Bulletin of the American Physical Society
2005 47th Annual Meeting of the Division of Plasma Physics
Monday–Friday, October 24–28, 2005; Denver, Colorado
Session LP1: Poster Session VI |
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Room: Adam's Mark Hotel Grand Ballroom I & II 2:00pm |
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LP1.00001: TURBULENCE, TRANSPORT, MHD, AND STABILITY |
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LP1.00002: Electron Transport and the Critical Gradient W. Horton, J.-H. Kim, J. Pratt, J.C. Perez, T. Hoang New experiments on Tore Supra with upgraded, higher power radio frequency heating systems with total powers up to 10~MW give new data points on the core temperature and temperature profiles versus injected power. The talk will review the evidence for the two space scales on which electron transport occurs: from (i) the large scale trapped electron modes (TEM-ITG) and (ii) the small scale electron temperature gradient (ETG) turbulence. Joint IFS - Tore Supra transport analysis for electron power balance gives a database for discharges driven by Fast Wave Electron Heating in for $\tau_E\le 100$~ms. The wide range of RF heating powers from near Ohmic 1~MW to above 10~MW produce an order of magnitude increase in the radial thermal flux. High resolution electron temperature data and true steady state conditions in TS allow give well defined electron thermal diffusivities for the classical circular cross section, large aspect ratio ($R/a = 2.2$~m/ 0.7~m) tokamak. The heat flux versus the temperature gradient relationship is presented and compared with standard theoretical models for the thermal flux $q_e(T_e)$. The extrapolation to zero heat flux of the flux-versus-gradient data yields a well-defined critical electron temperature gradient. Histograms of the anomalous thermal diffusivities before and after being normalized to the theoretical models are constructed to evaluate the quantitatively the prediction. Work supported by Dept. of Energy and the CEA-Cadarache [Preview Abstract] |
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LP1.00003: Investigation of micro-turbulence and particle transport in LHD Clive Michael, Kenji Tanaka, Leonid Vyacheslavov, Andrey Sanin, Osamu Yamagishi, Masayuki Yokoyama, Katsumi Ida, Kazuo Kawahata It is well known that particle and energy transport in fusion plasmas is driven strongly by micro-turbulence. Using a recently developed CO2 laser 2d imaging interferometer, spatially resolved density fluctuations in the range $5-15\rm{mm}^{-1}$ are measured. A comparison is made between discharges with significantly different particle transport coefficients, obtained from gas-puff modulation experiments. Different fluctuation branches are identified, according to their phase velocity. The amplitude of fluctuations with the ion diamagnetic drift velocity is larger when transport is enhanced, however, for other branches the tendency is opposite. The computed growth rate for ion temperature gradient (ITG) turbulence also scales with the amplitude of the i-dia branch. These results suggest that ITG turbulence plays an important role for anomalous transport. [Preview Abstract] |
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LP1.00004: Global Gyrokinetic Particle Simulation of Isotope Effects W.W. Lee, S. Ethier, W.X. Wang A series of global gyrokinetic particle simulations of ion temperature gradient (ITG) drift instabilities has been carried out using different hydrogen species (H+, D+ and T+) to study the isotope effects. These simulations with adiabatic electron approximations using the GTC code [1] have included the velocity-space nonlinearity for the ions. The inclusion of this nonlinearity in the earlier ITG simulations has impacted the resulting zonal flow and ion thermal diffusivity [2]. Most all all, with the addition of the new nonlinear channels, these simulations have shown to achieve the steady state at a much faster rate and maintain it for a long duration. Initial results based on the inclusion of this nonlinearity in a relatively large a/rho device have indicated that isotope effects are not as evident as those reported earlier [3]. Details will be reported. \newline \newline [1] Z. Lin, T. S. Hahm, W. W. Lee, W. M. Tang, and R. B. White SCIENCE 281 (5384), 1835 (1998). \newline [2] W. W. Lee, ``Steady State Global Simulations of Microturbulence,'' Bull. Am. Phys. Soc. 49 (8), 135 (2004). \newline [3] W. W. Lee and R. A. Santoro, Phys. Plasmas 4 (1), 169 (1997). [Preview Abstract] |
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LP1.00005: Shear Flow Stabilization of Turbulence in the Helimak Kenneth Gentle, Jakub Felkl, Kevin Lee, Dylan Miracle The Helimak is a good approximation to the infinite cylindrical slab, but the end plates allow application of radial electric fields that drive radial currents. A strong bifurcation occurs at a critical bias condition with reduction in the level of fractional density fluctuations, generally with both smaller fluctuations and higher density. The bifurcation occurs for both positive and negative bias voltages at a threshold current. The nature of the stabilization and the dependence of the threshold condition on plasma parameters will be described. Work supported by the Department of Energy Office of Fusion Energy Sciences DE-FG03-00ER54609. [Preview Abstract] |
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LP1.00006: Experimental Investigation of Turbulence and Transport in the Helimak Configuration K. Lee, D. Miracle, J. Felkl, K. Gentle Density fluctuations and transport in the Helimak device are investigated as a function of the connection length, $L_c$, and gas species. Radial equilibrium profiles for Ar and He show structure where $n$ and $T_e$ peak outside the electron cyclotron resonance radius. Due to the fact that the magnetic field on the LFS has unfavorable curvature, density fluctuation levels on this side are increased. For short $L_c$ results are consistent with the idea that these fluctuations are unstable drift waves. As $L_c$ increases, the LFS gradient becomes steeper and the fluctuation levels increase to more than $50\%$. Probability distribution functions in this region display highly non-Gaussian structure, and raw data suggests plasma here exhibits intermittency. In He plasmas a quasi-periodic oscillation occurs at longer $L_c$ which appears to consist of two or more low frequency components, $f_k<1$ kHz. Both radial and parallel transport are investigated with results indicating that at longer $L_c$ the particle fluxes are dominated by transport perpendicular to the magnetic field. Work supported by US Department of Energy grant DE-FG03-00ER54609 [Preview Abstract] |
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LP1.00007: Simulation of Blob Turbulence in Helimak J.C. Wiley, M. Kotschenreuther, P. Valanju, D. Miracle, K. Lee, J. Felkl, K. Gentle The Helimak appears to be an accessible realization representing only the essential features of the plasma condition described by the blob SOL model[1]. Comparing predictions of this model with experimental results tests the model's validity and provides a basis for using the model in divertor design. We have extended the model to include parallel physics while retaining the 2D character[2]. Here we compare results from a 2D finite element simulation including relevant atomic processes to measurements of profiles and turbulence in a Helimak. Results show systematic agreement with the experimental measurements for electron temperature and density profiles, and fluctuation power spectra as the helical pitch, applied power, and fill gas are varied. Profiles show in/out asymmetry consistent with blob transport. At short connection lengths which correspond to weaker turbulence, a distinct peak in the power spectra appears in both the experiment and simulation. This peak disappears with longer connection lengths and stronger turbulence. The frequency of this peak depends on the fill gas. \newline 1. D. A. D'Ippolito, et al., Phys. Plasmas {\bf 9} 222-233 (2002)\newline 2. J. C. Wiley, et al. Sherwood Theory Conf. 2005. [Preview Abstract] |
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LP1.00008: Controlled Suppresion of Turbulence in the Helimak Jakub Felkl, Kenneth Gentle, Kevin Lee, Dylan Miracle The Helimak design creates a helical magnetic field with cylindrical slab geometry. The plasma exhibits turbulence consistent with drift waves in argon. Insulated end plates permit one annular region to be biased with respect to another. The response to both positive and negative bias has a simple, strong bifurcation to a state of reduced turbulence, often with higher density. This transition is reversible with applied voltage/current and exhibits no hysteresis. Radial profiles of potential and density in argon, helium and hydrogen will be presented along with radial turbulent particle flux measurements. Work supported by the Department of Energy Office of Fusion Energy Sciences DE-FG03-00ER54609. [Preview Abstract] |
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LP1.00009: Instabilites in the Helimak Experiment D. Miracle, K. Lee, J. Felkl, K. Gentle, J. Wiley The Helimak offers a simple configuration for exploring plasma instabilities in the SOL that can be checked against simulation. Here we experimentally study the onset of instabilities that are predicted by the blob equations\footnote{D. A. D'Ippolito, et al., Phys. Plasmas \textbf{9} 222- 233 (2002)}. We study the turbulence in the Helimak configuration for various connection lengths in Helium and Argon plasmas. For long connection lengths ($\sim 100$m) the spectrum is broad, peaking at low frequencies. For short connection lengths ($\sim 20$m), a second higher frequency peak appears that scales with ion mass. We have found that in this regime our plasma displays statistical similarity to simulations of the blob equations. In addition we examine the 2D structure of the turbulence and compare with the structures from the simulation. We characterize the radial transport and isolate the fundamental instabilities that give rise to turbulence in the Helimak. [Preview Abstract] |
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LP1.00010: Hysteresis effects in edge poloidal flow generation B. Fetscher, N.D. Daniels, A.S. Ware, D.E. Newman, B.A. Carreras, C. Hidalgo Gas puffing at the edge of the TJ-II stellarator has been used to control the development of an edge poloidal velocity shear layer [1]. Recent experiments have been done to test for hysteresis in the development of the flow. In this work, a numerical transport model is used to examine for hysteresis in the development of an edge poloidal velocity shear layer due to a modeled gas puff. The transport model couples together density, ion temperature, electron temperature, poloidal flow, toroidal flow, radial electric field, and a fluctuation envelope equation which includes a shear-suppression factor. The physics of the model has been modified to include a turbulence growth rate for resistive ballooning modes as well as ion temperature gradient modes. The numerical integration in the model has been changed to a modified Runge-Kutta with adaptive time-stepping. For the cases run with parameters consistent with these TJ-II experiments (only RF heating, no neutral beams), the resistive ballooning mode growth rate is dominant in the edge region. In this work, we present results from a series of cases using parameters that are typical of TJ-II discharges and ramps (both up and down) of an edge density source term used to model a gas puff. The impact of the ramp on the generation of edge poloidal velocity is discussed. \\ \noindent {[1] C. Hidalgo, et al., Phys. Rev. E 70, 067402 (2004).} [Preview Abstract] |
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LP1.00011: Studies of the nonlinear turbulence stress response to arbitrary flows Klaus Hallatschek, Kimitaka Itoh To understand the nonlinear behavior of flows and Reynolds stress, the response of tokamak turbulence to a variety of zonal flow patterns has been measured in a series of computational experiments. Despite the seemingly erratic pattern, the Reynolds stress is surprisingly well described by a nonlinear functional, depending only on the turbulence level and the ambient zonal flows. The contributing terms can be understood qualitatively in terms of analytical models. [Preview Abstract] |
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LP1.00012: Kinetic electrons in global electromagnetic gyrokinetic particle simulations Y. Nishimura, Z. Lin, L. Chen, W. Wang Employing an electromagnetic gyrokinetic simulation model,\footnote{Z.~Lin and L.~Chen, Phys. Plasmas {\bf 8}, 1447 (2001).} kinetic electron dynamics in global tokamak geometry is investigated. The massless fluid electron model is developed as a base. We further evolve gyrokinetic equations for non-adiabatic kinetic electrons. To obtain the magnetic perturbation, the fluid-kinetic hybrid electron model$^1$ employs the inverse of the Faraday's law. Instead, the Ampere's law is used as a closure relation to avoid uncertainties in estimating $u_{e\|}$, the moment of the electron velocities. The physics goal is to investigate the finite beta effects on the turbulent transport, as well as $\alpha$ particle driven turbulence.\footnote{I.~Holod, Z.~Lin, {\it et al.}, this conference.} This work is supported by Department of Energy (DOE) Cooperative Agreement No. DE-FC02-03ER54695 (UCI), DOE Contract No. DE-AC02-76CH03073 (PPPL). [Preview Abstract] |
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LP1.00013: New Developments in Trapped Electron Mode Turbulence D.R. Ernst, K. Zeller, N. Basse, L. Lin, M. Porkolab, W. Dorland, A. Long The onset of TEM turbulence is believed to limit density gradients in Alcator C-Mod internal transport barriers.$^{1 }$ We have recently modified GS2 to make direct comparisons with phase contrast imaging measurements of density fluctuations in the ITB.$^{2}$ Further, the TEM critical density gradient is nonlinearly upshifted,$^{1}$ analogous to the Dimits shift for ITG turbulence.$^{3}$ In the ITG case, ion-ion collisions reduced the upshift by damping zonal flows.$^{4}$ In contrast, this new TEM nonlinear upshift persists in the presence of realistic ion-ion and electron-ion collisions, and increases with collisionality.$^{5}$ Quasi-periodic bursts arise near threshold, with a period dependent on the relative primary growth and zonal flow damping rates. Nonlinear simulations of this regime confirm the role of zonal flows. $^{1}$D. R. Ernst et al., Phys. Plasmas 11 (2004) 2637. $^{2}$D. R. Ernst \textit{et al.,} 2004 IAEA Fusion Energy Conference, paper IAEA-CN116/TH/4-1, http://www-naweb.iaea.org/napc/physics/fec/fec2004/datasets/TH 4-1.html, and A. Long \textit{et al}., this conference. $^{3}$A. M. Dimits \textit{et al.,} Phys. Plasmas 7(3)(2000) 969. $^{4}$Z. Lin \textit{et al}., Phys. Rev. Lett. 7(5) (2000) 1857. $^{5}$D.R. Ernst, K. Zeller, and W. Dorland, 2005 Sherwood Int'l Fusion Theory Conference, P3-33. [Preview Abstract] |
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LP1.00014: Barrier Formation in Mixed Electron and Ion Channel Systems K.K. D'Boyz, D.E. Newman, B.A. Carreras, P.W. Terry Simple dynamical models have captured much of the dynamics of the ion transport barriers found in a number of magnetically confined plasma experiments [1]. However, the mixed dynamics of ion and electron channel transport still has many open questions. It has been found that occasionally an electron channel barrier will form with a standard (``ion channel'') barrier, while at other times the electron channel barrier does not form even when a strong ``ion channel'' barrier forms. By adding to the simple barrier model an evolution equation for electron fluctuations we can investigate the interaction between the formation of the standard ion channel barrier and the somewhat less common electron channel barrier. Barrier formation in the electron channel is found to be even more sensitive to the alignment of the various gradients making up the sheared radial electric field then the ion barrier is. Electron channel heat transport is found to significantly increase after the formation of the ion channel barrier but before the electron channel barrier is formed. This increased transport is important in the barrier evolution and therefore the profile evolutions in the system. Various configurations will be explored and the implications for self heated plasmas explored.\\ \noindent [1] {D. E. Newman, B. A. Carreras, D. Lopez-Bruna, P. H. Diamond, and V. B. Lebedev, Phys. Plasmas, 5 (4) 938-952 (1998)}\\ [Preview Abstract] |
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LP1.00015: Predictive Transport Modeling of Hybrid Scenario Operation in KSTAR Tokamak J.M. Park, J.-Y. Kim High performance discharges of hybrid scenario in the KSTAR tokamak are predicted by an integrated transport code C2 (Coupled 2-D). The present simulations are focused on (a) finding optimum operation scenarios to establish and sustain a broad current profile with $q_0 \ge 1$ for preventing sawtooth which is believed to trigger large amplitude neoclassical tearing mode (NTM), and (b) estimating and control of edge pedestal parameters and divertor heat load which are influenced significantly by edge localized mode (ELM) observed usually in hybrid scenario operation. For this purpose, the C2 code is being integrated with the self-consistent transport models associated with various MHD activities including sawtooth oscillations, NTM island evolution, and ELM crash by peeling-ballooning mode. The simulation results show that the desired q-profiles can be obtained with the baseline heating and current drive systems of KSTAR by earlier central heating and subsequent off-axis current drive during the current rise phase, although the current ramp-up rates of KSTAR superconducting coils are too slow to adopt a conventional fast ramp-up method. The predicted temperatures at the top of the edge pedestal in the main heating phase are found to be reasonably in good agreement with the scaling laws obtained from the standard ELMy H-mode discharges. It is also shown that the maximum divertor heat load during ELMs can be reduced significantly by increasing plasma density with a careful control of gas-puffing rates. [Preview Abstract] |
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LP1.00016: Higher-Order Hurst Signatures: Dynamical Information in Time Series Willis Ferenbaugh, D.E. Newman Understanding and comparing time series from different systems requires characteristic measures of the dynamics embedded in the series. The Hurst exponent is a second-order dynamical measure of a time series which grew up within the blossoming fractal world of Mandelbrot. This characteristic measure is directly related to the behavior of the autocorrelation, the power-spectrum, and other second-order things. And as with these other measures, the Hurst exponent captures and quantifies some but not all of the intrinsic nature of a series. The more elusive characteristics live in the phase spectrum and the higher-order spectra. This research is a continuing quest to (more) fully characterize the dynamical information in time series produced by plasma experiments or models. The goal is to supplement the series information which can be represented by a Hurst exponent, and we would like to develop supplemental techniques in analogy with Hurst's original R/S analysis. These techniques should be another way to plumb the higher-order dynamics. [Preview Abstract] |
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LP1.00017: Investigations of turbulent structures in the TORPEX device S.H. Mueller, A. Fasoli, B. Labit, M. McGrath, G. Plyushchev, M. Podesta, F.M. Poli, V. Naulin Electrostatic turbulent structures are visualized on the TORPEX toroidal plasma experiment ($R=1\,$m, $a=0.2\,$m) using HEXTIP, an 86-tip, 2D Langmuir probe array covering the whole poloidal section. To characterize such turbulence imaging data statistically, thus providing a quantitative basis for comparison (theory-experiment, theory-theory, experiment-experiment), suitable observables like positions, shapes and velocities of structures must be defined. Several possible definitions are compared in terms of information content, discriminative power, robustness and computational requirements. The statistical distribution of these observables is experimentally measured on TORPEX as a function of control parameters, i.e. quantities set externally and not subject to the plasma feedback action. Among these, the magnetic field line pitch angle is shown to play a special role for the turbulence dynamics through its effect on parallel flows, important to oppose drift-induced charge separation. The TORPEX results thus provide a highly discriminative test environment for turbulence models. On the modeling side, a pseudo-3D variant of the two-fluid code ESEL has been developed, accounting for the effect of a non-zero field line pitch angle and permitting to replace formerly freely chosen dissipation parameters by a physical model of the parallel dynamics. [Preview Abstract] |
