underwater laser ionization reveal gas microbubble generation throughout the pump beam path. These microbubbles form in ps timescales with remarkably uniform initial diameters of a few-microns. Simulations using the HELCAP 4D nonlinear laser propagation code accurately predict measured filament fluence profiles and propagation, but also indicate complex, time-dependent and axially non-uniform plasma behavior. Results from recent experiments and simulations will be presented. [Preview Abstract] |
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NP8.00068: Amplification of 126 nm femtosecond seed pulses in optical-field-induced Ar plasma filamentation Shoichi Kubodera, Naoyuki Deshimaru, Masanori Kaku, Masahito Katto We have observed amplification of femtosecond (fs) VUV coherent seed beam at 126 nm by utilizing an optical-field-induced ionization (OFI) high-pressure Ar plasma filamentation. We have produced a low-temperature and high-density Ar plasma filamentation inside a high-pressure Ar cell by irradiating a high-intensity laser with an intensity of approximately 10$^{14}$ Wcm$^{-2}$. Argon excimer molecules (Ar$_{2}$*) as an amplifier medium were produced inside the high-pressure cell and were used to amplify a weak VUV ultrashort seed pulse at 126 nm, which was generated by harmonic generation of another short pulse infrared laser at 882 nm. We have measured the amplification characteristics and the OFI plasma diagnosis by utilizing the fs VUV pulses at 126 and 882 nm, respectively. The maximum optical gain value of 1.1 cm$^{-1}$ was observed. Temporal behaviors of the plasma temperature and density in the nano-second time scale indicated a high-density and low-temperature plasma produced by using the OFI. These plasma behaviors were utilized to reproduce the optical amplification characteristics with our OFI excimer simulation code. [Preview Abstract] |
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NP8.00069: RF Discharge Equilibrium, Transport, and Afterglow Radiation and Density Peaks in a Pre-Ionization Source for the Caltech MHD-Driven Jet Experiment Vernon Chaplin, Paul Bellan A novel pulsed battery-powered RF plasma source has been developed for pre-ionization in the Caltech MHD-driven jet experiment, enabling the formation of lower mass, faster jets than was possible with neutral gas breakdown alone. Results of jet experiments relevant to astrophysical and fusion plasmas will be presented, along with characterization of the custom 3 kW, 13.56 MHz RF amplifier and detailed studies of the argon RF plasma properties. The discharge conditions as a function of power input and axial magnetic field were monitored using Langmuir probes and optical spectroscopy; comparison of the data with a global discharge model indicated that the source was operating in a primarily inductively coupled mode with peak n$_{\mathrm{i}}$ \textgreater 3 x 10$^{\mathrm{19}}$ m$^{\mathrm{-3}}$. A 1D transport model has been developed to quantitatively explain the expansion of the RF plasma into the jet experiment chamber. The plasma transitioned from an ionizing to recombining phase during the course of the experiment, causing fast camera images to be a poor indicator of the density distribution. The optical brightness in the afterglow was proportional to the gas pressure and exceeded the main discharge brightness above 500 mTorr, and the downstream ion density also increased after power turn-off, likely due to metastable-metastable ionization. [Preview Abstract] |
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NP8.00070: Measurement of CO$_{2}$ laser absorption by tin plasma emanating extreme ultraviolet light for photo-lithography Hiraku Matsukuma, Kensuke Yoshida, Tatsuya Hosoda, Akifumi Yogo, Shinsuke Fujioka, Katsunobu Nishihara, Atsushi Sunahara, Toshihiro Somekawa, Hiroaki Nishimura Laser-driven tin plasma has been studied as a light source of extreme ultraviolet (EUV) at 13.5 nm ($+$/- 1{\%} in-band width) for the next-generation semiconductor manufacturing. By using CO$_{2}$ laser as a driver, high conversion efficiency (CE) has been attained in previous works by optimizing optical thickness for EUV radiation. Radiation hydorodynamic simulation predicts, however, that absorption coefficient for CO$_{2}$ laser is as high as 50{\%} for a tin plasma generated with a single laser pulse mainly due to short plasma scale. The relatively low absorption is a crucial problem for efficient generation of EUV light. In order to solve this problem and to increase the energy absorption, a double pulse method has been proposed where plasma scale length is extended by pre-pulse irradiation. Therefore, it is important to measure CO$_{2}$ laser absorption rate precisely in order to optimize plasma conditions. For this purpose we designed an integrating sphere for CO$_{2}$ laser. Laser absorption was measured for tin plasmas generated under various conditions including target geometries. We will show experimental results and discuss on guidelines for getting higher CE. [Preview Abstract] |
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NP8.00071: Development of a volume production type hydrogen negative ion source by using sheet plasma Satoki Matsumoto, Takaaki Iijima, Akira Tonegawa, Kohnosuke Sato, Kazutaka Kawamura Stationary production of negative ions are important to play an essential role in Neutral beam injection (NBI). Cesium seeded Surface-production of negative ion sources are used for NBI. However, Cesium seeded surface- production of negative ion sources are not desirable from the point of view of operating steady state ion sources. We carried out the development of negative ion sources by volume-production in hydrogen sheet plasma. Production of hydrogen negative ions through volume processes needs both high energy electron region and low energy electron region. The sheet plasma is suitable for the production of negative ions, because the electron temperature in the central region of the plasma as high as 10 -- 15eV, whereas in the periphery of the plasma, a low temperature of a few eV of obtained. The hydrogen negative ions density were detected using an omegatron mass analyzer, while the electron density and temperature were measured using a Langmuir probe. Negative ions current extracted from the grid are measured by Faraday-cup. [Preview Abstract] |
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NP8.00072: Microwave Assisted Helicon Source Plasma John McKee, Robert VanDervort, Mark Soderholm, Dustin McCarren, Earl Scime Helicon plasma sources are an efficient method by which plasmas of relatively high densities can be produced. However, the temperature of these plasmas is comparatively low. Electron temperatures are often only a few eV, with ion temperatures being a factor of ten below that. These temperatures are too low to study ion behavior in lighter noble gases, such as helium, using laser induced fluorescence (LIF) schemes as the energy difference between the ion ground state and excited levels is typically tens of eV. To bridge this energy divide, a 1.2 kW source of 2.45 GHz microwaves is used in addition to the normal rf helicon source. Through electron cyclotron resonance (ECR) heating, the electron temperature is raised and low lying ion energy states are populated. Here we present spectroscopic measurements of microwave assisted helium plasmas. [Preview Abstract] |
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NP8.00073: Modeling of helicon wave propagation and the physical process of helicon plasma production Shogo Isayama, Tohru Hada, Shunjiro Shinohara, Takao Tanikawa Helicon plasma is a high-density and low-temperature plasma generated by the helicon wave, and is expected to be useful for various applications. On the other hand, there still remain a number of unsolved physical issues regarding how the plasma is generated using the helicon wave. The generation involves such physical processes as wave propagation, mode conversion, and collisionless as well as collisional wave damping that leads to ionization/recombination of neutral particles. In this study, we attempt to construct a model for the helicon plasma production using numerical simulations. In particular, we will make a quantitative argument on the roles of the mode conversion from the helicon to the electrostatic Trivelpiece-Gould (TG) wave, as first proposed by Shamrai. According to his scenario, the long wavelength helicon wave linearly mode converts to the TG wave, which then dissipates rapidly due to its large wave number. On the other hand, the efficiency of the mode conversion depends strongly on the magnitudes of dissipation parameters. Particularly when the dissipation is dominant, the TG wave is no longer excited and the input helicon wave directly dissipates. In the presentation, we will discuss the mode conversion and the plasma heating using numerical simulations. [Preview Abstract] |
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NP8.00074: The MOCHI LabJet Experiment Setthivoine You, Jens von der Linden, Keon Vereen, Eric Sander Lavine, Evan Carroll, Alex Card, Manuel Azuara Rosales, Morgan Quinley The MOCHI LabJet experiment aims to simulate a magnetically driven jet launched by an accretion disk in the laboratory. The design uses three concentric planar electrodes linked by a vacuum magnetic field to drive azimuthal and axial shear flows in a jet configuration. Azimuthally symmetric gas sources reduce any anchoring effects on azimuthal rotation of the plasma. Two pulse-forming networks bias the electrodes to control the radial electric field profile and the azimuthal shear rotation profile. The dynamics of plasma jets are observed with 3D high-resolution magnetic probe arrays and computed vector tomography of ion Doppler spectroscopy. Vector tomography is capable of reconstructing 3D ion flow fields. Time-resolved measurements will determine if magnetic helicity is converted into ion kinetic helicity as predicted by the theory of canonical helicity transport. The theory suggests that fundamental tubes of magnetic flux with helical flows (canonical flux tubes) could be stabilized to large aspect-ratios by converting helical magnetic pitch into helical shear flows. [Preview Abstract] |