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LP1.00018: Critical Toroidal Rotation Profile for Resistive Wall Modes in Tokamaks K.C. Shaing, S.A. Sabbagh, M. Peng It is known experimentally and theoretically that resistive wall modes in tokamaks can be stabilized by toroidal plasma rotation. The critical toroidal rotation speed is usually a small fraction of the toroidal Alfven speed based on several theories. It is reduced when the enhanced plasma inertia is included in the polarization current density. Besides the usual safety factor dependence, the reduction factor depends on the aspect ratio when neoclassical dissipation is taken into account. The critical toroidal angular frequency is few kHz in the edge region for typical large tokamaks. Here, a model is developed to calculate the critical toroidal rotation profile. This is accomplished by including neoclassical dissipation and its corresponding inertia enhancement at each rational surface. [Preview Abstract] |
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LP1.00019: Modeling the Magnetorotational Instability in Differentially Rotating Plasmas Wilson Tillotson, William Dorland, Daniel Lathrop, Nicolas Mujica, Daniel Sisan When threaded by a weak magnetic field, a differentially rotating ionized fluid can be unstable to the Magnetorotational Instability (MRI). The MRI is a likely mechanism for enhanced angular momentum transport in accretion disks. We use an explicit, finite difference algorithm to model MRI turbulence in a plasma confined between differentially rotating, concentric cylinders. In addition to reproducing the stability boundaries and growth rates predicted by the local linear dispersion relation, we have investigated some nonlinear characteristics of MRI, including the potential for parasitic instabilities to saturate the instability and the effects of boundary layers on saturated flow profiles. (There are hydrodynamic instabilities associated with the boundary layers.) We also discuss precession characteristics of nonaxisymmetric modes and nonlinear mode interactions long after saturation. [Preview Abstract] |
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LP1.00020: Error field, torque, and plasma rotation L.J. Zheng, M. Kotschenreuther, J.W. Van Dam, F. Waelbroeck By calculating the torque on the error field coil structure, which is opposite to the torque exerting on the plasma, we find that the error-field-induced torque ($\tau_\phi$) can be expressed explicitly as the imaginary part of $ {\bf j}^\dag{\cal F}_1^{-1} (\delta W_b/\delta W_\infty) {\cal F}_2{\bf j}$, where ${\bf j}$ specifies the strength of the error field, $\delta W_b$ and $\delta W_\infty$ represent, respectively, the energy integrals with perfectly conducting wall and without wall, and ${\cal F}_1$ and ${\cal F}_2$ are regular equilibrium matrices. The kinetic version of the AEGIS code is being developed to calculate the torque in the numerically constructed equilibria. Experimental observations from DIII-D, JET, and C-Mod are examined and compared to our theoretical prediction based on the above torque expression. We will clarify the relationship between error field, torque, and plasma rotation. [Preview Abstract] |
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LP1.00021: First Results of a Linear MHD Stability Code for Axisymmetric Plasmas with Arbitrary Equilibrium Flow L. Guazzotto, R. Betti, J.P. Freidberg Fast toroidal plasma flows are routinely induced by neutral beam injection in current tokamaks such as NSXT and DIII-D. Flow and flow shear stabilize external modes such as the resistive wall mode, suppress turbulence when the flow shear is large enough, and also have a significant influence on the stability and nonlinear evolution of the internal kink and ballooning modes. Equilibria with poloidal and toroidal arbitrary flows can now be generated with the University of Rochester code \textit{FLOW},\footnote{ L. Guazzotto\textit{ et al.}, Phys. Plasmas \textbf{11}, 604 (2004).} but the tools available to tackle the stability problem are quite limited. We have undertaken the development of a new finite-element linear stability code named \textit{FLOS, }meant to investigate MHD stability with arbitrary flow. The code is based on a recent \textit{$\delta $}W formulation in which the energy principle (including arbitrary flow) is reduced to an eigenvalue problem of the kind (\textit{$\omega $}\textbf{A} -- \textbf{B)}\textbf{\textit{x}} = 0. Such a formulation is attractive because it allows a straightforward implementation of the finite element method. Presently, the code is limited to the analysis of internal modes. In the current work, we present the first results of the code relative to internal kink modes of tokamak plasmas in the presence of arbitrary flow. The implementation is also discussed in detail. This work was supported by the U.S. Department of Energy under Cooperative Agreement No. DE-FC52-92SF19460. [Preview Abstract] |
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LP1.00022: Axial Sheared Flow Stabilization of Conical Array Plasma Jets Lucas Wanex, Radu Presura Experiments with conical array implosions on a central axial wire at the Nevada Terawatt Facility offer evidence that axial shear provides a stabilizing influence in precursor plasma jets. Axial plasma flow converging on a central wire produces a stable precursor jet. Similar experiments without a central wire produce a very unstable plasma. By comparison cylindrical array implosions on a central wire do not generate axial flow in the plasma. Experimental evidence indicates that the kink instability is present in cylindrical array implosions. Analytical results show that axial shear is introduced into the jet by the no-slip boundary condition between the axial plasma flow and the central wire. A numerical linear analysis also complements our finding that axial sheared flow reduces instability growth rates in conical array precursor plasma jets. [Preview Abstract] |
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LP1.00023: Natural magnetic island rotation frequency in the presence of plasma turbulence C.C. Hegna A simple model is constucted to account for the interaction of plasma turbulence with a rotating magnetic island. The theory describes the self-consistent effect the turbulence has in determining the natural rotation frequency of the island. A number of magnetic island physics theories model the effect of plasma turbulence through the addition of phenomenological enhanced cross-field diffusion coefficients. What is neglected in this formulation is a description of how the island's helically deformed fields modify the turbulence. In particular, the helical electrostatic potential and plasma profile features modulate the turbulence induced mean field forces (Reynolds-Maxwell stresses). These forces then produce helical perpendicular plasma currents that alter the plasma quasineutrality condition, and hence affect the island growth and rotation properties. Speculations on the role of turbulence induced changes in the island rotation frequency and the seeding of neoclassical tearing modes will be addressed. [Preview Abstract] |
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LP1.00024: Ion sound effects on magnetic island stability in slab geometry Richard Fitzpatrick Two-fluid drift-MHD theory is used to calculate the ion polarization term in the Rutherford evolution equation of a quasi-static constant-psi magnetic island. The analysis is for cold ions, but takes the magnetosonic wave into account. It is found that an island rotating in a certain range of frequencies radiates electrostatic drift waves. These waves give rise to a radiative electromagnetic torque acting on the island. The magnetosonic wave acts to extend the range of frequencies in which drift-wave emission occurs. The sign of the polarization term remains the same as that predicted by MHD theory, even when the island emits drift-waves. [Preview Abstract] |
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LP1.00025: Rapid Evolution of the Magnetic Island in the Rotating Plasma Yasutomo Ishii, Masafumi Azumi, Andrei Smolyakov Magnetic island formation at the tearing stable resonant surface by the externally applied magnetic perturbation is investigated for rotating plasmas. When the magnetic island width exceeds the critical value, it grows rapidly with the reduction of the plasma rotation around the resonant surface. These features are consistent with the previous work [1]. The analytical theory show that the critical value depends on both the resistivity and viscosity. Our simulation results, however, show the weak dependence of the critical value on the viscosity. One of the purposes of this study is to investigate this critical value mainly by the MHD simulation in the wide parameter regime. Another purpose of this study is to investigate the whole process of the magnetic island evolution. During the rapid growth phase, the flow potential within the magnetic island changes from the dipole structure to almost the flux function. The change of the flow potential structure is important to the modified Rutherford equation for the neoclassical tearing mode [1]. Hence, we will investigate the detailed process of the magnetic flux and flow potential evolution in the rapid growth phase and the final state.\newline \newline [1] A.I.Smolyakov et al. PPCF43(2001)1661. [Preview Abstract] |
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LP1.00026: Island induced bootstrap current on island dynamics in tokamaks I.S. Land, K.C. Shaing When a magnetic island is imbedded in toroidally symmetric tokamaks, the toroidal symmetry in $\vert $\textbf{\textit{B}}$\vert $ is broken. Here, \textbf{\textit{B}} is the magnetic field. This broken symmetry induces an additional bootstrap current density in the vicinity of the island. It is illustrated that this island induced bootstrap current density modifies the island evolution equation and imposes a lower limit on the absolute value of the tearing mode stability parameter $\vert$ $\vert $ for the island to be unstable. This lower limit depends on the local poloidal plasma beta, the ratio of the plasma pressure to the poloidal magnetic field pressure. If is high enough, the magnetic island is stable or, in other words, self-healing. The theory provides an explanation as to why an $m$ = 2 island is not as commonly observed as $m$ =3, 4, or 5 island in tokamaks. Here, $m$ is the poloidal mode number. This mechanism also indicates an alternative route to stabilize the island in the long mean-free-path regime. [Preview Abstract] |
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LP1.00027: Saturated Widths of Magnetic Islands in Tokamak Discharges F. Halpern, G. Bateman, A.H. Kritz, A.Y. Pankin The new ISLAND module described in reference [1] implements a quasi-linear model to compute the widths of multiple magnetic islands driven by saturated tearing modes in toroidal plasmas of arbitrary aspect ratio and cross sectional shape. The distortion of the island shape caused by the radial variation in the perturbation is computed in the new module. In transport simulations, the enhanced transport caused by the magnetic islands has the effect of flattening the pressure and current density profiles. This self consistent treatment of the magnetic islands alters the development of the plasma profiles. In addition, it is found that islands closer to the magnetic axis influence the evolution of islands further out in the plasma. In order to investigate such phenomena, the ISLAND module is used within the BALDUR predictive modeling code to compute the widths of multiple magnetic islands in tokamak discharges. The interaction between the islands and sawtooth crashes is examined in simulations of DIII-D and JET discharges. The module is used to compute saturated neoclassical tearing mode island widths for multiple modes in ITER. Preliminary results for island widths in ITER are consistent with those presented [2] by Hegna. [1] F.D. Halpern, G. Bateman, A.H. Kritz and A.Y. Pankin, ``The ISLAND Module for Computing Magnetic Island Widths in Tokamaks,'' submitted to J. Plasma Physics (2005). [2] C.C. Hegna, 2002 Fusion Snowmass Meeting. [Preview Abstract] |
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LP1.00028: MHD Studies of Advanced Tokamak Equilibria E. Strumberger, S. Guenter, E. Schwarz, C. Tichmann Advanced tokamak scenarios are often characterized by an extremely reversed profile of the safety factor, $q$, and a fast toroidal rotation. ASDEX Upgrade type equilibria with toroidal flow are computed up to a toroidal Mach number of $M_{\rm ta} = 0.5$, and compared with the static solution. Using these equilibria, the stabilizing effect of differential toroidal rotation on double tearing modes (DTMs) is investigated. These studies show that the computation of equilibria with flow is necessary for toroidally rotating plasma with $M_{\rm ta} \ge 0.2$. The use of $\rho_{\rm tor}$ instead of $\rho_{\rm pol}$ as radial coordinate enables us also to investigate the stability of equilibria with current holes. For numerical reasons, the rotational transform, $\iota = 1/q$, has to be unequal zero in the CASTOR$\_$FLOW code, but values of $\iota_{\rm a} \ge 0.001$ ($q_{\rm a} \le 1000$) can be easily handled. Stability studies of DTMs in the presence of a current hole are presented. Tokamak equilibria are only approximately axisymmetric. The finite number of toroidal field coils destroys the perfect axisymmetry of the device, and the coils produce a short wavelength ripple in the magnetic field strength. This toroidal field ripple plays a crucial role for the loss of high energy particles. Therefore, three-dimensional tokamak equilibria with and without current holes are computed for various plasma beta values. In addition the influence of the plasma beta on the toroidal field ripple is investigated. [Preview Abstract] |
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LP1.00029: Comparison of Analytic and Numerical Unity-$\beta$ Equilibria Russell Neches, Steven Cowley, Pierre-Alexandre Gourdain, Jean-Noel Leboeuf, Simon Allfrey The characteristics of near unity-$\beta$ equilibria are investigated with two codes. CUBE is a multigrid Grad-Shafranov solver, and ACUBE was written to compute solutions using analytic unity-$\beta$ equilibria [S.C. Cowley {\em et. al.}, 1991]. Results from each method are qualitatively and quantitatively compared across a spectrum of mutually relevant parameters. These comparisons corroborate the theoretical results and provide benchmarks for high-resolution numerical results available from CUBE. These tools facilitate exploration of many properties of high-$\beta$ equilibria, such as a highly diamagnetic plasma and its ramifications for stability and transport as $\beta$ approaches unity. These tools are intended to facilitate the study of transitions from low to high $\beta$ that avoid MHD instabilities. [Preview Abstract] |
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LP1.00030: Internal Current-Driven Kink Modes in Line Tied Screw Pinch. V.V. Mirnov, C.B. Forest, C.C. Hegna In recent theoretical studies of external kink mode stability in line-tied geometry, a current channel was assumed to be isolated from a cylindrical conducting wall by a vacuum gap [1,2]. In a more general model [3], the current channel was surrounded by a low density external plasma yielding the no-wall marginal stability condition q$_{b }$=1 where b is the radius of the external plasma. Recent experimental results from the Rotating Wall Machine (RWM) show that during the initial ramp-up period of the plasma current, the m=1 mode becomes unstable before q$_{b }$ falls below unity at the plasma boundary. The instability has an internal character corresponding to the appearance of the resonance surface q(r$_{s})$ = 1 in the plasma region. Theoretical analysis based on an energy principle calculation for internal kink modes in line-tied geometry show that the line tying effects are strong enough to stabilize the ideal internal kink mode. This discrepancy motivates our interest in studying non-ideal effects in line tied geometry and non-MHD sheath mechanisms that can reduce line-tying stabilization and explain the experimental observations. [1] D.D.Ryutov, R.H.Cohen, L.D.Pearlstein, Phys. Plasmas, v.11, No 10, 4740 (2004). [2] C.C.Hegna, Phys. Plasmas, v.11, No 9, 4230 (2004).[3] V.V.Mirnov, C.B.Forest, C.C.Hegna, Bull. of the APS, v.49, No 8, p.231 (2004). [Preview Abstract] |
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LP1.00031: $m=1$ internal kink modes in a line-tied screw pinch Yi-Min Huang, Ellen G. Zweibel, Carl R. Sovinec It is well known that the radial displacement of the m=1 internal kink mode in a periodic screw pinch has a steep jump at the resonance surface where $\mathbf{k} \cdot \mathbf{B}=0$ (Rosenbluth, Dagazian, and Rutherford, Phys. Fluids, 1973). In a line-tied system, relevant to solar and astrophysical plasmas, the resonance surface is no longer a valid concept. It is then of interest to see how line-tying alters the aforementioned result of a periodic system. If the line-tied system also produces steep gradients, it may lead to strong heating, even with weak dissipation. Numerical solution of the eigenmode equations finds that the fastest growing mode in a line-tied system still possesses a jump in the radial displacement at the location coincident with the resonant surface of the fastest growing mode in the periodic counterpart. However, the inner layer is thicker in a line-tied system and the growth rate is smaller. As the system length approaches infinity, both the inner layer thickness and the growth rate approach the periodic ones. How the inner layer thickness and the growth rate scales with the system length will also be discussed. [Preview Abstract] |
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LP1.00032: Kinetic stability of internal kink in ITER standard operation Bo Hu, Riccardo Betti, Janardhan Manickam In tokamak discharges, the central safety factor reduces as the plasma current diffuses into the plasma center. When the safety factor at the magnetic axis falls below unity, the plasma is susceptible to $m/n=1/1$ internal kink. According to the predictions of fluid theory, the internal kink stability properties depend critically on the pressure gradient, magnetic shear and safety factor in the central region. Since the large-aspect-ratio low-beta expansion shows that the fluid contributions to the mode energy are of higher order ($\epsilon^4$) in the inverse aspect ratio compared to $m\neq 1$ modes ($\epsilon^2$), non-ideal contributions such as the kinetic effects of all the particle species can play an important role in determining the internal kink stability. We have developed a kinetic postprocessor of the MHD stability code PEST1 to calculate the kinetic contribution to the energy principle from the thermal particle species as well as the fusion alpha particles. The calculation is carried out for equilibria typical of ITER standard operation scenario. This work was supported by the US-DOE under Contracts DE-FG02-93ER54215 and DE-AC02-CH03073. [Preview Abstract] |
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LP1.00033: Suppression of Tearing Modes by Electron Cyclotron Heating in the TEXTOR tokamak. Avrilios Lazaros, Egbert Westerhof, Eshmaeil Farshi, Fred Hoekzema, Rudi Koslowski, Andreas Kraemer-Flecken, Oliver Zimmermann The 2/1 tearing mode triggered by the Dynamic Ergodic Divertor in the TEXTOR tokamak has been suppressed by properly localised ECRH. Because the mode suppression is observed to be relatively insensitive to the driven current or toroidal injection angle, heating must be responsible for the main effect. In addition, a clear benefit of power modulation was observed. This indicates that direct heating of the magnetic island and the consequent decrease of the resistivity at the O-point, rather than changes in the local temperature and current density profiles, is the dominant mechanism responsible for the suppression of the 2/1 magnetic island in this case. [Preview Abstract] |