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NP8.00075: Optically Isolated Control of the MOCHI LabJet High Power Pulsed Plasma Experiment Evan Carroll, Morgan Quinley, Jens von der Linden, Setthivoine You The MOCHI LabJet experiment designed to investigate the dynamics of astrophysical jets at the University of Washington, requires high energy pulsed power supplies for plasma generation and sustainment. Two $600$ $\mu F$, $10$ $kV$ DC, pulse forming, power supplies have been specifically developed for this application. For safe and convenient user operation, the power supplies are controlled remotely with optical isolation. Three input voltage signals are required for relay actuation, adjusting bank charging voltage, and to fire the experiment: long duration DC signals, long duration user adjustable DC signals and fast trigger pulses with $<$ $\mu s$ rise times. These voltage signals are generated from National Instruments timing cards via LabVIEW and are converted to optical signals by coupling photodiodes with custom electronic circuits. At the experiment, the optical signals are converted back to usable voltage signals using custom circuits. These custom circuits and experimental set-up are presented. [Preview Abstract] |
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NP8.00076: Dynamics of Laboratory Astrophysical Jets with Magnetized Helical Flows Jens von der Linden, Setthivoine You A triple electrode planar plasma gun (MOCHI LabJet) designed to study the dynamics of magnetized helical flows in plasma jets provides boundary conditions and dimensionless numbers relevant to astrophysical jets. The goal is to observe the effect of current and flow profiles on the collimation and stability of jets to address the questions: why are jets collimated and long? How are jet irregularities related to plasma instabilities? The current and azimuthal flow profiles of the jets are tailored by biasing the electrodes at different potentials. High-speed camera images, high-resolution $\dot{B}$ probe measurements, and 3D vector tomography of plasma flows will map a stability space for varying current and flow profiles. An analytical stability space is derived with Newcomb's variational analysis applied to collimated magnetic flux tubes with skin and core currents. Two numerical stability spaces are also computed by integrating the Euler-Lagrange equation and applying a shooting method to the ideal MHD eigenvalue problem. The eigenvalue problem is generalized to include azimuthal flows and computed with a monotonicity condition [1] for minimizing the required scanning of the complex eigenvalue space. \\[4pt] [1] J.P. Goedbloed, Plasma Phys. Control. Fusion \textbf{53}, (2011). [Preview Abstract] |
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NP8.00077: High-Resolution B Dot Probe for Measuring 3D Magnetic Fields in the MOCHI Labjet Experiment Manuel Azuara Rosales, Jens von der Linden, Setthivoine You The MOCHI Labjet experiment will use a triple electrode planar plasma gun to explore canonical helicity transport in laboratory astrophysical jets. Canonical helicity transport suggests that destabilizing magnetic energy can be converted into stabilizing shear flows at two-fluid spatial scales $l_{i} \sim \frac{c}{w_{pi}}$. A high-resolution $\dot{B}$ probe array, capable of measuring magnetic field dynamics at length and time scales important to canonical helicity transport is being built. The probe array consists of three tridents, made of $5.13$ mm OD and $4.32$ mm ID stainless steel tubes of $102$ cm length, enclosing a total of $1215$ commercial inductor chips with a three axis spatial resolution of 11 mm. The average value for the effective NA of each inductor chip is $1.21 \cdot 10^{-4}$ m$^{2}$. The probe array lays in a plane perpendicular to the jet, and is axially translatable. [Preview Abstract] |
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NP8.00078: Optical diagnostics in the MOCHI LabJet experiment Alexander Card, Keon Vereen, Chris Cretel, Setthivoine You The MOCHI LabJet experiment is designed to observe the dynamics of canonical flux tubes and measure the conversion of magnetic helicity into ion flow helicity. In addition to magnetic probes capable of measuring 3D magnetic fields,~Ion Doppler spectroscopy will~reconstruct 3D flow fields from computed vector tomography of line-integrated ion Doppler measurements. About 500 collimated lines-of-sight are distributed into 64 viewports regularly arranged around the 1.4m diameter spherical vacuum chamber. The custom fiber-bundles are arranged into a 2D array and coupled to a 1m focal length Czerny-Turner monochromator with custom matching optics. The spectral light is recorded with a dual-frame 1024x1024 intensified CCD camera with a 2 $\mu$s phosphor decay time capable of taking two measurements in a single plasma shot. A Mach-Zehnder HeNe interferometerer with unequal path lengths is also under construction for line-integrated plasma density measurements. This work is supported by US DOE Grant DE-SC0010340. [Preview Abstract] |
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NP8.00079: Investigating the Dynamics of Canonical Flux Tubes in Jet Geometry Eric Lavine, Setthivoine You Highly collimated plasma jets are frequently observed at galactic, stellar, and laboratory scales. Some models suppose these jets are magnetohydrodynamically-driven magnetic flux tubes filled with flowing plasma, but they do not agree on a collimation process. Some evidence supporting a universal MHD pumping mechanism has been obtained from planar electrode experiments with aspect ratios of $\sim$10:1; however, these jets are subject to kink instabilities beyond a certain length and are unable to replicate the remarkable aspect ratios (10-1000:1) seen in astrophysical systems. Other models suppose these jets are flowing Z-pinch plasmas and experiments that use stabilizing shear flows have achieved aspect ratios of $\sim$30:1, but are line tied at both ends. Can both collimation and stabilization mechanisms work together to produce long jets without kink instabilities and only one end tied to the central object? This question is evaluated from the point of view of canonical flux tubes and canonical helicity transport, indicating that jets can become long and collimated due to a combination of strong helical shear flows and conversion of magnetic helicity into kinetic helicity. The MOCHI LabJet experiment is designed to study this in the laboratory. [Preview Abstract] |
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NP8.00080: A Plasma Tweezer Concept to De-spin an Asteroid Keon Vereen, Iman Datta, Setthivoine You The Plasma Tweezer is a new concept for controlled de-spinning and deflection of space bodies without mechanical contact. The method shoots plasma jets or beams at the target from a pair of plasma thrusters located at the end of each lever arm of a ``tweezer'' structure. The main spacecraft body is at the fulcrum point of the tweezer and the target is located between the thrusters. This arrangement cancels out the impulse of two plasma jets on the spacecraft and applies forces on opposite sides of the target. Careful timing and orientation of the jets can then provide the necessary forces to despin and redirect the target. This concept is more efficient than the Ion Beam Shepherd method [Bombardelli C, Pelaez J, ``Ion Beam Shepherd for Asteroid Deflection,'' J. Guid. Control Dyn. (2011)] because it does not require a secondary thruster to cancel momentum and can benefit from angular momentum stored in the spacecraft's initial spin stabilization. [Preview Abstract] |
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NP8.00081: Injection of a coaxial-gun-produced magnetized plasma into a background helicon plasma Yue Zhang, Alan Lynn, Mark Gilmore, Scott Hsu A compact coaxial plasma gun is employed for experimental investigation of plasma bubble relaxation into a lower density background plasma. Experiments are being conducted in the linear device HelCat at UNM. The gun is powered by a 120-uF ignitron-switched capacitor bank, which is operated in a range of 5 to 10kV and 100 kA. Multiple diagnostics are employed to investigate the plasma relaxation process. Magnetized argon plasma bubbles with velocities 1.2Cs, densities 10$^{20}$ m$^{-3}$ and electron temperature 13eV have been achieved. The background helicon plasma has density 10$^{13}$ m$^{-3}$, magnetic field from 200 to 500 Gauss and electron temperature 1eV. Several distinct operational regimes with qualitatively different dynamics are identified by fast CCD camera images. Additionally a B-dot probe array has been employed to measure the spatial toroidal and poloidal magnetic flux evolution to identify plasma bubble configurations. Experimental data and analysis will be presented. [Preview Abstract] |
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NP8.00082: Correlating Metastable-Atom Density, Reduced Electric Field, and Electron Energy Distribution in the Early Stages of a 1-Torr Ar Discharge S.H. Nogami, J.B. Franek, M.E. Koepke, V.I. Demidov, E.V. Barnat Measurements of electron density, metastable-atom density, and reduced electric field are used to approximate reaction rates [1] for electron-atom collision excitation in a 1-Torr positive column of Ar plasma. This allows us to relate the observed 420.1nm to 419.8nm line-intensity ratio to plasma parameters by invoking a plausible assumption regarding the shape of the electron energy distribution function (EEDF) throughout the discharge [1]. We show that these reaction rates agree well with experimental observations in the late stages of the pulse and we address discrepancies in the initial and transient phases of the discharge. We examine three assumptions made in the model to see if they are violated in any stage of the discharge: (1) The stepwise excitation from the 1s4 and 1s2 resonant states is negligible; (2) The numerical model designed for a 5-mTorr plasma is applicable to a 1000-mTorr plasma; and (3) The EEDF is bi-Maxwellian and is modified only slightly due to the presence of electrons or metastable-atoms. We conclude that diagnostic signatures for electron density, metastable-atom density, and reduced electric field can be quantitatively interpreted by this correlation at all stages of the discharge.\\[4pt] [1] Adams, et al. Phys. Plas. 19, 023510 (2012). [Preview Abstract] |