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LP1.00034: Rotation dependence of tearing mode excitation by external perturbation fields on TEXTOR Yunfeng Liang, H.R. Koslowski, K. L\"owenbr\"{u}ck, O. Zimmermann, A. Kr\"amer-Flecken, R.C. Wolf, U. Samm, M. de Bock, M. von Hellermann, E. Westerhof The experimental study of the influence of plasma rotation on the 2/1 tearing mode excitation by external perturbation fields on TEXTOR shows that a large plasma rotation in either co- or counter-current direction has a stabilizing effect. However, the dependence of the mode onset threshold on the plasma angular frequency shows an asymmetry, i.e. counter-rotation is always stabilizing whereas co-injection of momentum first lowers the threshold before stabilization sets in. A possible explanation is the influence of the ion polarization current which can be stabilizing and destabilizing, depending on the mode frequency with respect to ion and electron diamagnetic frequencies. [Preview Abstract] |
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LP1.00035: Nonlinear $g$-Mode of Line-tied Magnetic Flux Tubes near Marginal Stability P. Zhu, C.C. Hegna, C. Sovinec The nonlinear dynamics of the convective instability induced by gravity (``g-mode'') of a line-tied magnetic flux tube near marginal stability is crucial to the understanding of the fast, large scale eruptions of flux tubes in a variety of plasma systems. Analytic calculations have predicted the presence of an explosive nonlinear phase of the instability [S. C. Cowley and M. Artun, Phys. Rep., 283, 185 (1997)]. However, explosive growth was not observed in recent direct MHD simulations [P. Zhu, A. Bhattacharjee, and K. Germaschewski, submitted (2005)]. Instead, the evolution of a line-tied flux tube driven by marginal g-modes is dominated by perpendicular convections of the flux tube at the density gradient scale from early to the intermediate nonlinear stage. The convection induced reconfiguration tends to move the system away from the marginal stable equilibrium where the previous detonation model may apply. In this work, a new perturbative nonlinear MHD model is being developed and compared with the NIMROD code to study the dynamics between the nonlinear line-bending and quasilinear reconfiguration in the development of g-mode of a line-tied magnetic flux tube near marginal stability. *Research supported by U.S. DoE under grant No. DE-FG02-86ER53218. [Preview Abstract] |
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LP1.00036: Ideal MHD stability code MARG2D for the analysis of external MHD modes in JT-60U plasma Nobuyuki Aiba, Shinji Tokuda, Tomoko Ishizawa, Takahisa Ozeki The MARG2D code has been developed to identify the stability of equilibrium against ideal MHD perturbations.[1] With the property of the two-dimensional Newcomb equation, this code realizes to determine whether the equilibrium is stable or unstable explicitly against a broad n range of ideal MHD modes, including a low-n kink-ballooning mode and a intermediate-n peeling-ballooning mode (1$\le$n$\le$100), where n is the toroidal mode number.[2] Moreover, since this code is developed as a parallel computing code, we can analyze the stability of an equilibrium in short time. The MHD stability of JT-60U plasma will be analyzed in detail with these advantages; in particular, a role of low-n external modes on constraining the achievable plasma pressure, and the property of a peeling-ballooning mode stability that is thought to be responsible for edge localized modes (ELMs) phenomena. Here we present results of recent progresses of the MARG2D code; emphasis is put on detailed code benchmarking results. [1] S. Tokuda et. al., Phys. Plasmas 6, 3012 (1999). [2] N. Aiba et. al., submitted to Plasma Phys. Control. Fusion. [Preview Abstract] |
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LP1.00037: Multivariable Feedback Controllers for MHD Instabilities Suppression in a Tokamak Based on Simple Model of Coupled Van Der Pole Generators System Igor Semenov, Yuriy Mitrishkin It was shown earlier that the behavior of coupled large scale resonant MHD perturbations in a tokamak has many common features with the excitation of oscillations in a system of coupled Van der Pole (VDP) generators. The VDP model was used to develop a multivariable controller for suppression of MHD perturbations. The controller designed is based on the combined principle of the feedback compensation of internal links between resonant magnetic surfaces and derivative damping of self-oscillations in each control channel. Numerical simulations of the nonlinear feedback control system containing the plant model with six VDP generators coupled and the controller designed have shown that it is possible to get fast suppression of self-oscillations in a wide range of system parameters. Thus the controller provides good robust properties of the feedback system. [Preview Abstract] |
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LP1.00038: Extended MHD Equations For NIMROD Simulations J.D. Callen, C.C. Hegna, C.R. Sovinec The usual derivations of plasma MHD equations either neglect dissipative effects (ideal MHD) or use collisional regime approximations and embed collisional (i.e., Braginskii) closures. Here, ``extended MHD" equations are derived that encompass all parallel (to $\mathbf{B}$) collisionality regimes for strongly magnetized plasmas ($\omega_c >> \nu$, $|\varrho \mathbf{\nabla}_{\! \perp}| << 1$). This is accomplished by developing extended MHD equations from complete two-fluid equations with electron collisional force density $\mathbf{R}_e$ $= - m_e n_e \nu_e [ - \mathbf{J}$$/ n_e e - (3/5) \mathbf{q}_e$$ / n_e T_e + \cdots]$ and arbitrary closure relations for the heat flux $\mathbf{q}$ and stress tensor $\stackrel{\leftrightarrow} {\pi}$ for both electrons and ions. Self-consistent procedures for determining the highly anisotropic closures are emphasized: parallel (along field lines) via kinetics using a Chapman Enskog-type approach\footnote{E.D.~Held, J.D.~Callen and C.C.~Hegna, Phys.~Plasmas {\bf 10}, 3933 (2003).}, cross (within flux surface) diamagnetic and gyroviscous, and perpendicular (across flux surfaces) from collisional relaxation of flows in flux surfaces. This set of extended MHD equations accounts for: viscous force effects in the plasma momentum equation, electron heat flow effects in Ohm's law, and an entropy evolution equation (from dissipative components of the closures) that determines evolution of the overall plasma pressure $P = p_e + p_e$ instead of the usual ideal (isentropic) equation of state. Possible use of these equations in NIMROD will be discussed. [Preview Abstract] |
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LP1.00039: A Finite Volume Approach to the Fully Compressible MHD Simulation of High $\beta $ Tokamak Plasmas Yasuhiro Kagei, Yasuaki Kishimoto, Takahiro Miyoshi A numerical approach based on the finite volume-spectral method is developed for the nonlinear and compressible magnetohydrodynamics(MHD) simulation of high $\beta $ tokamak plasmas. The finite volume method has an advantage over the conventional finite difference method for that it is available for both structured and adaptive unstructured grid schemes. Furthermore, being based on the conservative form, it naturally satisfies the divergence free condition of magnetic field. In this work, first simulation results of the linear and nonlinear evolution of resisitive MHD instabilities using the quadrilateral structured mesh code are shown. Nonlinear behavior of a high $\beta $ tokamak plasma with q$<$1 which is linearly unstable to both internal kink mode (n=1) and high n (n$>\sim $10) ballooning modes is investigated, and then finite beta effect on internal kink mode is discussed. [Preview Abstract] |
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LP1.00040: Two-Fluid Extensions to the M3D CDX-U Validation Study J. Breslau, W. Park, S. Jardin, H. Strauss, L. Sugiyama As part of a cross-code verification and validation effort, both the M3D code [1] and the NIMROD code [2] have qualitatively reproduced the nonlinear behavior of a complete sawtooth cycle in the CDX-U tokamak, chosen for the study because its low temperature and small size puts it in a parameter regime easily accessible to both codes. Initial M3D studies on this problem used a resistive MHD model with a large, empirical perpendicular heat transport value and with modest toroidal resolution (24 toroidal planes). The success of this study prompted the pursuit of more quantitatively accurate predictions by the application of more sophisticated physical models and higher numerical resolution. The results of two consequent follow-up studies are presented here. In the first, the toroidal resolution of the original run is doubled to 48 planes. The behavior of the sawtooth in this case is essentially the same as in the lower- resolution study. The sawtooth study has also been repeated using a two-fluid plasma model, with the effects of the $\omega^ {*}_i$ term emphasized. The resulting mode rotation, as well as the effects on the reconnection rate (sawtooth crash time), sawtooth period, and overall stability are presented. [1] W. Park, et al., Phys. Plasmas {\bf 6}, 1796 (1999). [2] C. Sovinec, et al., J. Comp. Phys. {\bf 195}, 355 (2004). [Preview Abstract] |
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LP1.00041: Two-fluid theory of error-field penetration in tokamak plasmas Andrew Cole, Richard Fitzpatrick The theory of error-field penetration in tokamak plasmas is extended to take {\em two-fluid}\/ physics into account. In particular, diamagnetic, semi-collisional, and Hall effects are all fully incorporated into the analysis. The new theory is used to examine the scaling of the penetration threshold with engineering parameters in ohmic tokamak plasmas. [Preview Abstract] |
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LP1.00042: Motion of Ablation Cloud in Torus Plasmas Ryuichi Ishizaki, Noriyoshi Nakajima, Masao Okamoto, Paul B. Parks Injecting small pellets of frozen hydrogen into torus plasmas is a proven method of fueling. Experimentally, it is known that the density distribution, after the pellet ablates by encountering the high temperature in plasmas, is not consistent with the distribution inferred from assuming that the ablated material remains on the flux surfaces where the ablation occurred. The subsequent redistribution of mass is considered to be due to $E \times B$ drift induced by toroidal drift~(1). It is this phenomenon which we seek to investigate. In this research, the basic equations are MHD equations including the ablation physics. The cubic interpolated pseudo-particle (CIP) method is used in the code~(2). As the first trial, the motion of the ablation cloud in a vacuum field is investigated by solving the ideal MHD equations. A vertical electric field is induced due to a toroidal drift in the cloud and in result the cloud has a $E \times B$ drift velocity toward the low field side across the flux surfaces. The ablation cloud drift in a tokamak equilibrium plasma will be investigated and discussed in the presentation. \\ (1) P. B. Parks {\it et~al.}, Phys. Plasmas {\bf 7}, 1968 (2000). \\ (2) T. Yabe and P. Y. Wang, J. Phys. Soc. Jpn {\bf 60}, 2105 (1991). [Preview Abstract] |
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LP1.00043: Hybrid Simulations of Alpha Particle Effects on MHD Modes in ITER Guoyong Fu Global hybrid simulations of alpha particle effects on MHD modes have been carried out using the M3D code in burning plasmas. In the hybrid model of M3D, the bulk plasma is described by full MHD while energetic ions are described by drift-kinetic equations. The code uses linear finite elements on unstructured mesh in poloidal planes and finite difference in toroidal direction. For ITER parameters and profiles, It is shown that the elongation of ITER cross-section significantly reduces the alpha particle stabilization of the internal kink mode. The fishbone mode is found to be stable at the nominal alpha beta value. Results will be presented on linear and nonlinear evolutions of alpha particle-driven Alfv\'en eigenmodes. [Preview Abstract] |
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LP1.00044: PLASMA SIMULATION II |
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LP1.00045: Dynamical Origin of Small-Scale Intermittent Density Fluctuations in Interstellar Medium K.W. Smith, P.W. Terry Pulsar radio signal broadening by interstellar turbulence is consistent with non Gaussian density fluctuations at small scales. We examine physical mechanisms responsible for the inferred intermittency from a model for kinetic Alfv\'{e}n wave (KAW) turbulence. KAW turbulence characterizes density fluctuations at scales smaller than the ion gyroradius. Intermittency at these scales requires a mechanism for supporting localized structures against turbulent mixing in the absence of significant flows or ion forces. We first investigate decaying turbulence by formulating a two-time scale description of slowly evolving localized current filaments in rapidly evolving KAW turbulence. If the magnetic field shear of a filament is sufficiently strong, refraction of random kinetic Alfv\'{e}n waves localizes them to the edge of the filament and strongly reduces the anomalous diffusion of the filament, allowing the filament to remain coherent. This process also applies to the density perturbation associated with the current filament. A condition for coherency is derived for comparison with simulation data. Work supported by NSF [Preview Abstract] |
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LP1.00046: Importance of Magnetic Field Configurations in the Structure of Self-gravitating Disks G. Bertin, B. Coppi, F. Rousseau Magnetic fields created by internal currents have been been found to be important in determining the physical characteristics and the geometry of self-gravitating disks in which a plasma component exists and the magnetic field pressure is comparable to the plasma pressure. The relatively strong radial modulation of the plasma density that has been found in the case of accretion disks, where the gravity of the central object is prevalent, indicates that this modulation has a significant effect on the gravity field that is prevalent in self-gravitating disks where the magnetic energy density is significant. The problem of finding the relevant axisymmetric equilibrium configurations requires the solution of three coupled equations: the gravitational Poisson's equation and the non-linear radial and vertical equilibrium equations according to the lines indicated in Refs.\footnote{B. Coppi, \textit {Phys. of Plasmas} \textbf{12}, 057302 (2005).} and \footnote {B. Coppi and F. Rousseau, M.I.T. LNS Report HEP 05/01, Cambridge, MA, June 2005. To be submitted to the $Astrophysical Journal$.}.\\ $^*$Sponsored in part by the U.S. Department of Energy [Preview Abstract] |
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LP1.00047: Plasma Accretion Disks with Comparable Thermal and Magnetic Energy Densities B. Coppi, F. Rousseau An important class of plasma accretion disks is that of relatively cold disks where the energy density of the magnetic field in which they are imbedded can be comparable with their thermal energy density (finite-$\beta$). We have considered, in particular, the important case where the field produced by currents inside the disk is comparable with the external magnetic field. We have found that the ``crystal structure'' of the magnetic field, identified in Ref.\footnote{B. Coppi, \textit{Phys. Plasmas} \textbf{12}, 057302 (2005).} for weak magnetic energy densities, can persist but that the plasma density becomes strongly modulated in the radial direction with periods that are fractions of that of the plasma current density. The vertical cross section of the magnetic crystal structure can be visualized as a string of spheromak configuration. The difference from real spheromaks is that here each pair of current filaments involves counterstreaming currents around the symmetry axis of the disk rather than by a single current channel streaming around the axis of spheromak. The gradient of the rotation frequency has a key role in determining the structure of this equilibrium configuration and can be considered the source of it as this corresponds to a marginally stable ballooning mode\footnote{B. Coppi and P.S. Coppi, \textit{Phys. Rev. Letters} \textbf{87}, 051101 (2001).} when the linearized approximation is valid. \\ $^*$Sponsored in part by the U.S. Department of Energy [Preview Abstract] |
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LP1.00048: On the Possibility of Observation of Magnetorotational Instability in High Temperature Plasmas V.I. Pariev, V.V. Mirnov, S.C. Prager Magnetorotational instability (MRI) is believed to be crucial for understanding of the origin and amplification of magnetic field in gravitationally collapsing objects such as accretion disks. Experimental verification of MRI in laboratory experiments is an active area of research. There are several efforts to observe MRI in liquid metal devices. High temperature laboratory plasmas attract less attention because strong magnetic fields suppress MRI in magnetically confined systems. We present linear eigenmode analysis of instabilities related to MRI and driven by sheared rotational flows in cylindrical plasmas. In the vicinity of the rational surfaces with k$_{\vert \vert }$ = 0, magnetic field is not strongly perturbed allowing for excitation of a spatially localized version of MRI under the condition similar to Suydam criteria, k$_{\vert \vert }^{2 }$V$_{A}^{2}<\vert $d $\Omega ^{2}$ /d lnR $\vert $. The shear of the magnetic field imposes a severe restriction on the rotational velocity needed for instability. We analyze modifications of this criteria and the role of resistive effects and large gradients of rotational flow in the dynamics of the instability. Applications for Madison Symmetric Torus reversed field pinch experiments are discussed. [Preview Abstract] |
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LP1.00049: Electron beam propagation in insulator: the effects of polarization current caused by electric field ionization Boris Frolov, Andreas Kemp, Sergei Krasheninnikov, Tom Cowan Experimental data show a strong filamentation of intense electron beam propagating through insulators [1] while the beam remains rather smooth propagating though conductors. A very strong electric field formed at the beam's head in an insulator, which causes its ionization due to tunneling effects. In Ref. 2 it was shown that the instability of the ionization front caused by the electric field ionization process might be the reason of such filamentation. However, in Ref. 2 the effects of the polarization current associated with the electric field ionization was not accounted for, while they may be important. Here we present the results of the study of these effects on the structure and stability of the ionization front. We also perform the scoping study of the impact of the parameters characterizing the insulator material on the ionization front speed. [1] L. Gremillet, et al., PRL 83, 5015 (1999), J. Fuchs, et al., PRL 91, 255002 (2003), R.B. Stephens, et al., PR E 69, 066414 (2004). ; [2] S. I. Krasheninnikov, A. V. Kim, B. K. Frolov, and R. Stephens, PoP, July (2005) [Preview Abstract] |
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LP1.00050: Ad-hoc KEEN-type Waves and their Occasional Resemblance to KdV Waveforms Yuriy Tyshetskiy, Tudor Johnston, Bedros Afeyan Nonlinear kinetic waves of the KEEN type [1] but constructed with two BGK recipes are tested with 1D Vlasov-Poisson simulation (1DVPS). One is that of Allis [2] as modified by Johnston (unpublished), the other is that of Eliasson and Shukla [3]. Strong kinetic waves survive well, but not weaker ones. The potential wave trains resemble those from the Korteweg-deVries equation. This proves to be natural when charge density variation with electrostatic potential is like a quadratic polynomial. For expositions on the physics of ponderomotively~ driven KEEN waves, consult presentations by Afeyan and Savchenko, this conference. (Part of this work was performed under the auspices of the U.S. Department of Energy under grant number DE-FG03-NA00059.) [1] B. Afeyan et al., ``Kinetic Electrostatic Electron Nonlinear (KEEN)~Waves and their interactions driven by the ponderomotive force of~crossing laser beams'', Proc. IFSA (Inertial Fusion Sciences and~Applications 2003, Monterey, CA), 213,~ B. Hammel, D. Meyerhofer, J.~Meyer-ter-Vehn and H. Azechi, editors, American Nuclear Society, 2004. [2] W.P. Allis, paper 3 (pp.21-42), in ``In Honor of Philip M. Morse'', ed. H. Feshbach and K. Ingard, MIT Press (1969). [3] B. Eliasson and P.K. Shukla, Phys. Rev. E 71, 046402 (2005) [Preview Abstract] |
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LP1.00051: KEEN Wave Dynamics: Self-Consistent Trapping Multimode Field-Trapped Particle Nonstationary Dynamical States Kirk Won, Bedros Afeyan, Vlad Savchenko, Philip Morrison, Tudor Johnston We use Vlasov-Poisson and Vlasov-Maxwell simulations to model the interaction of ponderomotively driven KEEN waves (1) and Electron Plasma Waves. We focus on categorizing and statistically describing the distinguished sets of particle orbits which make up KEEN waves. These involve nonlocal trapping-untrappping-retrapping oscillations in space and time. We also study the action conservation properties of trapped orbits and their energy exchange mechanism with their self consistent trapping electric field. These new nonlinear states have potential impact on the nonlinear saturation of parametric instabilities in laser-produced plasmas in short and long laser pulse regimes. \newline \newline (1) B. Afeyan et al., Proc. IFSA (Inertial Fusion Sciences and Applications 2003, Monterey, CA), 213, B. Hammel, D. Meyerhofer, J. Meyer-ter-Vehn and H. Azechi, editors, American Nuclear Society, 2004. [Preview Abstract] |
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LP1.00052: Simulation of Alpha-Channeling in Mirror Machines Andrey Zhmoginov, Nathaniel Fisch If suitable diffusion paths are defined in the coupled velocity-configuration space, an alpha-channeling effect can be obtained by shining waves resonating with alpha particles in mirror machines. To find the most efficient way to extract energy from alpha particles, different regimes of the wave-particle interactions and different parameters of the system are explored computationally. Computational models of different degrees of accuracy and complexity are used, the most rigorous of which is the computational solution of a full set of dynamic equations. The results obtained through different models are compared, and the applicability of standard approximations is determined. In light of these computations, the feasibility of implementing this concept is discussed. [Preview Abstract] |
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LP1.00053: Adaptive Mesh Refinement for ICF Calculations David Fyfe, C. Richard Devore This paper describes our use of the package PARAMESH to create an Adaptive Mesh Refinement (AMR) version of NRL's FASTRAD3D code. PARAMESH was designed to create an MPI-based AMR code from a block structured serial code such as FASTRAD3D. FASTRAD3D is a compressible hydrodynamics code containing the physical effects relevant for the simulation of high-temperature plasmas including inertial confinement fusion (ICF) Rayleigh-Taylor unstable direct drive laser targets. These effects include inverse bremmstrahlung laser energy absorption, classical flux-limited Spitzer thermal conduction, real (table look-up) equation-of-state with either separate or identical electron and ion temperatures, multi-group variable Eddington radiation transport, and multi-group alpha particle transport and thermonuclear burn. Numerically, this physics requires an elliptic solver and a ray tracing approach on the AMR grid, which is the main subject of this paper. A sample ICF calculation will be presented. \newline \newline MacNeice et al., ``PARAMESH: A parallel adaptive mesh refinement community tool,'' Computer Physics Communications, 126 (2000), pp. 330-354. [Preview Abstract] |
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LP1.00054: The formation stage of collisionless shocks Michael Marti, Gianfranco Sorasio, Ricardo Fonseca, Luis Silva, Warren Mori Recently we have shown that it is possible to launch collisionless shocks by interaction of ultra intense lasers with solid targets [1]. In this work, we explore the physics of the formation stage of collisionless shock structures by comparing the results of the numerical simulations carried out with osiris 2.0 with our theoretical predictions. The different driver mechanisms - i.e. laser vs. piston - are discussed and compared, and critical parameters such as jump conditions, electron temperature, electron trapping, and shock velocity are examined. The influence of the plasma parameters on the maximum shock velocity is explored in the regime of different plasma compositions and target shapes, and the results are theoretically investigated. The laser/piston - induced shocks are also probed in absence of a preformed plasma by shutting the laser on a non-ionized target. The effects of collisions on the shock structure and properties are investigated. Shock guiding and front control is explored through shaping of the density/temperature distribution in the targets. [1] Luis O. Silva, Michael Marti, Jonathan R. Davies, et al, Phys. Rev. Lett. 92, 015002 (2004) [Preview Abstract] |
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LP1.00055: Simulation of the Generation of Low Frequency Radiation From Argon Clusters lluminated by High-Intensity Short Pulse Lasers Clay Cordova, James Cooley, Ki-Yong Kim The interaction of high-powered lasers with small plasma clusters is of interest due to its range of applications including the generation of fast ions and electrons for advanced accelerators, self-focusing phenomenon in optics, and production of x-ray and extreme ultraviolet (EUV) radiation. We simulate the interaction of high-intensity lasers with solid density clusters using the fully electromagnetic PIC code TurboWAVE$^{2}$. We analyze a range of cluster sizes, laser intensities, and pulse durations to investigate the dependence of low frequency radiation production on these parameters. In this poster, we illustrate the results of this study. In particular, we present calculations of the energy absorbed and released from the cluster, as well as an analysis of the far-field radiation distribution, intensity, and power spectrum. Finally, we present conclusions that may guide future simulations and experiments. 1. ccor@lanl.gov 2. D. Gordon et al. IEEE TRANSACTIONS ON PLASMA SCIENCE, \textbf{28 }(4), 8/2000, 1135 [Preview Abstract] |
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LP1.00056: Simulations of particle beam heating of foils for studies of warm dense matter J.J. Barnard, G.E. Penn, J.S. Wurtele, A. Friedman, P. Santhanam, M.M. Marinak, S.S. Yu, R.M. More We present simulations of particle beam heating of target foils using the multiphysics radiation hydrodynamics code HYDRA$^{\ast \ast }$. We simulate possible targets for a near-term experiment at LBNL (the so-called Neutralized Drift Compression Experiment, NDCX) and possible farther-term experiments on a proposed facility (NDCX-II) for studies of warm dense matter. Simulation results are presented showing the degree of temperature uniformity and the maximum temperature expected. Various target materials (including aluminum, aluminum foam, water ice, and gas jets) and target configurations are presented. Strategies for characterizing the material equation of state, using data from the experiments together with simulations, will be discussed. Requirements on the NDCXII accelerator, based on target considerations will also be discussed. **M. M. Marinak, G. D. Kerbel, N. A. Gentile, O. Jones, D. Munro, S. Pollaine, T. R. Dittrich, and S. W. Haan, Phys. Plasmas \textbf{8}, 2275 (2001). [Preview Abstract] |
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LP1.00057: Interplay of collisions with quasilinear growth rates of relativistic e-beam driven instabilities in a superdense plasma C. Deutsch, P. Fromy, A. Bret, M.C. Firpo We focus attention on the rapidly growing electromagnetic (EM) instabilities arising in the interaction of intense and relativistic electron beams (REB) with supercompressed thermonuclear fuels. REB-target system is taken neutral in charge and current with a distribution function featuring beam and target temperatures. The EM filamentation (Weibel) instability is first considered in a linear approximation. Relevant growth rates then highlight density beam/target density ratio and beam transverse temperature. Significant refinements include mode-mode coupling and collisions with target electrons. The former qualify the quasilinear and weakly turbulent approach. Usually, it yields significantly lower growth rates than linear ones. Collisions enhance them slightly for small wavenumber k and damp them strongly at large k. In a low temperature target plasma, intrabeam scattering also contributes to instability taming while keeping it close to zero in a warm plasma. Our parameters study provides further support to the cone-angle (Osaka experiment) configuration with REB penetrating close to the compressed fuel dense core. [Preview Abstract] |
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LP1.00058: Bent Marshak Waves Omar Hurricane, James Hammer Radiation driven heat waves (Marshak Waves\footnote{Marshak, R.E., Phys. Fluids, {\bf 1}, 24, (1958)}) are ubiquitous in astrophysics and terrestrial laser driven high energy density plasma physics (HEDP) experiments. Generally, the equations describing Marshak waves are so nonlinear, that solutions involving more than one spatial dimension require simulation. However, in this paper we show how one may analytically solve the problem of the two-dimensional nonlinear evolution of a Marshak wave, bounded by lossy walls, using an asymptotic expansion in a parameter related to the wall albedo and a simplification of the heat front equation of motion.\footnote{J.H. Hammer and M.D. Rosen, Phys. Plasmas, {\bf 10}, 1829 (2003)} Three parameters determine the nonlinear evolution, a modified Markshak diffusion constant, a smallness parameter related to the wall albedo, and the spacing of the walls. The final nonlinear solution shows that the Marshak wave will be both slowed and bent by the non-ideal boundary. In the limit of a perfect boundary, the solution recovers the original diffusion-like solution of Marshak. The analytic solution will be compared to a limited set of simulation results and experimental data. [Preview Abstract] |
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LP1.00059: A Spherical Laser System For Inertial Confinement Fusion Catalin Filip, Patric Muggli The possibility to ignite nuclear fusion through implosion of a spherical pellet within a spherical laser cavity is examined theoretically. The resonator is formed by the pellet (and later the pellet plasma) and a spherical mirror (M). The gain medium is a spherical shell placed on the inside of this mirror. The medium is optically pumped from the outside on a nanosecond time-scale with pump beams that are uniformly distributed across mirror M. This mirror is a dichroic designed to transmit the pump pulses and trap the radially-propagating laser radiation within the resonator. In this system, as opposed to the direct drive scheme, the illumination uniformity of the pellet is determined by the resonator itself (not by the distribution of the laser beams), and the laser energy reflected by the plasma (that is normally lost) is recirculated and amplified to intensities $>10^{15} W/cm^{2}$, sufficient for ablation- driven compression of the fuel pellet to ignition conditions. [Preview Abstract] |
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LP1.00060: Laser-Driven Magnetic Field Compression N. Jang, J.P. Knauer, R. Betti, D.D. Meyerhofer An experiment was designed to compress magnetic fields to ultrahigh intensities through laser-driven implosions. A seed axial magnetic field is produced through two Helmholtz coils using a capacitor charged by the hot electrons produced by an intense laser pulse as a power supply. The seed-field generation circuit is designed to produce an initial field of several Tesla (5--10 T) inside a cylindrical CH shell. The plastic shell is then imploded by direct laser irradiation with a 23-kJ laser pulse. Two implosion pulse shapes have been considered: a square pulse and a shaped, low-adiabat pulse. One-dimensional simulations of the magnetic field compression resulting from the shell convergence show magnetic field amplifications of 300 for the square pulse and 1000 for the shaped pulse, thus leading to peak magnetic fields of 3 $\times $ 10$^{3}$ T and 10$^{4}$ T, respectively (for a 10-T seed). Details of the experimental design and simulations are presented, and the experimental plans for implementation are outlined. This experiment is intended to study ways to improve the hot-spot energy confinement through magnetic insulation. This work has been supported by the US-DOE under grant ER54768 and under cooperative agreement DE-FC52-92SF19460. [Preview Abstract] |
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LP1.00061: Design of a flux compression experiment on Atlas V. Makhin, B.S. Bauer, T.J. Awe, S. Fuelling, T. Goodrich, I.R. Lindemuth, R.E. Siemon, W.L. Atchison, R.J. Faehl, R.E. Reinovsky, D.W. Scudder, P.J. Turchi A possible plasma target for Magnetized Target Fusion is a stable diffuse z pinch like that of the MAGO experiments at VNIIEF. In this case plasma would reside in a toroidal cavity, eg., between two cylindrical walls with end planes. Compressing magnetic flux inside a chamber of that geometry is the purpose of the Atlas experiment. The outer wall or ``liner'' will be an aluminum cylinder $\sim $ 2-mm thick that is imploded by Atlas current flowing from the end walls (``glide planes'') to the outside of the liner. Flux for compression will be introduced by momentarily diverting a small amount of Atlas current using a shunt resistor, which avoids the cost and complexity of an auxiliary power supply. Modeling of experimental parameters will be described based on results from three codes: a) a semi-analytic ode incompressible liner model, b) the Los Alamos RAVEN 1D Lagrangian code, and c) the Los Alamos 1D or 2D MHRDR Eulerian code. [Preview Abstract] |
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LP1.00062: Dynamics of a thick liner for Magnetized Target Fusion R.E. Siemon, T.J. Awe, B.S. Bauer, S. Fuelling, T. Goodrich, I.R. Lindemuth, V. Makhin Liner inertia determines an important limit to the burn time and gain of an MTF system. Assuming as much material density as possible for the liner, the variable that determines inertia is liner thickness. Liner thickness is also important to achieve high liner velocity because of the burst condition (a limit on acceleration by magnetic fields because ohmic heating can boil the liner). Thick liners have interesting dynamics and compressional energy of the material becomes an important consideration. Early work by Gerwin and Malone (Nuclear Fusion \underline {19}, 155 (1979)) gives an analytic model that estimates the compressional energy and associated inefficiency. They find that an optimized system should allow 70{\%} of a liner's kinetic energy to convert into plasma energy, with 30{\%} going into liner compression. Numerical MHD models can also be used to calculate liner dynamics and the effects of compression. Results from numerical models will be compared with the analytic results, and the implications for high gain in MTF systems will be discussed. [Preview Abstract] |
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LP1.00063: Dense Hypervelocity Plasma Jets for Fusion Applications F. Douglas Witherspoon, Y.C. Francis Thio High velocity dense plasma jets are being developed for a variety of fusion applications, including refueling, disruption mitigation, High Energy Density Plasmas, magnetized target/magneto-inertial fusion, injection of angular momentum into centrifugally confined mirrors, and others. The technical goal is to accelerate plasma blobs of density $>$10$^{17}$ cm$^{-3}$ and total mass $>$100 micrograms to velocities $>$200 km/s. The approach utilizes symmetrical injection of very high density plasma into a coaxial EM accelerator having a tailored cross-section that prevents formation of the blow-by instability. AFRL MACH2 modeling identified 2 electrode configurations that produce the desired plasma jet parameters. The injected plasma is generated by up to 64 radially oriented capillary discharges arranged uniformly around the circumference of an angled annular injection section. Initial experimental results are presented in which 8 capillaries are fired in parallel with jitter of $\sim $100 ns. Current focus is on higher voltage operation to reduce jitter to a few 10's of ns, and development of a suite of optical and spectroscopic plasma diagnostics. [Preview Abstract] |
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LP1.00064: Theory, Simulation, and Design of a High-Brightness Heavy-Ion Beam System Ronak Bhatt, Tom Bemis, Chiping Chen, Jing Zhou A method is presented for the design of a high-brightness heavy-ion beam system. The recent elliptical beam diode theory of [1] is applied to form a laminar elliptical heavy-ion beam. A technique is presented to ideally match this beam from the diode into a periodic magnetic quadrupole focusing channel. A realization of the magnetic focusing system is implemented using Opera3D. The beam system design is verified with 3D OMNITRAK simulations. Applications of such beams in high-energy density physics research are discussed. [1] R. Bhatt and C. Chen, Phys. Rev. ST Accel. Beams 8, 014201 (2005). [Preview Abstract] |
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LP1.00065: Robust Method for Evaluating Ionization, Charge Exchange and Stripping Cross Sections in Atom-Ion Collisions Thomas Bender, Igor D. Kaganovich, Ronald C. Davidson Ion-atom charge-changing cross sections are needed in many applications employing the propagation of fast ions through matter. A hybrid method has been developed for calculation of the charge-changing cross sections of ions or atoms by fast ions by combining the quasi-classical approach and the Born approximation of quantum mechanics in the regions of impact parameters in which they are valid, and summing the results to obtain the total cross section [1,2]. As a result, typical computations take only few minutes. This has been tested by comparison with available experimental data and full quantum mechanical calculations. A new scaling formula for the ionization and stripping cross section of atoms and ions by fully stripped projectiles has also been developed [1]. \newline \newline [1] I. D. Kaganovich, E. A. Startsev and R. C. Davidson, ``Formulary and scaling cross sections for ion-atom impact ionization,'' http://arxive.org/abs/physics/0407140. \newline [2] Igor D. Kaganovich, et. al., Nucl. Instr. and Methods A \textbf{544}, 91 (2005). [Preview Abstract] |
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LP1.00066: Accuracy issues in spectroscopic modeling of K$_{\alpha}$ emission from M-shell ions in dense plasmas Stephanie Hansen, Hyun Chung, Mau Chen Although K$_{\alpha}$ emission originates from simple $1s-2p$ transitions, the many-electron ions of mid-$Z$ materials in warm, dense matter conditions introduce significant computational complexity to K$_{\alpha}$ spectroscopic modeling. First, complete models of M-shell ions in dense plasmas are inherently complex since they must include a large number of states with open $3p$ and $3d$ shells. Next, single-temperature models for collisional-radiative kinetics are inadequate since the thermal electrons that control the distribution of charge states in the M shell have insufficient energy to participate in inner-shell processes. Finally, near-solid densities introduce physical effects such as pressure ionization, the formation of quasi-bound states, and line broadening, which are not intrinsically included in the isolated-ion structure calculations used in most spectroscopic models. These issues are explored for K$_{\alpha}$ emission from M-shell Cu using several independent models. [Preview Abstract] |
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LP1.00067: BASIC PLASMA: RECONNECTION, LOW TEMPERATURE PLASMA, DUSTY PLASMA, STRONGLY COUPLED PLASMA |
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LP1.00068: Intense Electrostatic Field Structures in Low-$\beta$ Magnetic Reconnection William Daughton, Jack Scudder Kinetic simulations of low-$\beta$ reconnection indicate the formation of intense electrostatic fields that start at the x-point and form sheet-like structures that extend outward for large distances along the separatrices. The characteristic thickness of these layers is on the order of the local electron gyroradius and there are significant deviations from charge neutrality within the layer. The resulting electrostatic fields are primarily perpendicular and may exceed the reconnection electric field by a factor of 20. These intense electric fields are sufficient to demagnetize electrons across the layer and induce significant off- diagonal components in the electron pressure tensor. Many of these basic features are consistent with recent high-time resolution observations from the Polar satellite in the vicinity of suspected reconnection sites. In this work, we employ a combination of Vlasov theory in conjunction with large-scale kinetic simulations, to examine the formation mechanism and role these structures may play in setting the overall dissipation rate and/or accelerating particles. [Preview Abstract] |