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NP8.00083: Atomic modeling and spectral analysis of the Auburn Linear Experiment for Instability Studies (ALEXIS) N. Ivan Arnold, Ed Thomas, Stuart Loch, Connor Ballance Performing spectroscopic measurements of emission lines in relatively cold laboratory plasmas is challenging because the plasma is often neutral-dominated and is not in thermal equilibrium. However, these types of plasma do offer a unique opportunity for benchmarking the finer details of atomic physics, helping researchers gain a better understanding of fundamental atomic processes in plasmas. In this presentation, we report on a new set of atomic data, from which rate coefficients for the electron-impact excitation of neutral argon, along with dielectronic recombination of Ar$+$-Ar5$+$ are determined. This data is used to calculate synthetic emission spectra, which are compared to experimental measurements in the ALEXIS device. The goal is to identify emission lines that are sensitive to variations in temperature and density and to use this data to develop a new optical density and temperature diagnostic for a low temperature plasma. This presentation will discuss preliminary density and temperature measurements made using the atomic data, with comparisons to standard probe based measurements and will report on recent modifications to ALEXIS intended to allow for a more complete test of the atomic model. [Preview Abstract] |
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NP8.00084: Systematic Effects in Laser-Induced Fluorescence Measurments of Ion Density and Temperature Caused by Optical Pumping Thomas Langin, Trevor Strickler, Patrick McQuillen, Thomas Killian Ultracold neutral plasmas of strontium are generated by photoionizing laser-cooled atoms. The plasma evolution is probed by laser induced fluorescence (LIF) via the 5s$^{2}$S$_{1/2}$-5p$^{2}$P$_{1/2}$ ion transition. Spectra are obtained by recording LIF intensity at varying laser detunings. The ion temperature, $T$, is then measured by fitting a Voigt profile to obtain the Doppler width. However, for linearly (circularly) polarized light, 5p$^{2}$P$_{1/2}$ ions have a 7\% (33\%) chance of decaying to the dark metastable 5d $^{2}$D$_{3/2}$ state (dark opposite spin state). Near resonance, where ions are more likely to scatter multiple photons during the LIF process, the observed signal will be depressed due to optical pumping. This causes an artificial broadening in the spectra and thus artificially high $T$ measurements. Moreover, the loss of ions throughout the excitation process, if not corrected for, will result in artificially low density measurements. We have developed, and experimentally verified, a method for simulating the LIF process in order to determine the LIF-probe durations and intensities for which these effects becomes significant. [Preview Abstract] |
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NP8.00085: Periodic Evolution of a Xe I Population in an Oscillatory Discharge: Comparison between Time-Synchronized Laser-Induced-Fluorescence Measurements and A Dynamic Collisional-Radiative Model Andrea Lucca Fabris, Chris V. Young, Mark A. Cappelli We study the evolution of the Xe I $6s'[1/2]_{1} - 6p'[3/2]_{2}$ (834.68 nm air) transition lineshape in a plasma discharge oscillating at 60 Hz using time-synchronized laser induced fluorescence (LIF) measurements and a collisional-radiative model. Two different time-synchronized LIF techniques based on phase sensitive detection of the fluorescence signal are applied, yielding consistent results. The maximum observed peak fluorescence intensity occurs at low values of the discharge current, although the peak intensity drops to zero at zero discharge current. The peak intensity also decreases at the discharge current maximum. A dynamic collisional-radiative model of the weakly ionized xenon discharge is also implemented, based on a set of rate equations. The proper electron impact cross-sections and radiative decay rates are identified from the literature and used to compute accurate rate coefficients with the Boltzmann solver Bolsig+, including the time-varying electric field. The time evolution of the probed excited state density predicted by the model shows good agreement with the experimental measurements. [Preview Abstract] |
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NP8.00086: Ion velocity distribution function measurements in a dual-frequency rf sheath Nathaniel Moore, Walter Gekelman, Patrick Pribyl, Yiting Zhang, Mark Kushner Ion dynamics are investigated in a dual-frequency rf sheath above a 300 mm diameter biased silicon wafer in an industrial inductively coupled (440 kHz, 500 W) plasma etch tool. Ion velocity distribution (IVD) function measurements in the argon plasma are taken using laser induced fluorescence (LIF). Planar sheets of laser light enter the chamber both parallel and perpendicular to the surface of the wafer in order to measure both parallel and perpendicular IVDs at thousands of spatial positions. A fast (30 ns exposure) CCD camera measures the resulting fluorescence with a spatial resolution of 0.4 mm. The dual-frequency bias on the wafer is comprised of a 2 MHz low frequency (LF) bias and an adjustable 10-20 MHz high frequency (HF) bias. The bias voltages may be switched on and off (f$_{rep}$ up to 1 kHz, duty cycle 10-90\%). Several different bias and timing combinations were tested. Ion energy distribution function and ion flux measurements for each case are compared. For the LF case (no HF), the IVD was found to be uniform to within 5\% across the wafer. IVDs as a function of phase of the LF bias were also measured. The LF experimental results are compared with simulations specifically designed for this particular plasma tool. [Preview Abstract] |
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NP8.00087: Particle-in-Cell simulation of secondary electron emission effects in a trench geometry Y. Nishimura, C.W. Huang, Y.C. Chen, T.L. Lin A Particle-in-Cell simulation code is developed to investigate interaction between plasma and material surfaces in a one dimensional geometry and a two dimensional trench geometry. Both the electron and ion dynamics are incorporated. In the presence of secondary electron emission, the sheath potential can disappear. Repeatedly reflecting electron beam components can drive kinetic instabilities.\footnote{M.D.Campanell, A.V.Khrabrov, and I.D.Kaganovich, Phys. Rev. Lett. {\bf 108}, 25501, 2012.} Asymmetric effects of the reflecting wall conditions as well as Coulomb collisional effects are incorporated into the numerical simulation. This work is supported by National Science Council of Taiwan, NSC 100-2112-M-006-021-MY3 and NSC 103-2112-M-006-021-MY3. [Preview Abstract] |
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NP8.00088: PIC Simulation of RF Plasma Sheath Formation and Initial Validation of Optical Diagnostics using HPC Resources Casey Icenhour, Ashe Exum, Elijah Martin, David Green, David Smithe, Steven Shannon The coupling of experiment and simulation to elucidate near field physics above ICRF antennae presents challenges on both the experimental and computational side. In order to analyze this region, a new optical diagnostic utilizing active and passive spectroscopy is used to determine the structure of the electric fields within the sheath region. Parallel and perpendicular magnetic fields with respect to the sheath electric field have been presented. This work focuses on the validation of these measurements utilizing the Particle-in-Cell (PIC) simulation method in conjunction with High Performance Computing (HPC) resources on the Titan supercomputer at Oak Ridge National Laboratory (ORNL). Plasma parameters of interest include electron density, electron temperature, plasma potentials, and RF plasma sheath voltages and thicknesses. The plasma is modeled utilizing the VSim plasma simulation tool, developed by the Tech-X Corporation. The implementation used here is a two-dimensional electromagnetic model of the experimental setup. The overall goal of this study is to develop models for complex RF plasma systems and to help outline the physics of RF sheath formation and subsequent power loss on ICRF antennas in systems such as ITER. [Preview Abstract] |
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NP8.00089: An Experiment to Investigate Ion-Ion Two-Stream Instabilities in the Presheath Ryan T. Hood, Scott D. Baalrud, Frederick N. Skiff, Robert L. Merlino An experiment has been constructed to investigate ion-ion two-stream instabilities and their effect on ion flow velocities near sheaths. The device is a multidipole hot filament discharge operated in a mixture of argon and xenon. An emissive probe will be used to measure the electrostatic potential and laser induced fluorescence (LIF) will be used to measure the ion velocity distributions throughout the presheath of a negatively biased electrode. Optical tagging and LIF will be used to measure fluctuations in the ion distribution functions in an effort to search for evidence of ion-ion two-stream instability. This is thought to be responsible for anomalous friction causing a merging of ion speeds toward a common Bohm speed near the sheath edge. The results of measurements of the basic plasma parameters will be presented as well as a detailed description of the LIF system. [Preview Abstract] |