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LP1.00069: Electron Gyroradius Scale Electric Field Enhancements in Space: Sites for Demagnetization? Jack Scudder, William Daughton, F.S. Mozer Intense ($>$100mV/m), ms, quasi-perpendicular Electric Field Enhancements (EFEs) are surveyed from 3 years of NASA's Polar data. EFEs are 2-D, quasi-stationary with spatial scales $\lambda_{de} \rightarrow d_e$, with one measured scale of $7 \lambda_{De} \approx \rho_e$. EFE’s from this sample occur exclusively with $10^{-8} < \beta_e < 3 \times10^{-2}$, with a distribution of peak electric to magnetic force on a thermal electron ($\Gamma=c E_\perp /w_e B$) that breaks sharply at $\Gamma=0.11$. For a given B this threshold corresponds to a $\rho_e$ scale EFE with $E_\perp$ sufficient to “stretch” $P_{\perp e}$ by 50\%. All EFEs with $\Gamma \ge 0.1$ were considered possible agents for demagnetization of the electron fluid, or DEFEs. Although EFEs are found at all magnetic local times, magnetic latitudes and radial distances of the Polar orbit, they were confined within $10^\circ$ of the invariant latitudes of the Earth's cusps. By contrast DEFEs are strongly concentrated near local magnetic noon and at Polar's apogee, invariably in magnetopause current layers. DEFEs are only found in $10^{-4} < \beta_e < 3\times10^{-2}$ and where $\lambda_{De} \le \rho_e$. DEFEs are consistently understood as sites where electrons can be demagnetized by strongly inhomogeneous electric fields and are found at historically identified locales for magnetic reconnection. Simulations from Harris sheet equilibria are used to test the properties of fully resolved, analogue “PIC” EFEs for their (i) thickness along their normal; (ii) $\Gamma$ distribution; and (iii) ability to demagnetize electrons. [Preview Abstract] |
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LP1.00070: Gyrokinetic simulations of magnetic reconnection Barrett Rogers, Paolo Ricci, Tomo Tatsuno, William Dorland We present nonlinear gyrokinetic simulations of magnetic reconnection in the presence of a strong guide field. The simulations are based on the GS2 code and explore reconnection in a simple collisionless two-dimensional periodic slab geometry. The GS2 code treats both the electrons and the ions gyrokinetically, and includes effects such as trapped particles and out-of-plane magnetic field perturbations due to finite plasma beta. The linear and nonlinear gyrokinetic results are compared to two-fluid and particle simulations of the same system, and the linear growth rates are benchmarked against various analytic calculations. Important effects to be addressed include the dependence of the reconnection rate on the ion-to-electron temperature ratio, the plasma beta, the simulation box geometry, as well as the electron/ion energy branching ratio. [Preview Abstract] |
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LP1.00071: Guide field magnetic reconnection using the GS2 code T. Tatsuno, W. Dorland, J.F. Drake, M.A. Shay, B.N. Rogers, P. Ricci The processes that control magnetic reconnection in the limit of weak and strong guide field differ in a number of important ways. As the guide field becomes strong, the role of whistler dynamics is superseded by the physics of the kinetic Alfven wave [1], and the reconnection electric field becomes nearly aligned with the total magnetic field. Recent particle simulations of guide field reconnection [2] have shown that the formation of electron density cavities along the separatrices and the associated electric fields in these regions are an important source of high energy electrons. Other effects that are strongly influenced by the guide field include secondary island formation and diamagnetic stabilization. As a follow-up to the GEM challenge study of the zero guide field case [3], here we will elucidate the physics of guide- field reconnection using the GS2 code. This code was originally developed for tokamak microturbulence simulations and evolves the ion and electron distribution functions in a five dimensional phase space [4]. The GS2 simulations will be compared to the results from PIC and Hall MHD models. [1] B.N. Rogers et al., Phys. Rev. Lett. 87, 195004 (2001). [2] J.F. Drake et al., Phys. Rev. Lett. 94, 095001 (2005). [3] J. Birn et al., J. Geophys. Res. 106, 3715 (2001). [4] W. Dorland et al., Phys. Rev. Lett. 85, 5579 (2000). [Preview Abstract] |
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LP1.00072: Study of Two-fluid Effects during Magnetic Reconnection in a Laboratory Experiment M. Yamada, H. Ji, S. Gerhardt, M. Inomoto*, R. Kulsrud, A. Kuritsyn, Y. Ren This paper highlights the most recent findings on the two-fluid dynamics in the reconnection layer of the MRX (Magnetic Reconnection Experiment) plasmas. This paper primarily addresses the two-fluids MHD physics of magnetic reconnection and the results are compared with the recent space observations. As our experimental operation regime has moved from the collisional to the collision-free regime, two-fluid effects have become more evident. The recent progress of our understanding of our experimental research based on our two-fluid MHD analysis is presented to illuminated the physics of the Hall MHD in a collision-free reconnection layer. In particular, a clear experimental verification of an out-of-plane Hall quadrupole field has been made [1] in a Harris-like neutral sheet [2], with the width comparable to the ion skin depth, during magnetic reconnection. Also high frequency fluctuations observed in the reconnection layer exhibit two fluid effects demonstrating different kinematics for electrons and ions [3]. Finally, interrelationship between the observed fast reconnection rate, magnetic turbulence and the Hall quadrupole fields are discussed. Work supported by DoE, NSF, and NASA. [1] Y. Ren et al, Phys. Rev. Letts. August 12 issue (2005) [2] M. Yamada et al., Phys. Plasmas 7, 1781 (2000) [3] H. Ji et al., Phys. Rev. Letts. V.92, 115001 (2004) *Visiting from Osaka Univ. [Preview Abstract] |
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LP1.00073: Reconnection and Ion Acceleration Processes during Counter Helicity Merging of Spheromaks in the MRX M. Inomoto, S.P. Gerhardt, M. Yamada, H. Ji, B. McGeehan, A. Kuritsyn, Y. Ren, E. Belova A series of experiment studying FRC formation has recently been carried out in the Magnetic Reconnection Experiment (MRX) using counter-helicity merging of two spheromaks [1,2]. In MRX, there are two different counter-helicity cases: the two initial spheromaks have toroidal field polarities of $'$positive- negative$'$ and $'$negative-positive$'$ (toroidal field sign chosen with reference to the toroidal plasma current, which is in the same direction for both spheromaks). These two configurations show different features in magnetic probes, Langmuir probes and spectroscopic measurements. In one case, we observe X-point structure which is radially pushed in from the axis position of the spheromaks with significant radially outward ion flow, while in the other case, X-point structure was radially pushed out with almost no radially outward ion flow. Two-fluids effect is thought to play a significant role in determining dynamics of reconnection process in scales of or below ion skin depth. Detailed measurements using a newly fabricated high-resolution magnetic probe will be presented together with spectroscopic measurements and physics explanations. This work is supported by the US Department of Energy, the NSF, NASA and Japan Society for the Promotion of Science. [1] M. Yamada, et al., Phys. Fluids B 3, 2379-2386 (1991). [2] Y. Ono, et al., Phys. Plasmas 4, 1953-1963 (1997). [Preview Abstract] |
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LP1.00074: Three-dimensional Hall-MHD simulations of counter-helicity spheromak merging and FRC formation E.V. Belova, R.C. Davidson, H. Ji, M. Yamada, S.P. Gerhardt, M. Inomoto Initial results are presented of 2D and 3D simulations of counter-helicity spheromak merging using Hall-MHD version of the HYM code. In the code, the Hall electric field is subcycled on the ideal MHD time scale, which decreases the computational time by a factor of five compared to explicit numerical scheme. Calculations are performed for values of normalized ion skin depth comparable to that in counter-helicity spheromak merging experiments $d_i=0.03-0.05$. Hall-MHD simulation show significant differences of the radial current, toroidal magnetic field and radial flow profiles compared to the resistive MHD simulations. Depending on the polarity of the initial toroidal fields, the reconnection X-point is shifted downward or upward in radial direction, the radial current contours have ``V''-shaped (or inverted ``V'') structure, and radial component of ion flow is strongly non-symmetric relative to X-point. These results are explained by the structure of the electron flows, and found to be in agreement with MRX measurements. [Preview Abstract] |
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LP1.00075: Dissipative Processes Associated with the Oblique Lower Hybrid Drift Instabilities Yansong Wang, Russell Kulsrud, Hantao Ji Motivated by the observation of magnetic fluctuations in the current layer of the MRX an oblique LHDI theory has been developed which appears to explain them (Ji et al 2005). [A quasilinear estimate suggests that the waves are strong enough to explain the anomalous resistivity also observed in the MRX (Kulsrud et al 2005).] The theory is non dissipative. In order to understand the wave heating and nonlinear saturation, dissipative processes such as electron viscosity, thermal diffusion and possibly magnetic pumping are examined in this paper. In addition, the dissipation due to ion Landau damping is calculated. this is done for both the stable and the unstable modes as is required in a nonlinear theory. This work is supported by DOE Contract no.De-AC02-76-CH03073, NSF Award no. PHY-0215581 and NASA Grant no. SRT04-0000-0086.\\ Ji, Kulsrud, Fox, Yamada, JGR (in press) 2005.\\ Kulsrud, Ji, Fox and Yamada, PoP (in press)2005. [Preview Abstract] |
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LP1.00076: Experimental Studies of the Hall Effect and Fluctuations in MRX Y. Ren, M. Yamada, S.P. Gerhardt, H. Ji, R. Kulsrud, A. Kuritsyn, M. Inomoto Magnetic reconnection is being studied in the Magnetic Reconnection Experiment (MRX) in a well-controlled manner. The key question we wish to answer is: why the observed reconnection rate can be much larger than what the classical Sweet-Parker model predicts. In recent literature two mechanisms have often been cited for fast reconnection: anomalous resistivity generated by plasma turbulence and the Hall effect of two-fluid MHD theory. The first mechanism has been investigated in MRX and a positive correlation between resistivity enhancement and magnetic fluctuations has been found. An out-of-plane quadrupole magnetic field, the hallmark of the second mechanism, has also been observed during magnetic reconnection in MRX [1]. Recent reconnection experiments in MRX are focusing on both the Hall effect and fluctuations. Serval arrays of magnetic pickup coils (with resolution up to 1.25mm) are used to study the quadrupole magnetic field in more detail. Both electrostatic and magnetic fluctuations have been observed along with quadrupole field in the same low-collisionality discharges in MRX. The relationship between them is studied. In this paper, the recent data from MRX will be presented and a comparison between theory and experiment will be attempted. This work is supported by DOE, NASA and NSF. [1] Y. Ren et al., Phys. Rev. Lett., accepted (2005) [Preview Abstract] |
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LP1.00077: Physical Origin of the Quadrupole Out-Of-Plane Magnetic Field in Hall-MHD Reconnection Dmitri Uzdensky, Russell Kulsrud A quadrupole pattern of the out-of-plane component of the magnetic field inside a reconnection region is seen as an important signature of the Hall-MHD regime of reconnection. It has been first observed in numerical simulations (e.g., Mandt et al. 1994) and just recently confirmed in the MRX experiment (Ren et al. 2005). In this study, we analyze the physical origin of the quadrupole field and show that it can be traced to the current of electrons flowing in and out of the inner part of the reconnection region, as required by charge neutrality. We also discuss the role the quadrupole field plays in the overall dynamics of the reconnection process. [Preview Abstract] |
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LP1.00078: NIMROD Simulations of Reconnection in MRX and SSX Nicholas Murphy, Carl Sovinec Two-fluid effects are known to influence magnetic reconnection rates through non-MHD communication between the reconnection layer and the surrounding magnetic geometry [1]. To examine the interrelationship between local and global magnetic geometry, we perform NIMROD simulations of the Magnetic Reconnection Experiment (MRX) and the Swarthmore Spheromak Experiment (SSX). The geometry of MRX is nontrivial due to the presence of two flux cores, and it is necessary to depart from a logically rectangular finite element grid. The required steps to do this in NIMROD are outlined. We first consider reconnection rates in MRX using the resistive MHD model. Progress on 2-D two fluid simulations of MRX, taking advantage of a recently improved implementation [2], is reported. We also show preliminary resistive MHD simulations of spheromak merging in SSX and consider astrophysically relevant reconnection tests.\newline \newline 1. D. Biskamp, E. Schwarz, and J.F. Drake, Phys. Plasmas 4, 1002 (1997). \newline 2. C.R. Sovinec, H. Tian, D.D. Schnack, A.Y. Pankin, D.C. Barnes, this conference. [Preview Abstract] |
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LP1.00079: Theory and Role of the Inhomogeneous Electron Temperature Gradient Driven Mode C. Yarim, B. Coppi, V. Roytershteyn Magnetic shear plays a key role on the nature and topology [1] of the Inhomogeneous Electron Temperature Gradient (I.E.T.G.) mode that is radially localized around a given rational magnetic surface. The transverse wavelengths of this mode are, typically, smaller than or comparable with the ion gyroradius. Thus the resulting thermal energy transport may not be relevant to explain the observed energy confinement time but we consider it to be sufficient to affect the dynamics of drift-tearing modes [2] in high temperature regimes. In these regimes the related effects of electron Landau damping and finite electron thermal conductivity hinder the onset of drift-tearing modes in the presence of a finite electron temperature gradient [3]. Thus the transverse electron thermal conductivity resulting from I.E.T.G. modes localized in the “singular” region where magnetic reconnection can take place is shown to interfer with the effects of the longitudinal electron thermal conductivity and to broaden the reconnection region. [1] B.Coppi, M. Rosenbluth and R.Z. Sagdeev, \textit{Phys. Fluids}, \textbf{10}, 582 (1967) [2] B.Coppi, \textit{Phys. Fluids}, \textbf{8}, 2273 (1965). [3] B. Coppi, \textit{et al., Phys. Rev. Letters}, \textbf{42}, 1058 (1979); J. Drake, \textit{et al., Phys. Fluids}, \textbf{25}, 2509 (1983) [Preview Abstract] |
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LP1.00080: Multiscale Modeling Techniques for Plasma: 1D Scaling Results and Application to Magnetic Reconnection Michael Shay, J. Drake, G. Stantchev, B. Dorland We examine a novel simulation scheme called ``equation free projective integration'' which has the potential to allow global simulations which still include microscale physics, a necessary ingredient in order to model multiscale problems. Such codes could be used to examine the global effects of reconnection and turbulence in tokamaks, the Earth's magnetosphere, and the solar corona. Using this method to simulate the propagation and steepening of a 1D ion acoustic wave, we have already achieved excellent agreement between full particle codes and equation free with a factor of 20 speed-up. In this method of simulation, the global plasma variables stepped forward in time are not time-integrated directly using dynamical differential equations, hence the name ``equation free.'' Instead, these variables are represented on a microgrid using a kinetic simulation. This microsimulation is integrated forward long enough to determine the time derivatives of the global plasma variables, which are then used to integrate forward the global variables with much larger timesteps. Results will be presented of the successful application of equation free to 1-D ion acoustic wave steepening with a PIC code serving as the underlying kinetic model. Initial results of this technique applied to magnetic reconnection will also be discussed. [Preview Abstract] |
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LP1.00081: Near-wall sheath in a plasma with non-local fast electrons Vladimir Demidov, Charles DeJoseph, Jr., Anatoly Kudryavtsev It is demonstrated that the presence of a small number of fast, non-local electrons can dramatically change the thickness and electric field in the near-wall sheath. Even if the density of the non-local ``fast group,'' $n_f$, is much less than the density of the bulk electrons, $n_b$, ($n_f \sim 10^{-5} n_b$), the near wall potential can increase dramatically resulting in a comparable increase in the sheath thickness. Due to this low fractional density, the average energy (electron temperature, $T_e$) of all electrons is little changed from that of the bulk, yet the near-wall potential drop can increase to 10's of $T_e/e$. More importantly, due to the non-local nature of this group of electrons, the near-wall sheath potential does not depend on $T_e$ at all and is determined only by the energy of the fast group. In the local approximation, the near-wall sheath potential is essentially determined by the average electron energy and can be influenced by fast electrons only if they significantly alter $T_e$. This work was supported by The Air Force Office of Scientific Research. [Preview Abstract] |
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LP1.00082: Particle and energy balance in low-density plasma discharges Zoltan Sternovsky, Scott Robertson For nearly-collisionless, unmagnetized plasma discharges, it is shown that electron particle and energy balance can be found from a model that includes 1) the energy distribution of the newborn electrons, 2) the rate of heating of confined electrons by collisions with more energetic electrons, and 3) the rate at which confined electrons are lost over the confining potential barrier. The ion flux density at the wall is reduced by charge-exchange collisions, thus these ion collisions must be included in particle balance. The model is applied to a simple, low-density, hot-filament discharges such as those in the double plasma device. The plasma density, electron temperature, and confining potential are shown to have approximately the values given by the model. [Preview Abstract] |
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LP1.00083: Plasma Compression in the Periodically Oscillating Plasma Sphere R.A. Nebel, L. Chacon, E. Evstatiev, A. Marocchino, G. Lapenta, J. Park Theoretical studies P$^{1,2P}$ have suggested that a tiny oscillating ion cloud immersed in a uniform electron background may undergo a self-similar collapse that can result in the periodic and simultaneous attainment of ultra-high densities and temperatures. The oscillating ion cloud (referred to as the Periodically Oscillating Plasma Sphere or POPS) is in local thermodynamic equilibrium at all times independent of the collisionality of the plasma (i.e. these self-similar solutions are exact solutions of the Vlasov equation). Recent experimental work has demonstrated the existence of the POPS resonance. P$^{3,4P}$ However, the effect of space charge neutralization on the plasma compression remains a significant issue. An analytic formalism has shown that it is possible to program the distribution function of the injected electrons to completely mitigate space charge effects during the ion cloud collapse. If this can be achieved, the required compression ratios for POPS can be reduced and the requirement that the average electron density must be much greater than the average ion density may be elimintaed. This new formalism has been incorporated into the 1-D particle simulation code. P$^{P}$ Results will be presented. 1. R. A. Nebel, D. C. Barnes, Fusion Technology 34, 28 (1998). 2. D. C. Barnes, R. A. Nebel, Physics of Plasmas 5, 2498 (1998). 3. J. Park, R. A. Nebel, S. Stange, S. K. Murali, Physics of Plasmas 12, 056315 (2005)\textbf{.} 4. J. Park, R. A. Nebel, S. Stange, S. K. Murali, accepted for publication in Phys. Rev Lett. [Preview Abstract] |