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NP8.00090: Expansion into vacuum of a plasma with a two-temperatures electron distribution function Abdourahmane Diaw Recent developments in laser-plasma ion acceleration has renewed interest in plasma expansion into vacuum theory. This phenomenon is usually described by simple 1-D model using the acceleration of ions under the isothermal or adiabatic pressure of hot electrons. However, the electron energy spectra obtained when a short pulse laser interacts with a solid target is generally composed of two temperatures electrons: a hot electrons population (with low density) and a cold electrons population (with a higher density). We will give an overview of a plasma expansion into a vacuum with a two-temperature electron distribution function. Characteristics (amplitude, microscopic structure) of the rarefaction shock which occurs in the plasma when the hot- to the cold-electron temperature ratio is larger than $5+\sqrt{24}$ are investigated using a semi-infinite plasma. Asymptotic expressions of the quantities of the flow are established in the limit of large temperature ratios. Spatial structures of the ion and electron densities and velocities are presented, together with the prediction of the maximum ion velocity. A second illustration corresponds to the expansion of a thin-foil into a vacuum with a two- temperature electron distribution function. [Preview Abstract] |
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NP8.00091: VAN ALLEN 100 |
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NP8.00092: Onset of reconnection in the near magnetotail: PIC simulations Yi-Hsin Liu, Joachim Birn, William Daughton, Michael Hesse, Karl Schindler Using 2.5-dimensional particle-in-cell (PIC) simulations of magnetotail dynamics, we investigate the onset of reconnection in realistic tail configurations. Reconnection onset is preceded by a driven phase, during which magnetic flux is added to the tail at the high-latitude boundaries, followed by a relaxation phase, during which the configuration continues to respond to the driving. We found a clear distinction between stable and unstable cases, dependent on the deformation amplitude and ion/electron mass ratio. The threshold appears consistent with electron tearing. The evolution prior to onset as well as the evolution of stable cases, are largely independent of the mass ratio, governed by the integral entropy conservation as imposed in MHD. This suggests that ballooning instability in the tail should not be expected prior to the onset of tearing and reconnection. The onset time and other onset properties depend on the mass ratio, consistent with expectations for electron tearing. At onset, we found electron anisotropies $T_\perp/T_\| = 1.1-1.3$, raising growth rates and wave numbers. Our simulations have provided a quantitative onset criterion that is easily evaluated in MHD simulations, provided the spatial resolution is sufficient.\footnote{Liu et al. submitted to JGR} [Preview Abstract] |
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NP8.00093: Whistler Anisotropy Instabilities as the Source of Banded Chorus: Van Allen Probes Observations and Particle-in-Cell Simulations S. Peter Gary, Xiangrong Fu, Misa M. Cowee, Reinhard H. Friedel, Herbert O. Funsten, George B. Hospodarsky, Craig Kletzing, William Kurth, Brian A. Larsen, Kaijun Liu, Elizabeth A. MacDonald, Kyungguk Min, Geoffrey D. Reeves, Ruth M. Skoug, Dan Winske Magnetospheric banded chorus events are enhanced whistler waves with frequencies $\omega_r < \Omega_e$ where $\Omega_e$ is the electron cyclotron frequency, and a characteristic spectral gap at $\omega_r \simeq \Omega_e/2$. Here two-dimensional particle-in-cell simulations in a magnetized, homogeneous, collisionless plasma test the hypothesis that banded chorus is due to two branches of the whistler anisotropy instability excited by two distinct, anisotropic electron components. The electron densities and temperatures are derived from HOPE instrument measurements on the Van Allen Probes A satellite during a banded chorus event on 1 November 2012. Observations show a three-component electron model consisting of a dense, cold (a few tens of eV) population, a less dense, warm (a few hundred eV) anisotropic population, and a still less dense, hot (a few keV) anisotropic population. Simulations show that the warm component drives quasi-electrostatic upper-band chorus, and the hot component drives electromagnetic lower-band chorus; the gap near $\Omega_e/2$ follows from growth of the two distinct instabilities. [Preview Abstract] |
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NP8.00094: An Experimental Concept for Probing Nonlinear Radiation Belt Physics Bill Amatucci, Guru Ganguli, Chris Crabtree, Manish Mithaiwala, Carl Siefring, Erik Tejero The SMART sounding rocket is designed to probe the nonlinear response of a known ionospheric stimulus. High-speed neutral barium atoms generated by a shaped charge explosion perpendicular to the magnetic field in the ionosphere form a ring velocity distribution of photo-ionized Ba+ that will generate lower hybrid waves. Induced nonlinear scattering of lower hybrid waves into whistler/magnetosonic waves has been theoretically predicted, confirmed by simulations, and observed in the lab. The effects of nonlinear scattering on wave evolution and whistler escape to the radiation belts have been studied and observable signatures quantified. The fraction of the neutral atom kinetic energy converted into waves is estimated at 10-12\%. SMART will carry a Ba release module and an instrumented daughter section with vector wave magnetic and electric field sensors, Langmuir probes and energetic particle detectors to determine wave spectra in the source region and detect precipitated particles. The Van Allen Probes can detect the propagation of the scattered whistlers and their effects in the radiation belts. By measuring the radiation belt whistler energy density, SMART will confirm the nonlinear scattering process and the connection to weak turbulence. [Preview Abstract] |
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NP8.00095: PIC simulations of whistler wave generation using plasma conditions from the RAM-SCB model Yiqun Yu, Lei Zhao, Bo Peng, Gian Luca Delzanno, Vania Jordanova, Stefano Markidis Wave-particle interactions play an important role in the Earth's inner magnetospheric dynamics. We study the whistler wave generation with an implicit particle-in-cell code (iPIC3D) within unstable equatorial regions identified by the kinetic ring current model RAM-SCB. During storm time, RAM-SCB shows that hot electrons on the dayside demonstrate high temperature anisotropy and are unstable to whistler wave excitation. By using plasma parameters from RAM-SCB, we carry out iPIC3D simulations assuming a bi-Maxwellian distribution for electrons. We find that with an electron temperature anisotropy of 4, electron density of 6 cm$^{-3}$, and parallel temperature of 1 keV on the dayside around $L\sim5.5$, whistler waves are rapidly excited and propagate along the background magnetic field. Comparisons with linear theory show good agreement. The electron velocity distribution is significantly changed after wave generation, with smaller anisotropy due to the pitch-angle scattering. Furthermore, test particles are tracked in the whistler wave environment and the pitch-angle diffusion coefficient is extracted. The coefficient generally agrees with quasi-linear theory prediction with slight deviation even when the wave amplitude is as large as $5\%$ of the background magnetic field. [Preview Abstract] |
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NP8.00096: Bounce averaged diffusion coefficients in a physics based magnetic field geometry from RAM-SCB Lei Zhao, Yiqun Yu, Gian Luca Delzanno, Vania K. Jordanova In this work we explore wave-particle interaction in the radiation belt. By applying quasilinear theory, we obtain the particle diffusion coefficients in both pitch angle and energy for different configurations of the Earth's magnetic field. We consider the Earth's magnetic dipole field as a reference, and compare the results against non-dipole field configurations corresponding to quiet and stormy conditions. The latter are obtained with RAM-SCB, a code that models the Earth's ring current and provide a realistic modeling of the Earth's magnetic field. The bounce averaged electron pitch angle diffusion coefficients are calculated for each magnetic field configuration. The equatorial pitch angle, wave frequency and spectral distribution of whistler waves are shown to affect the bounce averaged diffusion coefficients. In addition, wave-particle resonance is significantly influenced by the magnetic field configuration: in storm conditions, diffusion is strongly reduced for some equatorial pitch angles. [Preview Abstract] |
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NP8.00097: Wave-particle interactions in the radiation belts: effect of wave spectra Dimitris Vassiliadis, Mattias Tornquist, Mark Koepke Particle acceleration in Earth's radiation belts is often explain in terms of radial diffusion theory. Some of the most important contributions to diffusive transport are stochastic as well as resonant interactions with low-frequency (Alfven/magnetosonic) waves. While spectra of such waves are traditionally assumed to be broadband and spectrally white, a number of recent studies [Rae et al., 2012; Ozeke et al., 2012] indicate that the spectra of ground geomagnetic pulsations are significantly more complex. We examine power-law spectra in particle simulations in a realistic magnetospheric field configuration and report on their effect on the transport and energization of the pre-storm electron population. [Preview Abstract] |
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NP8.00098: LPI SHORT PULSE \& BEAMS |
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NP8.00099: Analysis of ionization with intense laser radiation Bahman Hafizi, Daniel Gordon, John Palastro, Michael Helle Laser-plasma experiments routinely rely on field ionization for plasma formation. While several analyses of non-relativistic ionization have been carried out [1], they often fail to reproduce experimental observations [2]. Moreover for large laser intensities or for high-Z atoms relativistic effects become important. We have undertaken a numerical study of ionization processes employing three-dimensional, time dependent, deBroglie/Compton wavelength-resolved, parallel algorithms for the Schr\"{o}dinger and Klein-Gordon equations [3]. Along with the numerical analysis we have performed analytic modeling, employing the Schr\"{o}dinger, Klein-Gordon and Dirac equations. Results of the analysis and numerical studies will be presented. In particular we discuss ionization of hydrogen-like Xe, the momentum distribution of ejected electrons and the related Bohmian trajectories. \\[4pt] [1] L.V. Keldysh, Sov. Phys. JETP \textbf{20}, 1307 (1965).\\[0pt] [2] C.I. Moore, \textit{et al}., Phys. Rev. Lett. \textbf{82}, 1688 (1999); A. Ting, et al., Phys. Plasmas 12, 010701 (2005); D. Kaganovich, \textit{et al}., Phys. Rev. Lett. \textbf{100}, 215002 (2008).\\[0pt] [3] D. Gordon {\&} B. Hafizi, J. Comp. Phys. \textbf{231}, 6349 (2012); D. Gordon, B. Hafizi {\&} A. Landsman (to be published). [Preview Abstract] |
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NP8.00100: Enhancing Bremsstrahlung Radiation using Front Surface Target Structures Sheng Jiang, Andrew Krygier, Douglass Schumacher, Richard Freeman, Kramer Akli X-ray or $\gamma $-ray sources generated by laser solid interactions have many potential applications in different fields including X-ray radiography, pair production and photonuclear physics. Recent studies with 3D PIC modeling have shown that large scale front-surface target structures can significantly increase the energy and narrow the angular distribution of hot electrons compared to that for a regular flat target.\footnote{S. Jiang, A. G. Krygier, et al., Phys. Rev. E 89, 013106 (2014)} These characteristics of electrons are crucial for further Bremsstrahlung production using a high-Z converter target. The corresponding Bremsstrahlung radiation covers a wide energy range and can be as high as 100 MeV. By performing the Monte-Carlo simulations we find that the peak $\gamma $-ray brightness is 6.0*10$^{19}$ s$^{-1}$mm$^{-2}$mrad$^{-2}$ at 10MeV and 1.4*10$^{19}$ s$^{-1}$mm$^{-2}$mrad$^{-2}$ at 100MeV (0.1{\%} bandwidth), which is comparable to other tunable $\gamma $-ray sources. The brightness for high energy $\gamma $-rays (\textgreater 50MeV) is one or a few orders of magnitude higher using the structured target than the flat target. Simulation and preliminary experimental results will be presented. [Preview Abstract] |
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NP8.00101: Guiding of high-energy electrons in high-intensity-laser interactions with wire targets through surface wave excitation A. Maksimchuk, P. Belancourt, P. Kordell, M.J.-E. Manuel, L. Willingale, A.G.R. Thomas, R.P. Drake, K. Krushelnick, A. Brantov, V.Yu. Bychenkov Experiments investigating the interaction of an ultra-short pulse laser (intensity of up to 2x10$^{19}$ W/cm$^{2})$ with thin metal wires of different diameter, length and conductivity at different angles of incidence were performed. The generation of a highly collimated electron beam with a charge of several nC, electron energies in the range of 1-7 MeV and efficiency of few percents were demonstrated. The beam was confined and guided along a thinnest wire of 15 microns to a distance of 130 cm. The spatial structure for different components of the spectrum demonstrated a better confinement of the lower energy electrons. The experimental results were interpreted through the generation of a strong Sommerfeld surface wave propagating along the wire with phase velocity close to $c$, which is produced due to electron expulsion from the focal region and generation of magnetic fields near the surface of the wire during the laser-plasma interaction. 2D PIC simulation combined with test particle simulations shows that Sommerfeld surface wave provides the crucial conditions for collimating and confining the laser-produced electron beams along the wire. [Preview Abstract] |
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NP8.00102: Recent High-Intensity Experiments at the Trident Laser James Cobble, Sasikumar Palaniyappan, Cort Gautier, Yongho Kim, Chengkun Huang With near-diffraction-limited irradiance of 2 x 10$^{20}$ W/cm$^{2}$ on target and prelase contrast better than 10$^{-8}$, we have accessed the regime of relativistic transparency (RT) at the Trident Laser. The goal was to assess electron debris emitted from the target rear surface with phase-contrast imaging (PCI) and current density measurements (hence, the total electron current). Companion diagnostics show whether the experiments are in the target-normal-sheath-acceleration mode or in the RT regime. The superb laser contrast allows us to shoot targets as thin as 50 nm. PCI at 527 nm is temporally resolved to 600 fs. It has shown the evolution of electron behavior over tens of ps, including thermal electrons accompanying the ion jet, accelerated to many tens of MeV earlier in time. Faraday-cup measurements indicate the transfer of many microC of charge during the laser drive. As a ride-along experiment using a gas Cherenkov detector (GCD), we have detected gamma rays of energy \textgreater 5 MeV. This radiation has a prompt component and a lesser source, driven by accelerated ions, that is time resolved by the GCD. The ion time of flight is compared to Thomson parabola data. Electron energy spectra are also collected [Preview Abstract] |
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NP8.00103: Self-focusing instability of stochastic laser pulses Vladimir Malkin, Nathaniel Fisch The propagation of coherent laser pulses through plasmas and other focusing Kerr-like media is known to be limited by the transverse filamentation instability, occurring at powers exceeding the so-called ``critical power of self-focusing.'' It appears, however, that the self-focusing instability threshold for stochastic laser pulses might be much higher than for coherent laser pulses. Furthermore, the instability of over-threshold stochastic pulses might develop much slower than the instability of coherent pulses of the same intensity. These effects might be used advantageously to suppress the transverse filamentation instability of amplified pulses in ultra-powerful plasma-based backward Raman amplifiers. [Preview Abstract] |
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NP8.00104: Generation of lower harmonic radiation by a strong laser plasma interaction with asymmetrically bundled carbon nanotubes Toshihiro Taguchi, Masahiko Inoue, Thomas Antonsen We have investigated a generation of low frequency radiation by an interaction between a strong laser field and nano particles, such as clusters or carbon nanotubes (CNTs), using PIC simulations. As known well, a single mode laser irradiation is not enough to generate the lower harmonics and the second harmonic laser must be added. The main reason of this problem is that the single mode laser can only induce odd harmonic oscillations on electrons. The odd harmonic generation is due to the symmetric shape of the targets, spherically for clusters and cylindrically for CNTs. The symmetric ion structure produces a symmetric electrostatic potential in a target and the potential exerts an antisymmetric forces on electrons. This is why only the odd harmonics are generated by the monochromatic laser. This indicates that the possibility of the even harmonic excitation exists when the potential structure is not symmetric. Since carbon nanotubes are synthesized on metal catalysts, the shape of the bundled CNTs can be changed by the shape of the catalysts. We performed PIC simulations of laser plasma interaction with a carbon nanotube of a shape like an egg. As a result, we found that even modes are generated as well as odd modes and lower harmonic oscillation is also excited. [Preview Abstract] |
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NP8.00105: Spot size dependence of laser accelerated protons in thin multi-ion foils Tung-Chang Liu, Xi Shao, Chuan-Sheng Liu, Bengt Eliasson, Jyhpyng Wang, Shih-Hung Chen We present a numerical study of the effect of the laser spot size of a circularly polarized laser beam on the energy of quasi-monoenergetic protons in laser proton acceleration using a thin carbon-hydrogen foil. The used proton acceleration scheme is a combination of laser radiation pressure and shielded Coulomb repulsion due to the carbon ions. We observe that the spot size plays a crucial role in determining the efficiency of proton acceleration. Using a laser pulse with fixed input energy and pulse length impinging on a carbon-hydrogen foil, a laser beam with smaller spot sizes can generate higher energy but fewer quasi-monoenergetic protons. We studied the scaling of the proton energy with respect to the laser spot size and obtained an optimal spot size for maximum proton energy flux. In particular, we provided a theoretical model interpreting the acceleration mechanism for non-penetration cases and the calculated optimal spot size agreed well with the 2D PIC simulation results. Using the optimal spot size, we can generate an 80 MeV quasi-monoenergetic proton beam containing more than $10^{8}$ protons using a laser beam with power 250 TW and energy 10 J and a target of thickness 0.15 wavelength and 49 critical density made of 90\% carbon and 10\% hydrogen. [Preview Abstract] |
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NP8.00106: Mass Limited Target Effects on Proton Acceleration with Femtosecond Laser Plasma Interactions Calvin Zulick, A. Raymond, A. McKelvey, L. Willingale, V. Chvykov, A. Maksimchuk, A.G.R. Thomas, V. Yanovsky, K. Krushelnick Experiments at the HERCULES laser facility have been performed to measure the effect of reduced mass targets on proton acceleration through the use of foil, grid, and wire targets in femtosecond laser plasma interactions. The target thickness was held approximately constant at $12.5$ $\mu$m, while the lateral extent of the target was varied. The electron current density was measured with an imaging Cu K$_{\alpha}$ crystal. Higher current densities were observed as the target mass was reduced which corresponded to an increase in the temperature of the accelerated proton beam. Additionally, a line focusing feature was observed in the spatial distribution of protons accelerated to from the wire target, believed to be a result of azimuthal magnetic fields generated by electron currents in the wire. Particle-in-cell and Vlasov-Fokker-Plank simulations were performed in order to investigate the focusing magnetic field as well as the complex sheath formation dynamics on the mesh target. [Preview Abstract] |