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LP1.00084: Resonant collisionless heating in a non-uniform plasma Constantine Theodosiou, Oleg Polomarov, Igor Kaganovich The electron dynamics in low-pressure plasmas is non-local and collisionless [1]. Electron heating occurs due to resonant wave- particle interactions, namely, the transit and bounce resonances for non-magnetized plasmas and the ECR and transmission resonances for magnetized plasmas. The effects of resonances on the properties of non-uniform plasmas have been studied analytically and numerically using fast kinetic non- local approach [2]. A drastic enhancement of the power transfer into the plasma takes place at the bounce resonance if the non-uniformity of the density profile is accounted for [3]. The conditions of effectiveness of ECR and transmission resonance heating in magnetized ICP plasmas have been identified. Analytical expressions for plasma parameters to achieve the transmission resonances have been derived [4]. I. D. Kaganovich, O. V. Polomarov, C. E. Theodosiou and D. Economou. [1] Phys. Plasmas 11, 2399 (2004). [2] ``Revisiting the anomalous rf field penetration into a warm plasma'' and ``Resonant effects in a non-uniform ICP plasma'' submitted to special issue of IEEE Trans. Plasma Sci.(2005). [3] ``Enhanced collisionless heating in a non-uniform plasma'' and [4] ``Effectiveness of ECR and transmission resonant heating in ICP plasmas,'' Phys. Plasmas, accepted (2005). [Preview Abstract] |
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LP1.00085: Ohmic heating of electrically pulsed biological cells in suspension Allen L. Garner, David W. Jordan, Wilkin Tang, Y.Y. Lau, Ronald M. Gilgenbach, Michael D. Uhler Intense, ultrashort pulsed electric fields have been studied for applications ranging from killing cells in suspension to reducing tumor size [1]. Recent work at the University of Michigan has focused on applying multiple ultrawideband pulsed electric fields with chemotherapeutics to obtain a synergistic effect in killing cancer cells [2]. While bulk temperature measurements indicate that these experiments have slight to no change in temperature, there remain questions about quantifying the thermal effects so that their importance compared to the electrical effects can be assessed. To estimate the thermal effects induced by applying electrical pulses of different pulse duration and intensity to cells in suspension, we apply a recent model of the Ohmic heating of spherical particulates with an arbitrary electrical conductivity exposed to an rf electric field [3]. The possible relevance of these results with regard to pulse-induced thermal effects will be discussed. [1] K. H. Schoenbach, et al. Proc. IEEE. \textbf{92}, 1122 (2004). [2] D. W. Jordan, et al. Electromed 2005, Portland, OR, pp. 89-90. [3] W. Tang, et al. J. Appl. Phys. \textbf{97}, 114915 (2005). [Preview Abstract] |
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LP1.00086: Non-Local Property of a Hall-MHD Contact Discontinuity Eliezer Hameiri A contact discontinuity may model a transport barrier or a plasma-vacuum interface. One outstanding question concerning such a discontinuity is whether, in a two-fluid plasma, the electron fluid is allowed to cross the discontinuity even if the ion fluid does not (because of the ``contact'' nature). Indeed we find that such a situation is possible in Hall-MHD, implying that the discontinuity problem is a global problem where in order to preserve charge neutrality in the volume bounded by the contact discontinuity, electrons leaving the volume at one point have to be compensated for by electrons crossing in the opposite direction at another point. We carry out a full linear treatment of a toroidal plasma, deriving also stability criteria that conflict with some previous results$^{1,2}$ which ignored the issue of non-locality and its implication for the appropriate jump conditions. \begin{enumerate} \item U. Schaper, J. Plasma Phys. {\bf 30}, 169 (1983). \item P. Rosenau et al., J. Plasma Phys. {\bf 21}, 385 (1975). \end{enumerate} [Preview Abstract] |
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LP1.00087: Effects of different electrode configurations in the plasma rotation of Magneto Bernoulli eXperiment (MBX) H.J. Quevedo, P.M. Valanju, Jeremy Murphy, Roger D. Bengtson MBX is an experiment that combines a small mirror machine and a radial electric field to create a rotating plasma. In the present stage of the experiment a low density plasma has been successfully rotated at supersonic speeds using a 1kV-80mF capacitor bank with currents of order 10 amps. However, certain conditions turn out to be inefficient in rotating the plasma because the external potential does not fully penetrate inside the plasma. The magnetic lines do not behave as equipotentials which seriously limits the rotation speed of the bulk plasma. Different configurations of electrodes have been used and results of the characteristics of the plasma profiles achieved will be shown. We will present data describing asymmetry between electrode polarities, smothing effects of electrode sheaths, fluctuations generated by shear flow, and overall penetration of the applied potential. The set of plasma measurements consists of Langmuir, Mach and floating potential probes, photo diodes and spectroscopy. [Preview Abstract] |
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LP1.00088: Suppression of Neoclassical Tearing Modes with ECCD Francois Waelbroeck Electron Cyclotron Current Drive (ECCD) is routinely used on several tokamaks to suppress the Neoclassical Tearing Mode (NTM). The conventional explanation of the suppression is that it is achieved through the replacement by the ECCD of the bootstrap current lost through the flattening of the pressure profile inside the island. This explanation appears to conflict, however, with the observation that both co and counter ECCD have a stabilizing effect of comparable magnitude. In order to apply the ECCD method of NTM suppression to next-step devices it is clearly necessary to better understand the mechanism responsible for the suppression. This is necessary, in particular, in order to deduce reliable estimates of the required power in ITER. Here we reexamine the effect of ECCD and Electron Cyclotron Heating (ECH) on rotating magnetic islands using the Braginskii model. We consider in particular the effect of the EC wave absorption on the island rotation frequency and the possible effect of the modification of this frequency on island suppression. [Preview Abstract] |
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LP1.00089: Numerical Simulation of Two-Fluid Dynamo Effects Driven by Tearing Instabilities Hao Tian, Carl Sovinec Simulations of the nonlinear evolution of current-driven tearing instabilities in slab geometry are run to investigate the two-fluid dynamo effects. The study is conducted with the NIMROD code [1] applied in the large guide-field limit. In the $\beta =0$ cold plasma case, the reported simulations used an unphysically small equilibrium scale length $L$ that is much smaller than the ion skin depth $c/\omega _{pi} $ to emphasize two-fluid effects. The hot plasma case with finite $\beta $ values and ion sound gyroradius larger than the resistive skin depth is also considered. We verify the role of dispersive whistler and kinetic Alfven dynamics in decoupling the ion and electron flow at small scales, which allows the fast connection in the two-fluid plasmas. Linear simulation results are compared with two-fluid quasilinear calculations [2], which show that the dynamo effect driven by the $<\vec {J}\times \vec {B}>$ Hall term is greater than the contribution from the MHD dynamo term, $<\vec {V}\times \vec {B}>$, in the narrow electron layer. We also present two-fluid dynamo effects through the nonlinear saturation stage where the Hall contribution broadens to the ion spatial scale. \newline [1] C.R.Sovinec \textit{et al}., J.Comput.Phys, 355 (2004) \newline [2] V.V.Mirnov \textit{et al}., Phys.Plasmas 11, 4468 (2004) [Preview Abstract] |
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LP1.00090: General expression of the gyroviscous force J.J. Ramos Assuming only small gyromotion periods and Larmor radii compared to any other time and length scales, and retaining the lowest significant order in the small Larmor radius asymptotic expansion, the general expression of the ion gyroviscous stress tensor is presented. This expression covers both the ``MHD'' ordering where the time derivative and the ion gyroviscous stress are first order in the ratio between the ion Larmor radius and other lengths relative to the ion gyrofrequency and scalar pressure respectively, and the ``drift'' ordering where the time derivative and ion gyroviscous stress are respectively second order. This general stress tensor applies to arbitrary collisionality and does not require the distribution function to be close to a Maxwellian. Its exact divergence (gyroviscous force) is written in closed vector form, allowing for arbitrary magnetic geometry, parallel gradients and flow velocities. Considering in particular the contribution from the velocity gradient (rate of strain) term, the final form of the momentum conservation equation after the ``gyroviscous cancellation'' and the ``effective renormalization of the perpendicular pressure by the parallel vorticity'' is precisely established. [Preview Abstract] |
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LP1.00091: QIP3D: A Code to Solve the Quiet Implicit PIC (QIP) Moment Equations in Toroidal Geometry Dave Nystrom A computer program, QIP3D, has been developed to solve the Quiet Implicit Particle-in-cell (QIP) moment equations in three-dimensional toroidal geometry. This model provides an efficient algorithm for computing the time evolution of the full two-fluid (ion/electron) plasma. A Fourier, pseudospectral representation is employed for the poloidal and toroidal angles and finite differencing for the radial coordinate. The QIP equations are differenced implicitly in time and solved using a predictor-corrector algorithm. The implicit electric field equation (and other elliptic equations) are solved using preconditioned Krylov space iterative methods. Calculations have been performed to study the linear theory of the ideal internal kink mode as a function of poloidal beta and aspect ratio. These results will be presented along with plans for the future development of QIP3D. [Preview Abstract] |
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LP1.00092: A High Resolution Wave Propagation Scheme for Two-Fluid Plasma Equations with applications to Field Reversed Configurations Ammar Hakim, Uri Shumlak Algorithms for the solution of the five-moment ideal Two-Fluid equations are presented. The ideal Two-Fluid model is more general than the often used magnetohydrodynamic (MHD) model. The model takes into account electron inertia, charge separation and the full electromagnetic field equations and allows for separate electron and ion motion. The algorithm presented is the high resolution wave propagation scheme. The wave propagation scheme is a finite volume method of the Godunov type based on solutions to the Riemann problem at cell interfaces. The scheme presented here does not use dimensional splitting and is hence able to capture accurately flow not aligned along grid lines. The scheme is stable to Courant number of unity. Operator splitting is used to incorporate the Lorentz and electromagnetic source terms. To preserve the divergence constraints on the electric and magnetic fields two different approaches are used. In the first approach Maxwell equations are rewritten in their mixed-potential form. In the second approach the so called perfectly-hyperbolic form of Maxwell equations are used which explicitly incorporate the divergence equations into the time stepping scheme. Collisionless magnetic reconnection and Field Reversed Configurations are studied. Two-Fluid physics described by the ideal Two-Fluid model is highlighted. [Preview Abstract] |
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LP1.00093: Two Fluid Steady States in Magnetically Confined Plasmas L.E. Sugiyama, H.R. Strauss, W. Park, G.Y. Fu Two-fluid plasma models, which allow the electron and ion fluids to move independently, have been shown to have important consequences for magnetically confined plasma behavior and steady states compared to MHD, particularly when axisymmetry is broken. Two-fluid steady states can be described in terms of the canonical momenta and generalized vorticities of the two species (eg [1]). Analytical solutions typically oversimplify and do not describe the conditions of actual plasmas nor their nonlinear numerical simulations. The radial electric field, required to balance the two flows, depends on the full velocities and therefore on the complete dissipation picture, necessarily present numerically. The plasma edge region, which supplies boundary conditions for the global solution, is almost always strongly idealized, while actual plasmas may have large localized flows and radial electric fields as well as pressure gradients, as in H-mode, perhaps due to non-fluid effects. These questions are investigated, in part with the help of the M3D initial value code, that can also study non-axisymmetric configurations.\\ \leftline{[1] L.C. Steinhauer, \emph{Phys. Plasmas} \textbf{6} 2734 (1999).} [Preview Abstract] |
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LP1.00094: Relationship of electric field and charged particle density fluctuations to air turbulence in the mesosphere Scott Robertson A theoretical model is developed for the electric field fluctuations that arise in the polar summer mesosphere as a result of the coupling of the charged species to the neutral air turbulence. The motions of electrons, ions, and charged aerosol particles are described as harmonic oscillators both driven and damped by the drag force exerted by the neutral air. The relative fluctuations in the ion density are found to be nearly the same as those in the neutral air as a consequence of the ions' high momentum-transfer relaxation frequency. The aerosol particle (dust) density fluctuations follow those of the neutral air at frequencies below their relaxation frequency, which is in the acoustic range. The electrons move primarily in response to the electric force to partially cancel the net charge density of ions and aerosol particles except at wavelengths shorter than the Debye length. Electric field and charge-density fluctuations are calculated for several sets of conditions. In bite-out regions in which the electron density is reduced as a consequence of attachment to the aerosol particles, the electric field fluctuations are found to be enhanced, which is consistent with observations. [Preview Abstract] |
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LP1.00095: Potential Barrier around an Emitting Body an a Plasma Antonio Bruno, Gian Luca Delzanno, Gianfranco Sorasio, Giovanni Lapenta We present a self-consistent, kinetic theory for the charging and shielding of an object at rest in a collisionless plasma [1]. The body is an electron emitter according to thermionic emission, photoemission or secondary emission. The theory is formulated for positively charged bodies, derived under the assumption of spherical symmetry so that conservation of energy and angular momentum can be used to calculate the plasma distribution functions at any given point in phase space. Far away from the body the plasma is assumed unperturbed, described by a Maxwellian distribution function at rest. Thus, the unperturbed plasma acts as a source of particles balancing the absorptions from the body and a steady state is eventually reached. The theory is shown to be in good agreement with PIC simulations [1-2]. Further on, several cases (focusing on parameters typical of laboratory experiments) are presented for the three different emission mechanisms, showing that shielding potentials having an attractive well are possible for all of them.\newline [1] G. L. Delzanno, A. Bruno, G. Sorasio, G. Lapenta, Phys. Plasmas 12, 062102 (2005).\newline [2] G. L. Delzanno, G. Lapenta, M. Rosenberg, Phys. Rev. Lett. 92 (3), 035002 (2004). [Preview Abstract] |
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LP1.00096: Dust confinement and dust acoustic waves in a magnetized plasma A. Piel, T. Trottenberg, D. Block Systematic laboratory experiments on dust acoustic waves require the confinement of dust particles. Here we report on new experiments in a magnetized plasma region in front of an additional positively biased disk electrode in a background plasma which is generated in argon at 27MHz between a disk and grid electrode. The plasma diffuses through the grid along the magnetic field. The three-dimensional dust distribution is measured with a horizontal sheet of laser light and a CCD camera, which are mounted on a vertical translation stage. Depending on magnetic field and discharge current, cigar or donut-shaped dust clouds are generated, which tend to rotate about the magnetic field direction. Measurements with emissive probes show that the axial confinement of dust particles with diameters between 0.7-2~$\mu$m is achieved by a balance of ion-drag force and electric field force. Dust levitation and radial confinement is due to a strong radial electric field. Dust acoustic waves are destabilized by the ion flow or can be stimulated by a periodic bias on the disk electrode. The observed wave dispersion is compared with fluid and kinetic models of the dust acoustic wave. [Preview Abstract] |
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LP1.00097: Structure and confinement of Coulomb balls O. Arp, D. Block, A. Piel, V. Golubnychiy, M. Bonitz Coulomb balls [2] are spherical dust clouds of a few hundred micrometer sized particles embedded in a plasma environment. Due to their large negative charge these particles are strongly coupled and can form crystalline structures. Coulomb balls have an unusual crystal structure with nested spherical shells. This contribution presents experiments and simulations on structural properties and trapping of these Coulomb balls. By means of particle imaging velocimetry the contribution of different forces to the confinement is investigated. It is shown that a proper combination of gravity, thermophoresis and electric fields leads to a stable confinement potential. Further, a comparison of experiments with molecular dynamics simulations shows that the structural properties of Coulomb balls require a description based on Yukawa interaction of individual particles.\\ $[1]$ O. Arp et al, Phys. Rev. Lett. \underline{63}, 165004 (2004) [Preview Abstract] |
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LP1.00098: Dust confinement, voids and self-exited waves in complex plasmas under microgravity M. Klindworth, O. Arp, A. Piel Complex plasmas under microgravity conditions are generally affected by the void instability - a dust-free region in the center of the dust cloud. Here, we report on recent experiments on parabolic flights. The IMPF-K devices is a parallel plate rf-discharge with segmented electrodes and a pair of retractable glass tubes for dust confinement. It is shown that by means of these dielectric walls void free dust clouds can be produced. The acting forces are derived from probe measurements and discharge simulations. For high dust density, large amplitude density waves are found between the void edge and the plasma boundary. These compressional waves are characterised by their frequency and wave number. The observations are completed by Langmuir probe measurements which yield the background plasma parameters. The wave phenomenon is compared with theoretical dust acoustic wave dispersion relations including the growth rate of the instability from which conclusions on the driving mechanism can be drawn. [Preview Abstract] |
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LP1.00099: Measurements and simulations of thermal properties of dusty plasmas Jeremiah Williams, Edward Thomas A dusty plasma is a four-component system composed of ions, electrons, neutral particles and charged microparticles. The presence of the microparticles gives rise to new plasma phenomena and allows the study of fundamental aspects of plasma physics on the kinetic level. Using stereoscopic particle image velocimetry (stereo-PIV), one can measure the velocity of the microparticles in three dimensions and extract a three-dimensional distribution of velocities. From this velocity distribution, it is possible to extract a kinetic temperature for the microparticle component. To understand how this distribution of velocities is related to the underlying velocity space distribution function of the microparticle component, extensive simulations of the PIV measurement have been made. It is found that the width of the distribution, and the resulting kinetic temperature, is smaller than the underlying distribution and strongly depends on the density of the microparticle component. This presentation describes ongoing numerical studies on the application of stereo-PIV to the measurement of thermal properties of dusty plasmas. This work is supported by NSF Grant PHY-0354938. [Preview Abstract] |