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NP8.00107: Plasma generation in mass-limited water targets Jungmoo Hah, Kirk Liberty, John Nees, Karl Krushelnick, Alexander Thomas One major problem associated with high repetition-rate experiment is obtaining a suitable new target for each shot, while maintaining shot-to-shot spatial stability. For high repetition-rate laser experiments with solid targets, rotating stage is usually used for moving a target point, which causes stability and size problems. To solve these problems, some researchers have tried to replace solid targets with liquid stream or droplet. Here, we use a syringe pump, a piezoelectric device and a tungsten needle to make continuous and stable water droplets with a diameter of $\sim$ 2 $\mu$m. These mass-limited water droplets as a target have some advantages. First, heat dissipation is blocked, so the target is entirely heated. Second, effective spatial contrast is improved by reducing the interaction between lower intensity spatial wings of the beam and a single-micron target. Third, at the relativistic laser intensities, a smaller target allows for higher electron densities at the target's back surface, which enhances field's strength for ion acceleration. For these advantages, it is required that we understand plasma generation processes. Therefore, we investigate the processes by irradiating fs laser pulses to mass-limited droplets and these interactions are captured by CCD. [Preview Abstract] |
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NP8.00108: Study of energy partitioning in mass limited targets using the 50 TW Leopard short-pulse laser Brandon Griffin, Hiroshi Sawada, Yasuhiko Sentoku, Toshinori Yabuuchi, Hui Chen, J.-B. Park, Harry McClean, Prav Patel, Farhat Beg Mass limited Cu targets were used to study the energy distribution in the interaction of an ultra-intense, short-pulse laser by measuring characteristic x-rays and energetic particles. At the Nevada Terawatt Facility, Leopard delivered 15 J to an 8 $\mu $m spot size in a 350 fs pulse, achieving a peak intensity of 10$^{19}$ W/cm$^{2}$ at 20$^{\circ}$ incidence. The 2 $\mu $m thick Cu foil targets varied in size from 1 mm$^{2}$ to 75 $\mu $m by 60 $\mu $m. A spherical crystal imager and a Bragg crystal x-ray spectrometer were used to measure 8.05 keV monochromatic x-ray images and 7.5-9.5 keV x-rays respectively. A magnet-based electron spectrometer in the rear monitored escaping electrons. Results show a decrease in the absolute yield of both escaped electrons and Cu K-shell x-rays as targets sizes are reduced, while He$\alpha $ emission remains nearly constant. In the smallest target, a bulk temperature of about 150 eV was inferred from the ratio of K$\beta $ to K$\alpha $. The interaction of the Leopard laser with the targets was simulated with 2-D implicit Particle-in-cell code PICLS. Comparisons of the simulation and experiment will be presented. [Preview Abstract] |
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NP8.00109: Ion Acceleration from Pure Frozen Gas Targets using Short Pulse Lasers Edward McCary, Florian Stehr, Finn O'Shea, Xuejing Jiao, Ronald Agustsson, Robert Berry, Dennis Chao, Donald Gautier, Samuel Letzring, Hernan Quevedo, Kaley Woods, Bjorn Hegelich A system for shooting interchangeable frozen gas ice targets was developed and tested on the Trident laser system at Los Alamos National Lab. A target holder which could hold up to five substrates used for target growing was cryogenically cooled to temperatures below 14 K. The target substrates consisted of holes with diameters ranging from 15$\mu $m-500$\mu $m and TEM grids with micron scale spacing, across which films of ice were frozen by releasing small amounts of pure gas molecules directly into the vacuum target chamber. The thickness of the ice targets was determined by using alpha spectroscopy. Accelerated ion spectra were characterized using a Thomson Parabola with magnetic field strength of 0.92T and electric field strength of 30kV and radio-chromic film stacks. Hydrogen targets were additionally characterized using stacks of copper which became activated upon exposure to energetic protons resulting in a beta decay signal. The beta decay was imaged on electron sensitive imaging plates to provide an energy spectrum and spacial profile of the proton beam. Results of the interchangeable, laser-based ion accelerator will be presented. [Preview Abstract] |
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NP8.00110: Proton acceleration in the interaction of high power laser and cryogenic hydrogen targets Rohini Mishra, Frederico Fiuza, Siegfried Glenzer High intensity laser driven ion acceleration has attracted great interest due to many prospective applications ranging from inertial confinement fusion, cancer therapy, particle accelerators. Particle-in-Cell (PIC) simulations are performed to model and design experiments at MEC for high power laser interaction with cryogenic hydrogen targets of tunable density and thickness. Preliminary 1D and 2D simulations, using fully relativistic particle-in-cell code PICLS, show a unique regime of proton acceleration, e.g. $\sim$ 300 MeV peak energy protons are observed in the 1D run for interaction of $\sim$ 10$^{20}$ W/cm$^{2}$, 110fs intense laser with 6n$_{\mathrm{c}}$ dense (n$_{\mathrm{c}}=$10$^{21}$ cm$^{-3}$) and 2 micron thin target. The target is relativistically under-dense for the laser and we observe that a strong (multi-terawatt) shock electric field is produced and protons are reflected to high velocities by this field. Further, the shock field and the laser field keep propagating through the hydrogen target and meets up with target normal sheath acceleration (TNSA) electric field produced at the target rear edge and vacuum interface and this superposition amplifies the TNSA fields resulting in higher proton energy. In addition, the electrons present at the rear edge of the target continue to gain energy via strong interaction with laser that crosses the target and these accelerated electrons maintains higher electric sheath fields which further provides acceleration to protons. We will also present detailed investigation with 2D PICLS simulations to gain a better insight of such physical processes to characterize multidimensional effects and establish analytical scaling between laser and target conditions for the optimization of proton acceleration. [Preview Abstract] |
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NP8.00111: X-Ray Imaging of Ultrafast Magnetic Reconnection Driven by Relativistic Electrons Anthony Raymond, Andrew McKelvey, Calvin Zulick, Anatoly Maksimchuk, Alexander Thomas, Louise Willingale, Vladimir Chvykov, Victor Yanovsky, Karl Krushelnick Magnetic reconnection events driven by relativistic electrons are observed between two high intensity laser/plasma interaction sites. The two laser focuses were on average 20$\mu$m FWHM containing 50TW of power each, delivered with a split f/3 paraboloid onto copper foil targets at a focused intensity of 4x10$^{18}$ W/cm$^2$. A spherically bent k-alpha X-ray Bragg crystal was utilized to image the interactions, and by motorizing one half of the paraboloid vertically the focal separation was varied between 0-200$\mu$m. While these k-alpha images demonstrated a ring structure surrounding a single focus (due to electrons returning from vacuum to the rear of the target surface), splitting the focuses revealed the rings of either spot interacting and enhancing between the focuses, evidencing magnetic reconnection driven by the relativistic electron currents. Imaging the transversely propagating electrons with a filtered LANEX screen demonstrated relativistic currents with spatial nonuniformities potentially directly originating from reconnection events, and varying target geometries were used to investigate the resulting effects on the spatial electron profiles. At present PIC simulations are being conducted to better understand and attempt to reproduce the measured electron outflow dynamics. [Preview Abstract] |
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NP8.00112: Quasi-Monoenergetic Dense and Uniform Electron Bunch Generation from Laser Driven Double-Layer Thin Films C. Wang, R. Roycroft, E. McCary, A. Meadows, J. Blakeney, K. Serratto, D. Kuk, C. Chester, L. Gao, H. Fu, X.Q. Yan, J. Schreiber, I. Pomerantz, A. Bernstein, H. Quevedo, G. Dyer, E. Gaul, T. Ditmire, D.C. Gautier, J. Fernandez, B.M. Hegelich We demonstrate that dense, uniform quasi-monoenergetic relativistic electron bunches can be generated from the interaction of a high-intensity laser pulse with a double-layer thin film target. The first layer of the target is a freestanding, nanometer-scale, diamond-like carbon production layer. The second layer is a thin plastic reflection layer which reflects the drive-laser pulse, but allows electrons to pass through. Although no electron bunch is generated from the second layer alone, by adding it behind the first layer we obtained a quasi-monoenergetic bunch along the laser axis, 35 times denser than a bunch from the single layer target. Comparing the angular distribution of the electron spectra from a double-layer target with that of a single-layer target, we observed an increase of the electron cutoff energy at larger angles, which improves the uniformity of created electron bunches. [Preview Abstract] |
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NP8.00113: Energy Loss of High Intensity Focused Proton Beams Penetrating Metal Foils C. McGuffey, B. Qiao, J. Kim, F.N. Beg, M.S. Wei, M. Evans, P. Fitzsimmons, R.B. Stephens, S.N. Chen, J. Fuchs, P.M. Nilson, D. Canning, D. Mastrosimone, M.E. Foord Shortpulse-laser-driven intense ion beams are appealing for applications in probing and creating high energy density plasmas. Such a beam isochorically heats and rapidly ionizes any target it enters into warm dense matter with uncertain transport and stopping properties. Here we present experimental measurements taken with the 1.25kJ, 10ps OMEGA EP BL shortpulse laser of the proton and carbon spectra after passing through metal foils. The laser irradiated spherically curved C targets with intensity 4x10$^{18}$ W/cm$^{2}$, producing proton beams with 3 MeV slope temperature and a sharp low energy cutoff at 5 MeV which has not been observed on lower energy, shorter pulse intense lasers. The beam either diverged freely or was focused to estimated 10$^{16}$ p$+$/cm$^{2}$ps by a surrounding structure before entering the metal foils (Al or Ag and a Cu tracer layer). The proton and ion spectra were altered by the foil depending on material and whether or not the beam was focused. Transverse proton radiography probed the target with ps temporal and 10 micron spatial resolution, indicating an electrostatic field on the foil may also have affected the beam. We present complementary particle-in-cell simulations of the beam generation and transport to the foils. [Preview Abstract] |
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NP8.00114: Stochastic heating of electrons by intense laser radiation in the presence of electrostatic potential well Sergei Krasheninnikov Previous model used for the study of synergistic effects of electrostatic potential well and laser radiation where electric field in electrostatic potential was slowing down electrons moving in the direction of the laser field propagation, is extended for the opposite case, where electric field of the well is accelerating electrons moving in the direction of the laser field propagation. It was found that in both cases the rate of stochastic heating of energetic electrons remains virtually the same. [Preview Abstract] |
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NP8.00115: Fast ion distribution in the presence of flow Seth Davidovits, Nathaniel Fisch Experiments and simulations in multiple ICF related configurations have observed signs of bulk flow near stagnation. These configurations include both laser driven implosions such as at the NIF, as well as Z-Pinches. We investigate the possibilities for enhancement or depletion of fast ion tails in simplified flow models, with an eye towards applicability to ICF experiments. Small effects on the tail populations may substantially affect fusion output, as the fast ions in these tails have much larger fusion cross sections than thermal ions and make up the majority of fusion production for typical ICF temperatures. While in collisional plasma the bulk of the distribution function is driven toward Maxwellian in a few collision times, the high velocity tails can take much longer to form. Furthermore, the long mean free paths of the fast ions means they may sample differing regions of flow, while thermal particles only sample the local flow. [Preview Abstract] |
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NP8.00116: Electron Transport and Related Nonequilibrium Distribution Functions in Large Scale ICF Plasma W. Rozmus, T. Chapman, A.V. Brantov, B. Winjum, R. Berger, S. Brunner, V. Yu. Bychenkov, A. Tableman Using the Vlasov-Fokker Fokker-Planck (VFP) code OSHUN [M. Tzoufras \textit{et a}l. Phys. Plasmas\textbf{ 2}0, 056303 (2013)] and higher order perturbative solutions to the VFP equation, we have studied electron distribution functions (EDF) in inhomogeneous and hot hohlraum plasmas of relevant to the current ICF experiments. For these inhomogeneous ICF plasmas characterized by with the temperature and density gradients consistent with the high flux model [M.D. Rosen,\textit{ et al}., HEDP 7, 180 (2011)], nonequilibrium EDF often display unphysical properties related to first and second order derivatives at larger velocities. These EDF strongly modify the linear plasma response, including Lanadau damping of Langmuir waves, electrostatic fluctuation levels, and instability gain coefficients We have found that Langmuir waves propagating in the direction of the heat flow have increased Lanadau damping compared to damping calculated from a Maxwellian EDF, while Langmuir waves propagating in the direction of the temperature gradients are far less damped as compared to damping calculated from the Maxwellian EDF. These effects have been discussed in the context of stimulated Raman scattering, Langmuir decay instability and Thomson scattering experiments. [Preview Abstract] |
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NP8.00117: Revisiting hot electron generation in ignition-scale hohlraums William Kruer, Cliff Thomas, David Strozzi, Nathan Meezan, Otto Landen, Harry Robey Recent work\footnote{H. F. Robey, \textit{et. al}., Phys. Plasmas 21, 022703 (2014)} invoking hot electron preheat in NIC ignition experiments is motivating a fresh look at hot electron generation in ignition-scale hohlraums. Various mechanisms for high energy electron generation are considered, with particular attention to their time dependence and the potential role of the two plasmon decay instability in the main laser pulse.\footnote{William L. Kruer, Nathan Meezan, S. P. Regan, \textit{et. al},, Journal of Physics Conference Series 244, 022020 (2010)} The energy at risk calculations\footnote{S. P. Regan, \textit{et. al},. Phys. Plasmas 17, 020703 (2010)} are updated to include the effects of cross beam energy transfer on the time-dependent energy and intensity of the inner beams as well as improvements in the calculated plasma conditions. The generation of hot electrons by the Raman-scattered light driving the two plasmon decay instability and the effect of the Weibel instability on the propagation of the hot electrons are also briefly considered. Uncertainties in interpreting the energy in hot electrons from hard x-ray measurements and techniques to reduce hot electron generation are discussed. [Preview Abstract] |
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NP8.00118: Laser backscatter damage risk assessments of NIF Target Experiments R.K. Kirkwood, D.J. Strozzi, P.A. Michel, D.A. Callahan, B. Raymond, G. Gururangan, B.J. MacGowan The National Ignition Facility (NIF) performs target experiments of many types. Laser power and target configurations and materials vary between experiments; in many cases the variation is incremental from a previously fielded experiment but can also involve a major change that represents a new operating regime for laser plasma interactions with a large plasma. A review methodology has been established to assess the potential risk of damage to the facility optics from scattered light for all proposed experiments. This process uses a defined set of rules and criteria to efficiently assess the risks of each proposed experiment. This assessment allows rapid approval of most experiments and identifies others that need more detailed analysis and/or modifications to their design to produce acceptable risk. The methodology uses comparisons to an extensive data base of existing experiments and some simple rules to extend beyond the parameters for which we have data. This poster will describe our assessment methodology and data sets that provide the experimental basis. [Preview Abstract] |
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NP8.00119: The Langmuir Decay Instability and Stimulated Raman Scattering in ICF-Relevant Plasmas B.J. Winjum, A. Tableman, F.S. Tsung, W.B. Mori Kinetic simulations of stimulated Raman scattering (SRS) in ICF-relevant plasmas with long-scale-length density gradients have shown that SRS can grow strongly when the laser propagates above that density for which $k\lambda_D \approx 0.30$, where $k$ is the wavenumber of the daughter electron plasma wave and $\lambda_D$ is the electron Debye length. Simulations and experiments have shown that SRS saturation is dominated by kinetic effects for $k\lambda_D > 0.30$ and the Langmuir decay instability (LDI) for $k\lambda_D < 0.30$, but few kinetic simulations of SRS have explicitly explored the role of LDI in this regime or the transition in SRS behavior across this $k\lambda_D$ boundary. Here we present one- and two-dimensional PIC simulations of LDI in the midst of SRS dynamics for both single-laser-speckles as well as for lasers propagating up long-scale-length density gradients covering a range of $k\lambda_D$. We show the effect of LDI on SRS reflectivity and hot electron generation for variable $ZT_e/T_i$ and $k\lambda_D$, as well as the effect of LDI on the spatio-temporal behavior of electron plasma wave packets generated by SRS. [Preview Abstract] |
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NP8.00120: Simulations of 3D LPI's relevant to IFE using the PIC code OSIRIS F.S. Tsung, W.B. Mori, B.J. Winjum We will study three dimensional effects of laser plasma instabilities, including backward raman scattering, the high frequency hybrid instability, and the two plasmon instability using OSIRIS in 3D Cartesian geometry and cylindrical 2D OSIRIS with azimuthal mode decompositions. With our new capabilities we hope to demonstrate that we are capable of studying single speckle physics relevant to IFE in an efficent manner. [Preview Abstract] |
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NP8.00121: Three-Dimensional Full-Beam Simulation of Ultrashort Laser Pulse Amplification by Brillouin Backscattering in the Strong Coupling Regime Kathleen Weichman, Richard Berger, Thomas Chapman, Steven Langer, Caterina Riconda Laser amplification by stimulated Brillouin scattering (SBS) has been previously proposed as a method of achieving high intensity sub-picosecond laser pulses. The 3D fluid simulation code pF3D is used to simulate the SBS interaction of two counterpropagating laser pulses in parameter regimes similar to current experiments [1,2]. The optimal operating regime is explored by variation of the pump and seed intensity, pulse duration, and plasma properties. The sensitivity of seed intensity amplification, pulse compression, and wavefront quality are investigated with regards to spontaneous laser beam instabilities such as filamentation and amplified spontaneous emission. The influence of the spatial and temporal coherence of the pump and seed on the amplification process is presented.\\[4pt] This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344 and partly funded by the Laboratory Research and Development Program at LLNL under project tracking code 12-ERD-061. \\[4pt] [1] L. Lancia \emph{et al}., Phys. Rev. Lett. \textbf{104}, 025001 (2010).\\[0pt] [2] L. Lancia \emph{et al}., Oral presentation at the 44$^{\mathrm{th}}$ annual Anomalous Absorption Conference, Estes Park, 2014. [Preview Abstract] |