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LP1.00100: Boundary analysis of a dusty plasma John McKee, Jeremiah Williams, Edward Thomas In the field of dusty plasmas, direct measurements of fundamental quantities such as the dust grain charge or the potential structure of the dust cloud has proven challenging. For example, the use of probes often alters the spatial and electrical structure of the cloud, thereby perturbing the quantities that are being measured. Therefore, optical techniques have been the primary method for diagnosing dusty plasmas. However, a relatively non-intrusive alternative may have presented itself through the use of the transport of the microparticles themselves as a diagnostic tool. Here, particle image velocimetry (PIV) and high speed imaging techniques are used to measure particle transport in the cloud. This presentation focuses on the plasma - particle cloud interface region. Transport measurements suggest that particles at the cloud boundary have much higher velocity than interior particles. This presentation will present data on these two regions and discuss possible mechanisms for the differences between interior and boundary particles. [Preview Abstract] |
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LP1.00101: Measurement of dust particle density by a laser extinction method C.R. Seon, H.Y. Park, W. Choe, S. Park, Y.H. Shin, K.H. Chung Measurement of dust particle density was performed using the laser extinction method, in which particle density was obtained by the relation between the particle density and the difference between two He-Ne laser beam intensities with and without passing through a dusty plasma. The multi-pass laser beam is provided by two spherical mirrors of which radius of curvature is 8 m with small holes, and the design value of the number of reflections is 32. Before applying the method to the dusty plasma, the measurement limit of the extinction ratio of the setup and the accuracy of the calculated scattering cross-section were experimentally investigated using a solution of particles and distilled water. The measurement limit was about 2{\%} with the single-pass system. In the single-pass experiment, the 120 nm diameter dust particle density in a Ar-diluted SiH$_{4}$ CCP (20 mtorr, 30 W RF power, 200 s after plasma on) was about 10$^{8}$ cm$^{-3}$. The lowest measurable particle diameter of the single-pass setup was about 90 nm. However, in the multi-pass setup with 20 passes, $\sim $5x10$^{6}$ cm$^{-3}$ of 120 nm diameter particles is expected to be measured, and the lowest measurable size is expected to be about 30-40 nm in diameter. [Preview Abstract] |
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LP1.00102: 3-D Plasma Crystals in the lab and in space Peter Huber, V.E. Fortov, A.V. Ivlev, A.M. Lipaev, V.I. Molotkov, G.E. Morfill, M. Rubin-Zuzic, H.M. Thomas The crystallisation of complex plasmas under microgravity conditions performed in the PKE-Nefedov laboratory on the ISS was restricted to a small area close to the sheath region at the electrodes. Now, it is possible to grow large crystals of different type (fcc, bcc etc.) by a special developed annealing procedure. The latter will be explained and the 3-D analysis of the resulting crystal will be presented. Even larger crystals of more than 100 particle distances ($\Delta $ = 0.1 mm) in every direction can be formed in the PK-3 Plus lab on Earth due to the usage of particles of $\sim $1$\mu $m diameter. PK-3 Plus is the follow-up experiment to PKE - Nefedov and will be launched to the ISS end of 2005. Beside the smaller particles PK-3 Plus provides larger electrodes and better plasma conditions also necessary for this large crystal formation. By depth scans of the usual 2-D particle diagnostics, we are able to study the crystallization process and phase transition in 3-D. [Preview Abstract] |
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LP1.00103: Dust transport and force equilibria in magnetized dusty DC discharges Victor Land, Edward Thomas, Jeremiah Williams In many plasma applications and in plasmas in space dust is present. In both types of plasmas magnetic fields can be present. Dust particles collect ions and electrons and the plasma parameters become very different from those in dust-free plasmas. Magnetic fields change the plasma parameters even more. Electrons gyrate around magnetic field lines. This changes the density and potential profiles in the discharge. Furthermore, these electrons move with the ExB drift. In a DC discharge we used different probes to measure the response of a plasma to the introduction of a magnetic field. We also introduced micrometer sized dust particles and recorded the response of the dust to the change in plasma parameters with different optical techniques. We observed a plasma response in the direction of the ExB drift. The dust particles followed this response, however, this response consisted of an initial fast response, on the ion diffusion time scale, followed by a final slow response. Finally we observed an important role for the ion drag force in the vertical force balance. [Preview Abstract] |
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LP1.00104: Charging of dust in a negative ion plasma Robert Merlino, Ross Fisher, Su-Hyun Kim, Nathan Quarderer We investigate experimentally the charging of dust particles in a plasma consisting of positive ions, negative ions and electrons. In typical laboratory plasmas containing electrons and positive ions, dust grains acquire a negative charge. In negative ion plasmas, charging due to the negative ions, in addition to positive ions and electrons, must be taken into account. Calculations show that if a significant fraction of the electrons are attached to negative ions, the magnitude of the charge on the dust particles is reduced. If the ratio p = n$_{e}$/n$_{+}$ of the electron density to positive ion density is sufficiently small and the positive ions are lighter than the negative ions, then the dust charge can be positive. This possibility is investigated in a Q machine potassium (K$^{+})$ plasma, into which is added the highly electronegative SF$_{6}$ gas which attaches low energy electrons to produce (SF$_{6})^{-}$ negative ions. The relatively cold electrons in the Q machine plasma (T$_{e}$ = 0.2 eV) enhances the attachment probability allowing values of p $<$ 10$^{-3}$ to be attained. [Preview Abstract] |
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LP1.00105: Theory and Simulation of a Nonlinear Fluid Model for Void Formation in Dusty Plasmas C.S. Ng, A. Bhattacharjee, S. Hu, Z.W. Ma We present new developments in the theory and numerical simulation of a recently proposed [Phys. Rev. Lett. 90, 075001 (2003)] nonlinear time-dependent fluid model for void formation in dusty plasmas, which has been observed recently in a number of experiments in laboratory as well as under microgravity conditions. A void is typically a small and stable centimeter-size region within the plasma that is completely free of dust particles and characterized by sharp boundaries. This model describes an initial instability caused by the ion drag, rapid nonlinear growth, and a nonlinear saturation mechanism that realizes a quasi-steady state with void. New simulations in 1D/2D, as well as 3D spherically symmetric cases, are performed based on a realistic ion drag force operator derived recently [S. Khrapak et al, Phys. Plasmas 12, 042308 (2005)]. Qualitative features of these simulations are similar to our previous results using a model ion drag force operator, and compare well with observations. [Preview Abstract] |
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LP1.00106: Hypervelocity Dust Injection for Plasma Diagnostic Applications Catalin Ticos, Zhehui Wang, Leonid A. Dorf, Glen A. Wurden Hypervelocity micron-size dust grain injection was proposed for high-temperature magnetized plasma diagnosis. Multiple dust grains are launched simultaneously into high temperature plasmas at several km/s or more. The hypervelocity dust grains are ablated by the electron and ion fluxes. Fast imaging of the resulting luminous plumes attached to each grain is expected to yield local magnetic field vectors. Combination of multiple local magnetic field vectors reproduces 2D or even 3D maps of the internal magnetic field topology. Key features of HDI are: (1) a high spatial resolution, due to a relatively small transverse size of the elongated tail, and (2) a small perturbation level, as the dust grains introduce negligible number of particles compared to the plasma particle inventory. The latter advantage, however, could be seriously compromised if the gas load from the accelerator has an unobstructed access to the diagnosed plasma. Construction of a HDI diagnostic for National Spherical Torus Experiment (NSTX), which includes a coaxial plasma gun for dust grain acceleration, is underway. Hydrogen and deuterium gas discharges inside accelerator are created by a $\sim$ 1 mF capacitor bank pre-charged up to 10 kV. The diagnostic apparatus also comprises a dust dispenser for pre-loading the accelerator with dust grains, and an imaging system that has a high spatial and temporal resolution. [Preview Abstract] |
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LP1.00107: Measurement of the Electron-Ion Thermal Equilibration Rate J.M. Taccetti, R.P. Shurter, B.R. Haberle, J.P. Roberts, J.F. Benage We are conducting a laboratory experiment aimed at measuring the temperature equilibration rate between ions and electrons in a strongly-coupled plasma. Theory indicates that this rate could be significantly ($\approx$ 50 times) lower than that given by the usual weakly coupled model (Landau/Spitzer) due to coupled collective modes present in the dense plasma. The plasma under study is formed by heating a hypersonic SF$_{6}$ gas jet with a short pulse ($\approx$ 10 ps) laser, resulting in warm electrons ($\approx$ 100 eV) and cold ions ($\approx$ 5 eV). The electron and ion temperatures of the resulting plasma will be independently measured during and after heating, using collective Thomson scattering for electrons and a high-resolution x-ray spectrometer for the ions (measuring Doppler-broadened absorption lines). Determining how the equilibration rate varies from Landau/Spitzer requires very fast diagnostics, since Landau/Spitzer equilibration would occur within $\approx$ 100 ps. We will present our most recent experimental results. [Preview Abstract] |
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LP1.00108: Combined effects of drift waves and neoclassical transport on density profiles in tokamaks W.A. Houlberg, P. Strand, A. Eriksson, H. Nordman, J. Weiland The relative importance of neoclassical and anomalous particle transport depends on the charge number of the species being studied. The detailed particle balance including the EDWM [1] drift wave model for anomalous transport that includes ITG, TEM and in some cases ETG modes, and the neoclassical model NCLASS [2], are illustrated by simulations with the DEA particle transport code. DEA models the evolution of all ion species, and can be run in a mode to evaluate dynamic responses to perturbations or to conditions far from equilibrium by perturbing the profiles from the experimental measurements. The perturbations allow the fluxes to be decomposed into diffusive and convective (pinch) terms. The different scaling with charge number between drift wave and neoclassical models favors a stronger component of neoclassical transport for higher Z impurities through the effective pinch term. Although trace impurities illustrate a simple Ficks Law form, the main ions as well as higher concentrations of intrinsic impurities exhibit non-linear responses to the density gradients as well as off-diagonal gradient dependencies, leading to a more complicated response for the particle fluxes.[1] H. Nordman, et al., Plasma Phys. Control. Fusion 47 (2005) L11. [2] W.A. Houlberg, et al., Phys. Plasmas 4 (1997) 3230. [Preview Abstract] |
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LP1.00109: Collisionless neoclassical polarization drift in a spatio-temporally sheared radial electric field in a tokamak plasma Hoyul Baik, Seunghoe Ku, C.S. Chang Neoclassical polarization drift of plasma ions is of critical importance in the dynamics of a radial electric field $E_r$. Neoclassical polarization drift speed $V_{\rm NP}$ of collisionless single ions is studied using a guiding center code in a time-varying, spatially sheared $E_r$ in a realistic tokamak geometry. It is found numerically that $V_{\rm NP}$ is not only a function of the time derivative $dE_r/dt$, as conventionally understood, but also a strong function of the radial shear $dE_r/dr$ if the shear length is on the same order as the ion banana width. If the radial shear $(\Delta r) dE_r/dr$ has the same sign as $E_r$, where $\Delta r$ is the banana excursion width, then the radial shear effect adds to $V_ {\rm NP}$; but if $(\Delta r) dE_r/dr$ has the opposite sign to $E_r$, then its effect opposes $V_{\rm NP}$. Due to this effect, $V_{\rm NP}$ can even be in the opposite direction from the $dE_r/dr=0$ case for fat banana ions. An analytic investigation reveals that this effect is simply due to the finite banana modification to the orbital average $E_r$. An approximate analytic formula has been presented for majority ions in a conventional tokamak plasma. [Preview Abstract] |
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LP1.00110: Real geometry gyrokinetic PIC computations of ion turbulence in advanced tokamak discharges with SUMMIT/PG3EQ{\_}NC Jean-Noel Leboeuf, Viktor Decyk, Terry Rhodes, Andris Dimits, Dan Shumaker Development of the PG3EQ{\_}NC module within the SUMMIT gyrokinetic PIC FORTRAN 90 framework is largely completed. It provides SUMMIT with the capability of performing 3D nonlinear toroidal gyrokinetic computations of ion turbulence in real DIII-D geometry. PG3EQ{\_}NC uses local, field line following, quasi-ballooning coordinates and direct interface with DIII-D equilibrium data via the EFIT and ONETWO codes. In addition, Holger Saint John's PLOTEQ code is used to determine the (r,z) position of each flux surface. The thus initialized SUMMIT computations have been carried out for shot {\#}118561 at times 01450 and 02050 at many of the 51 flux surfaces from the core to the edge. Linear SUMMIT results will be compared to available data from calculations with the GKS code for the same discharges. Nonlinear SUMMIT results will also be compared with scattering measurements of turbulence, as well as with accessible measurements of fluctuation amplitudes and spectra from other diagnostics. [Preview Abstract] |
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LP1.00111: Relating parallel and perpendicular flows in a magnetized toroidal plasma Abinadab Dieter, Richard Hazeltine We present a general relationship between the transport of heat and particles across flux surfaces predicted by neoclassical theory and the parallel flows within those flux surfaces. We make no assumptions regarding collisionality, obtaining results valid in all collisionality regimes. Our results are constructed using variational solutions to Spitzer problems with appropriate source terms. While these Spitzer functions are particularly relevent to analysis of high collisionality regimes, here they are simply mathematical tools. We compare our results to previous work specific to the collision dominated Pfirsch-Schl\"{u}ter regime\footnote{Hinton, F.L., and R.D. Hazeltine, 1976, Reviews of Modern Physics, 48, 239} and to low collisionality ion transport.\footnote{Dieter, A., and R.D. Hazeltine, 2004, Physics of Plasmas} [Preview Abstract] |
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LP1.00112: Global gyrokinetic particle simulation of trapped electron mode turbulence Zhihong Lin, Yasutaro Nishimura, Ihor Holod Kinetic electrons have been implemented in our gyrokinetic toroidal code (GTC) using the fluid-kinetic hybrid electron model. Global nonlinear simulations of TEM turbulence have been carried out with contribution of kinetic electrons to zonal flows properly retained. The nonlinear electron dynamics is found to be constrained by the conservation of the second invariant, resulting in simultaneous diffusions of electron banana orbits in both energy and real space, which have not been studied in analytical theories or local simulations. Zonal flows with short radial wavelength are found to be generated in the TEM turbulence, and the electron contribution to the zonal flow generation is found to be larger than the ion contribution. The key difference between TEM and ITG is that the perpendicular wavelength of TEM can be on the order of ion gyroradius, which is much shorter than that of the ITG. E x B nonlinearity and ion polarization nonlinearity are therefore on the same order of magnitude, which invalidates the nonlinear analysis of ITG turbulence assuming time scale separation between these two nonlinearities. [Preview Abstract] |
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LP1.00113: Heat and momentum transport in arbitrary mean-free path plasma with a Maxwellian lowest order distribution function Andrei N. Simakov, Peter J. Catto Expressions for ion perpendicular viscosity, electron and ion parallel viscosities, gyroviscosities, and heat fluxes, as well as electron-ion energy and momentum exchange terms are derived for arbitrary mean-free path plasmas, in which the lowest order distribution function is a Maxwellian. The latter assumption often holds for plasmas confined by magnetic fields with closed flux surfaces in the absence of strong external driving forces [1], such as neutral beams or radio-frequency waves. In particular, it is always employed in the neoclassical theory. The results are given in terms of a few velocity space integrals of the gyrophase averaged correction to the Maxwellian by assuming the gyroradius is small compared to the shortest perpendicular scale length. The general expressions make possible a hybrid fluid-kinetic description, and correctly reproduce known results in the collisional limit [2].\newline \newline [1] R. D. Hazeltine and J. D. Meiss, Plasma Confinement (Addison-Wesley, Redwood City, CA, 1991).\newline [2] P. J. Catto and A. N. Simakov, Phys. Plasmas 11, 90 (2004). [Preview Abstract] |
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LP1.00114: Transport Studies of Electric Field in Helical Plasmas S. Toda, K. Itoh In the compact helical system (CHS), the steep gradient of the radial electric field has been observed in the inner plasma region and the transport barrier was found in the ECRH plasma. The similar structure in the electron temperature profile has been observed in the Large Helical Device (LHD). A pulsating behavior of electrostatic potential (or the radial electric field) was also observed in CHS. We have analyzed the one-dimensional transport equations which describe the temporal evolutions of the density, the electron and ion temperatures, and the radial electric field in a cylindrical configuration. The transport model for anomalous diffusivities was used to describe the turbulent plasma. At first, we compare the analysis results with the experimental results in LHD to discuss the validity of the physical process employed in one-dimensional theoretical study. Next, we examine the two dimensional (radial and poloidal) profile of the electric field. Two dimensional transport equations which include the temporal evolution of the electric field are used. The electric filed is assumed to be determined by the ambipolar condition which is constituted with the neoclassical particle flux for the non-axisymmetric part. We include the effect of the inward particle pinch to examine the temporal oscillation of the electric field. The parameter regime in which the temporal oscillation of the radial electric field is predicted will be shown. [Preview Abstract] |