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NP8.00122: Hohlraum Modeling of Hybrid Shock Ignition Target E.S. Dodd, J.A. Baumgaertel, E.N. Loomis Hybrid Shock Ignition (HSI) combines a hohlraum driven capsule with a directly driven shock for heating. Unlike standard Shock Ignition, the capsule is imploded with X-rays from a laser driven hohlraum to compress the fuel, which is too cold to ignite. However, as in Shock Ignition, the compressed fuel is subsequently heated to ignition temperatures with a directly-driven shock. The use of indirect and direct drive in the same target necessitates complex beam geometry, and thus HSI is being pursued with spherical hohlraums. More importantly for the NIF, the beam repointing required for polar direct drive will not be needed for the implosion phase with this target. Spherical hohlraums have been fielded previously at the OMEGA laser [1] as a part of the Tetrahedral Hohlraum Campaign. They were originally proposed as an alternative to cylindrical hohlraums to achieve highly symmetric radiation drive. The new HSI hohlraums will require six laser entrance holes in hexahedral symmetry to accommodate all beams. This presentation will show radiation-hydrodynamic calculations of the current hexahedral OMEGA hohlraum design, as well as benchmark calculations of the old tetrahedral targets.\\[4pt] [1] J. M. Wallace, T. J. Murphy, N. D. Delamater, et al., Phys. Rev. Lett., 82 3807 (1999). [Preview Abstract] |
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NP8.00123: Capsule Design for Hybrid Shock Ignition J.A. Baumgaertel, E.S. Dodd, E.N. Loomis Hybrid Shock-Ignition (HSI) is an alternate fusion energy concept that combines indirect drive and shock ignition schemes in order to access new regimes in National Ignition Facility (NIF) hohlraum physics. Building off of tetrahedral hohlraum experiments [1] at the OMEGA laser facility, we have preliminary designs for spherical hohlraums that combine symmetrically arranged laser entrance holes for indirect-drive beams (to initially compress the capsule) and holes for direct-drive beams to drive a strong ignitor shock (to further compress and ignite the fuel). A LANL Eulerian hydrodynamic code is being used to find optimal laser drive, hohlraum, and capsule specifications, via criteria such as implosion symmetry, implosion time, and neutron yield. At first, drive will be modeled using a radiation source to mimic the hohlraum drive, and later, ignitor beams will be added. Initial capsule designs will be presented for experiments to develop the HSI platform on the sub-ignition scale OMEGA laser facility in FY15. \\[4pt] [1] J. M. Wallace, T. J. Murphy, N. D. Delamater, et al., Phys. Rev. Lett., 82 3807 (1999) [Preview Abstract] |
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NP8.00124: Shocks waves in high power laser plasma interactions R. Bingham, R.A. Cairns, P.A. Norreys, R. Trines Some recent experiments on the interaction of high power lasers with plasmas have shown evidence of shock like structures with very high electric fields existing over very short distances. In inertial confinement fusion capsules the existence of fields of more than $10^{10}$ V/m over distances of the order of 10-100 nm have been observed. In other experiments with intense lasers interacting with over dense plasmas high energy proton beams with small energy spread are observed. We propose a theory to describe laminar ion sound structures in a collisionless plasma. Reflection of a small fraction of the upstream ions converts the well-known ion acoustic soliton into a structure with a steep potential gradient upstream and with downstream oscillations. The strong electric field is also responsible for separation in the shocked region in a deuterium, tritium mix, while accelerating the deuterium and tritium ions at the shock front. The possibility of using these accelerated ions to heat the fuel in a fast ignition scheme will be discussed. [Preview Abstract] |
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NP8.00125: Fast Ignition by Photon-Pressure Accelerated Ion Beam Tomoyuki Johzaki, Yasuhiko Sentoku, Atsushi Sunahara, Takamasa Morikawa, Takuma Endo For enhancing the core heating efficiency in fast ignition, the ion beam generated by radiative pressure acceleration with circularly-polarized ultra-intense laser pulse is used as a core heating driver. In the present study, on the basis of the integrated simulations (PIC simulations for beam generation and Fokker-Planck simulations for core heating) and demonstrated the potential probability for C6+ beam driven fast ignition. From the coupled transport and hydro simulations, it is found that the beam particle (C6+) energy of 100 $\sim$ 200 MeV minimizes the beam energy required for ignition and the beam duration of $\sim$ 1 ps is suitable for ignition in terms of beam generation and core heating. From 2D PIC simulations for ion beam generation it is found that fast ion beam with ion energy of 210 MeV is obtained when the carbon target with the ion density of 90 ncr (ncr is the laser critical density) is irradiated with the CP laser with the intensity of 6x10$^{22}$W/cm$^2$. In this case, 12\% energy convergence efficiency of laser to ion beam is obtained. If assuming the laser spot of 24 micron diameter and pulse duration of 700fs, the required laser energy for beam generation is $\sim$190 kJ and the resultant beam energy of 23 kJ, which satisfy the beam condition required for ignition. [Preview Abstract] |
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NP8.00126: Unique capabilities for ICF and HEDP research with the KrF laser Stephen Obenschain, Jason Bates, Lop-Yung Chan, Max Karasik, David Kehne, John Sethian, Victor Serlin, James Weaver, Jaechul Oh, Bruce Jenkins, Robert Lehmberg, Frank Hegeler, Stephen Terrell, Yefim Aglitskiy, Andrew Schmitt The krypton-fluoride (KrF) laser provides the shortest wavelength, broadest bandwidth and most uniform target illumination of all developed high-energy lasers. For directly driven targets these characteristics result in higher and more uniform ablation pressures as well as higher intensity thresholds for laser-plasma instability. The ISI beam smoothing scheme implemented on the NRL Nike KrF facility allows easy implementation of focal zooming where the laser radial profile is varied during the laser pulse. The capability for near continuous zooming with KrF would be valuable towards minimizing the effects of cross beam energy transport (CBET) in directly driven capsule implosions. The broad bandwidth ISI beam smoothing that is utilized with the Nike KrF facility may further inhibit certain laser plasma instability. In this presentation we will summarize our current understanding of laser target interaction with the KrF laser and the benefits it provides for ICF and certain HEDP experiments. Status and progress in high-energy KrF laser technology will also be discussed. [Preview Abstract] |
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NP8.00127: Progress in LPI Experiments at the NikeLaser J. Weaver, D. Kehne, S. Obenschain, A. Schmitt, V. Serlin, J. Oh, R. Lehmberg, F. Tsung, P. McKenty, J. Seely The experimental program at the Nike laser facility at NRL is studying laser plasma instabilities (LPI) in the quarter critical region and cross-beam energy transport (CBET). The Nike krypton-fluorine (KrF) laser has unique characteristics that allow parametric studies of LPI. These features include short wavelength (248 nm), large bandwidth ($\sim$ 2-3 THz), beam smoothing by induced spatial incoherence (ISI), and full aperture focal spot zooming during the laser pulse. Nike also has a unique beam geometry that combines two widely separated beam arrays (145$^{\circ}$ in azimuth) with close beam-beam spacing (as low as 3.5$^{\circ}$) within the main drive array. Particularly relevant for the CBET studies, recent campaigns have demonstrated the capability to alter the laser bandwidth by a factor of $\sim$ 10 as well as shifts in the peak laser wavelength. An extensive LPI diagnostic suite is available for observation of stimulated Raman scattering, two-plasmon decay, stimulated Brillouin scattering, the parametric decay instability, and hard x-ray emission due to hot electrons. An overview of the observations of scattered laser light made during the previous studies of instabilities in the quarter critical region will be presented. Ongoing analysis of observed LPI emission from rotated targets will also be included. Plans for upcoming experiments related to quarter critical instabilities and CBET will be discussed. [Preview Abstract] |
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NP8.00128: Presheath and boundary effects on helicon discharge equilibria Cory Jackson, M. Umair Siddiqui, Justin Kim, Noah Hershkowitz Two distinct discharge equilibria are observed in a 500 W argon helicon plasma with uniform magnetic fields of 900 G at neutral pressures between 3 and 4 mTorr. Both modes exhibit localized populations of relatively hot electrons. For one discharge equilibria a downstream density peak is observed, similar to observations by other authors [Chen et al., Plasma Sources Sci. Technol. \textbf{5}, 173 (1996)]. For the other mode the hot electrons are confined by a localized potential structure and no density peaks are observed. The determination of the discharge mode and the location of the potential structure and hot electron population is modulated by the position of the downstream conducting boundary and the length of its presheath. [Preview Abstract] |
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