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LP1.00115: Experimental characterization of edge turbulence with GPID in RFX-mod P. Scarin, R. Cavazzana, G. Serianni, M. Agostini, N. Vianello The new Gas Puffing Imaging Diagnostic (GPID) system\footnote{\textit{Cavazzana R. et al.} 2004, Rev. Sci. Instrum. \textbf{75} 4152} has been installed on RFX-mod to investigate the dynamical structure of plasma edge turbulence in different plasma conditions in the Reversed Field Pinch (RFP) configuration. The system consists of a gas-puffing nozzle and 32 optical channels to measure the local H$_{\alpha}$ emission from an area normal to the local magnetic field. The optical lines are 5 mm spaced, cover an area of about 70 mm in the toroidal direction and 40 mm in the radial one. The effective bandwidth of the electronics is 2 MHz and signals are sampled at 10 MSamples/s. Some results obtained during different experiments like externally driven $m=0$ mode rotation discharges, OPCD/PPCD and QSH discharges are reported. The comparison of different regimes of discharges is carried out in terms of power spectrum, toroidal propagation of fluctuations and spectral analysis of wave number. Furthermore, the probability distribution functions (pdf) of fluctuations at different time scales have been analysed, revealing deviation from gaussianity and the intermittent character of the fluctuations. Moreover a characterization of the region of the intense plasma-wall interaction, caused by $m=1$ localised kink perturbation (wall locking), in terms of turbulence parameters will be shown. [Preview Abstract] |
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LP1.00116: Global gyrokinetic simulations of MHD-modes in tokamak plasmas Igor Holod, Zhihong Lin, Yasutaro Nishimura, Liu Chen Linear stability properties of global MHD modes, e.g., TAE and EPM, are determined by competition of dissipative kinetic effects and drive mechanisms, with energetic particles playing a significant role. The simulations of global MHD modes are performed using gyrokinetic toroidal particle-in-cell code (GTC) where ions and energetic particles are treated kinetically while hybrid scheme is implemented for electrons [Z. Lin, L. Chen, Phys. Plasmas 8, 1447 (2001)]. Linear dispersion from global kinetic simulations will be compared to theory and MHD simulations. The work was supported by DOE grants DE-FC02-04ER54796 and DE- FG02-03ER54724. [Preview Abstract] |
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LP1.00117: Dynamic Sub-Grid Scale Modeling of Tearing Mode Interaction with Turbulence Christopher McDevitt, Patrick Diamond A self-consistent treatment of both a long wave-length tearing mode and electrostatic background micro-turbulence is developed with consideration of applications to the Quasi Single Helicity (QSH) state of the RFP. Turbulent viscosities are derived for the case of homogeneous micro-turbulence as well as for micro- turbulence developing in a sheared magnetic field. A negative viscosity is found in both cases, motivating the possiblity of generation of large scale flows. The nonlinear evolution of this system is under analysis, with special emphasis on the interaction of a tearing mode with the background vortices driven via the negative viscosity. Viscoresistive magnetohydrodynamic simulations predict a transition into the QSH state for low Hartmann Numbers [1]. Comparison with estimates of experimental collisional values of the resistivity and viscosity lead to a Hartmann Number far in excess of the critical value needed to access the QSH state. While the magnitude of the viscosity is found to be enhanced by a factor of roughly two to three orders of magnitude (roughly that needed to access the QSH state), the sign is negative on large scales. This leaves the meaning of a Hartmann Number criterion for the transition to the QSH state unclear. The results of ongoing considerations of this issue will be reported. [1] S. Cappello, D.F. Escande, Phys. Rev. Lett. 85, 3838 (2000) [Preview Abstract] |
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LP1.00118: Toroidal rotation measurements in TJ-II and its relation with the bootstrap current David Rapisarda, Bernardo Zurro, Victor Tribaldos, Alfonso Baciero, David Jimenez In this work, measurements have been made of toroidal rotation velocities for several ions in order to make an experimental estimation for the ionic contribution to the neoclassical bootstrap current in the TJ-II stellarator, because they are not consistent with the drift caused by a positive radial electric field. The experimental system and the absolute calibration method employed here to determine flow velocities averaged along lines of sight have been described previously [1], and the measurements were performed by means of passive emission spectroscopy. Typically, data from ECRH discharges show flow velocities, depending on the ion type, that range between -7 and 10 km/s across the plasma. The measures for rotation profiles obtained using the shot-to-shot technique suggest that the shape of these profiles might be related to, or influenced by, the local current density, and the orders of magnitude are in good agreement with bootstrap current neoclassical theory. In order to deduce the local current profiles we are used two different schemes, giving similar results (within a factor of two) but showing differences for small angles, i.e. near the magnetic axis. \newline \newline [1] D. Rapisarda \textit{et al}, Proc. 31st EPS Conf. Vol\textbf{. 28G}, P-4.173 (2004). [Preview Abstract] |
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LP1.00119: Non-local Effects on Temperature Evolution in the Presence of Magnetic Islands John James, Eric Held We review diffusive and integral methods of closing the plasma temperature evolution equation for the conductive heat flow and list the assumptions and orderings made in deriving and using these closures. An outline of the numerical implementation of these terms in the plasma fluid code NIMROD\footnote{C. R. Sovinec, et al., J. Comput. Phys. {\bf 195}, 355 (2004).} will be given with particular emphasis on the development of a general, integral form for $\vec{q}_\|$\footnote{E. D. Held, J. D. Callen, C. C. Hegna and C. R. Sovinec, Phys. Plasmas {\bf 8}, 1171 (2001).} with comment on the efficiency and accuracy of each method for relevant parameter regimes and magnetic field topologies. We calculate and compare several quantities of interest using each form of the closure including effective radial heat diffusivities and on-axis pressure vs. $L_\nu / L_T$. These calculation will be made in slab geometry with a single magnetic island and with over lapping magnetic islands using the NIMROD code. [Preview Abstract] |
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LP1.00120: Transport reduction in the edge of the RFX reversed-field pinch G. Spizzo, F. Auriemma$^1$, A. Canton$^1$, S. Cappello$^1$, A. Cravotta$^1$, D.F. Escande, R. Lorenzini$^1$, L. Marrelli$^1$, P. Martin$^1$, R.B. White, P. Zanca$^1$ Magnetic field lines and particle orbits are calculated with the code \textsc{Orbit} for a typical multiple helicity (MH) chaotic field, provided by a MHD numerical simulation of the reversed-field pinch (RFP). The result (confirmed by an analytical Hamiltonian calculation) is that $m=0$ and $m=1$ modes allow for the formation of magnetic islands which induce transport barriers at $r/a \simeq 0.7 \div 0.8$. This model has been cross-checked with experimental data coming from the Padua experiment RFX. A particle transport analysis has been done, by means of the 1D transport code TED, to investigate the dependence of the particle diffusion coefficient $D$ on mode amplitude. TED runs show that there is a decrease of $D$ at $r/a \simeq 0.7$. \textsc{Orbit} runs are consistent with TED results. Finally, we present preliminary data showing the active control of $m=0$ modes in the recently rebuilt RFX-mod, aiming at reproducing (with a suitable choice of externally applied $m=0$ amplitudes and phases) an ideal no-resonance, no-island condition, which corresponds, in the Hamiltonian formulation, to the presence of good flux surfaces at the $q=0$ radius. [Preview Abstract] |
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LP1.00121: Nonlinear Particle Pinch in Collisionless Trapped Electron Mode Turbulence P.W. Terry, D.A. Baver, R. Gatto Collisionless trapped electron mode turbulence is shown to have an anomalous particle pinch fundamentally unlike pinches identified previously. It arises from a nonlinear fluctuation eigenmode, placing it outside the purview of quasilinear theory. The nonlinear eigenmode develops because the nonlinearity excites a damped linear eigenmode, changing the density- potential correlation. The flux is solved from spectrum balance equations in a complete basis spanning the fluctuation space under a joint expansion in collision frequency and instability threshold parameter. The solution accounts for saturation by anisotropic energy transfer to zonal wavenumbers of the damped eigenmode. To lowest order the pinch is a convective-like flux driven by temperature gradient. It arises from the damped eigenmode energy and the real part of the correlation between damped and growing eigenmodes. The pinch is slightly smaller than the outwardly directed flux associated with the growing eigenmode, making the flux a small fraction of the quasilinear value. Work supported by US DOE. [Preview Abstract] |
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LP1.00122: Turbulence Spreading through Shear Flow Layer T.S. Hahm, W. Wang, G. Rewoldt, P.H. Diamond, O. Gurcan, Z. Lin A recent study of turbulence spreading [1] has shown that a significant level of turbulence can penetrate from the linearly unstable region to the region dominated by self-generated zonal flows. In this work, we study turbulence spreading through a mean ExB shear layer using both simple analytic model and nonlinear gyrokinetic particle (GTC) simulations. \newline \newline [1] T.S. Hahm, P.H. Diamond, Z. Lin et al., On the dynamics of edge-core coupling, To appear in Phys. Plasmas (2005). [Preview Abstract] |
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LP1.00123: Sources and Sinks in the Zonal Flow Energy Balance in Tokamak Microturbulence Andris Dimits, William Nevins, Dana Shumaker The relative contributions of various terms to the driving and damping of the zonal flows in toroidal ion temperature gradient turbulence are investigated using nonlinear gyrokinetic simulations. The simulation diagnostic was discussed and its correctness, in that the implemented terms numerically recover the net rate of change of the zonal flows, was demonstrated previously (A.M. Dimits et al. APS-DPP02 meeting). Application of this diagnostic to gyrokinetic ITG simulations in the turbulent finite-transport regime shows that zonal flow energy is generated primarily through the Reynolds' stress term and dissipated by the transit time damping terms. The source and sink rates of other terms, such as the diamagnetic Reynolds stress, are finite but lower. The effect of sampling the fields at a small number of positions vs. full flux-surface averaging, relevant to the applicability of such a diagnostic to experiments, is addressed. [Preview Abstract] |
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LP1.00124: Next-Generation Methods for Bispectral Analysis D.A. Baver, P.W. Terry Bispectral analysis provides a method of determining linear growth rates and nonlinear transfer rates of turbulence from third-order correlations in fluctuation measurements. Traditionally, its usefulness has been limited by its requirement of large data sets in order to detect useful statistical correlations, and thus by the availability of experimental diagnostics capable of providing such large amounts of data. In terms of time series length, this requirement is greatly reduced by use of the basis function method. However, most diagnostics are limited in spatial resolution as well. To address this problem, we propose methods of reconstructing the fluctuation data where experimental data is insufficient. In order to avoid growth rate and transfer rate fits which reflect the reconstruction technique rather than actual physics, this reconstruction must be integrated into and concurrent with the fitting procedure. Using this approach we propose and test methods which are adapted to the limitations of experimental diagnostics and thus might provide a practical tool for data analysis. Work supported by USDOE. [Preview Abstract] |
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LP1.00125: Unaveraged parallel viscous force and its effects on shear flows and turbulence A.L. Garcia-Perciante, C.C. Hegna, P.W. Terry Recent calculations have demonstrated that the varying part of $\mathbf{B}\cdot\nabla\cdot{\bf \Pi_{\parallel}}$ exceeds the averaged closure by a factor of $1/\epsilon$ in a large aspect ratio expansion. We investigate the effect of the large poloidally varying parallel viscosity on shear flow evolution and turbulence by extending a phase transition model developed by Diamond et. al. [Phys. Rev. Lett. 16, 2565]. In the modified model, averaged and poloidally varying parts of the shear flow are advanced separately. These two equations are coupled to each other through the viscous force $\mathbf{B}\cdot\nabla\cdot{\bf \Pi_{\parallel}}$ (which damps both $m=0,\,1$ components of the poloidal flow) and to the fluctuation level through the Reynolds stress tensor. The fluctuation levels for steady states are shown to be lower when the total viscous drive is taken into account and the transition from a no-flow state to a state with poloidal flow occurs faster. These results can be used to explore the implications of the anisotropic viscous damping of the poloidal flow on the L-H transition. [Preview Abstract] |
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LP1.00126: Studies of the particle-continuum method for large-scale simulations of ETG and edge turbulence Scott Parker, Yang Chen, C.S. Chang, Wei-li Lee Delta-f methods result in a huge reduction in particle number over conventional full-f particle-in-cell methods in situations where fluctuations in delta-f are small compared to field fluctuations. We note delta-f can be arbitrarily large when using the delta-f method. This just requires using as many particles as one would for full-f for the same problem. There are situations where delta-f can become quite large for timescales of interest. Such situations can be dealt with by simply increasing particle number. However, an alternative approach, which may be much more efficient, is to use the particle-continuum method. Such a scheme has been shown to solve the so-called ``growing weight problem.'' Here, we discuss the particle-continuum method and our progress on implementing it in five-dimensions. We will test the method in flux-tube geometry on electron-temperature-gradient turbulence. ETG turbulence is a good test bed because the streamer dominated turbulence has a high flux level that drives the particles' delta-f to a relatively large value. For ETG simulations we are confident in our converged simulation results. However, simulations of time evolution of the edge pedestal may require application of the particle-continuum method or even full-f particle-in-cell simulation using Denavit's ``hybrid'' method where no distinction is made between the equilibrium and perturbed distribution function. Work is supported by DOE SciDAC. [Preview Abstract] |
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LP1.00127: Global Particle Simulation of RF-driven Rotation and Er in a Tokamak Plasma Jae-Min Kwon, C.S. Chang Numerical simulation of ICRH driven radial transport of minority ions is performed using particle-in-cell guiding center code in a circular tokamak geometry. Generation of radial electric field and plasma rotation are evaluated self-consistently with minority ion heating, finite orbit excursion effects and transport. The simulation is performed over global toroidal plasma with conserving main ion collisions and minority-main plasma collisions. It is found that RF-driven radial transport can induce radial electric field profile which significantly exceeds the conventional neoclassical level under a moderate RF-power. Together with the generation of the radial electric field, spin up of both minority ions and bulk plasma are observed. Time evolution of minority ion distribution function will be described in detail. [Preview Abstract] |
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LP1.00128: Comparison of Turbulent Transport Models and Transport Simulation of Internal Transport Barrier Formation Mitsuru Honda, Atsushi Fukuyama In order to predict the performance of ITER plasma, it is important to validate the existing theory-based turbulent transport models by systematicallycomparing them with the experimental observations. Taking experimental data from the ITPA profile database, we have carried out transport simulations with the CDBM, GLF23 and Weiland models by the one-dimensional diffusive transport code TASK/TR. The results are evaluated by the six figures of merit as specified in ITER Physics Basis$^1$. From the simulation on 55 discharges, it is found that each model has unique dependence on devices and operation modes and the CDBM model gives the most satisfactory results. We have incorporated the dependence on the elongation on the CDBM model$^2$ and confirmed that the accuracy of the prediction is improved for H-mode discharges. Single-particle-species heat transport simulations have indicated that the CDBM model reproduces $T_{\mathrm{i}}$ profiles more accurately than $T_{\mathrm{e}}$ profiles. We will also show the results of the predictive simulations coupling TASK/TR and TASK/EQ, two-dimensional equilibrium code, for high performance plasmas with internal transport barriers like the high $\beta_{\mathrm{p}}$ and reversed shear plasmas. [1] ITER Physics Basis Expert Groups, Nucl. Fusion, \textbf{39}, 2175 (1999) [2] M. Yagi et al., J. Phys. Soc. Japan, \textbf{66}, 379 (1997) [Preview Abstract] |
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LP1.00129: Modulated Electron Cyclotron Heating of TCV Plasmas In the presence of Heat Waves Nonlinear Coupling Ilya Pavlov, Jean-Marc Moret Heat pulse propagation experiments for studying electron heat transport have been performed on TCV tokamak in various plasma conditions. Detailed analysis demonstrates the simultaneous propagation and non-linear interaction of heat pulses induced by Modulated Electron Cyclotron Heating (MECH) and sawtooth activity. The effect of the nonlinear coupling of these heat waves on the measured temperature perturbation will be illustrated. This is undertaken by analyzing sawtoothing plasmas at different MECH localized deposition, and different modulation frequencies. Since both types of heat waves are periodic events of approximately fixed frequency, and since both heat pulse propagations are subject to essentially the same transport physics, non-linear coupling plays an important role when studying perturbative transport. In particular, non-linear coupling in MECH experiments can contaminate the time traces of temperature profiles. The latter are used to extract the electron heat diffusivity for so-called transient transport. Methods to treat such kind of signals on the basis of Higher Order Spectral or polyspectra analysis will be presented. [Preview Abstract] |
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LP1.00130: Status of TRANSP / PTRANSP Douglas McCune, Robert Andre, Eliot Feibush, K. Indireshkumar, Christiane Ludescher-Furth, Lew Randerson The status of the TRANSP integrated tokamak modeling effort is described. This will include the status of TRANSP FusionGrid operations and client software as well as development of the core physics model. Physics modeling topics to be covered include: (1) status of effort to parallelize the NUBEAM Monte Carlo fast ion model; (2) installation of RF and related modules in TRANSP: TORIC, GenRay, and CQL3D; (3) status of equilibrium solver upgrades particularly for ST tokamaks; (4) status of predictive upgrades to TRANSP (i.e. PTRANSP). Examples of recent TRANSP modeling results and applications will be shown. [Preview Abstract] |
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LP1.00131: Particle, momentum and energy conserving Monte Carlo model for ion transport Jes Christiansen, Jack Connor A Monte Carlo model based on guiding centre drift motion and Coulomb collisions has been developed to study collisional ion transport in an axisymmetric tokamak equilibrium. The model features momentum conservation for test particles colliding with the background plasma. Calculations have shown an 8\% particle loss rate during an energy confinement time. The Monte Carlo model has been extended with a recycling scheme in order to conserve particles and plasma energy. Recycling of lost particles occurs as neutrals from either a limiter or the SOL. Conservation of energy is enforced by an ad hoc prescription which assigns the lost particle energy E to the next particle in increments E/100000 per step of motion. This prescription is meant to simulate energy gained by the electrons from the axial electric field. Extensive calculations are made to study the resulting density and temperature profiles; these are accumulated from the Monte Carlo test particle motions. The profiles will be compared with the assumed profiles of the background plasma to establish self-consistency. [Preview Abstract] |
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