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 KI1: Turbulence and Transport: Experiment and Simulation |
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Chair: Charles Greenfield, General Atomics Room: Adam's Mark Hotel Plaza Ballroom ABC |
Wednesday, October 26, 2005 9:30AM - 10:00AM |
KI1.00001: Experimental Test of the Neoclassical Theory of Poloidal Rotation in Tokamaks Invited Speaker: Plasma rotation plays many critical roles in fusion plasmas such as the suppression of turbulence, the formation of transport barriers and the stabilization of both resistive wall modes and neoclassical tearing modes. Despite the importance of rotation, our present understanding of momentum transport is clearly inadequate. Our lack of understanding is in part related to the difficulty of performing accurate poloidal rotation measurements. Recently, detailed measurements of poloidal rotation have been obtained in the core of a wide range of \hbox{DIII-D} discharges using charge exchange recombination spectroscopy, and these measurements have been compared with predictions based on the neoclassical theory of poloidal rotation from the code NCLASS. The analysis shows that the neoclassically predicted poloidal rotation is in many cases too low by an order of magnitude compared to the actual measurements. The inferred poloidal rotation is based on careful consideration of the effective energy-dependent cross-section (which is modified by the presence of excited beam neutrals) and of the gyro motion of the ions. These effects can generate apparent poloidal velocities that are many times greater than the neoclassical predictions. For consistency, the radial electric field is independently computed from different impurity species and also compared against motional Stark effect spectroscopic measurements. The large discrepancy between the measured and predicted poloidal rotation suggests that (i)~the neoclassical theory of poloidal rotation may need substantial revision and (ii)~knowledge of poloidal rotation is essential for quantitative determination of the radial electric field from charge exchange measurements. [Preview Abstract] |
Wednesday, October 26, 2005 10:00AM - 10:30AM |
KI1.00002: A New Paradigm of Plasma Transport and Zonal Flows Invited Speaker: First, a set of experiments to explore the isotope scaling paradox is described and a new paradigm is proposed. Most tokamak experimental results indicate dependence of the ion thermal conductivity on the isotopic mass close to $\chi_ {\perp}\sim A_i^{-0.5}$ , i.e., inverse gyro-Bohm. This is in stark contradiction to most present theoretical models predicting Bohm $(A_i^0)$ or gyro- Bohm $(A_i^{0.5})$ scaling. A basic physics experiment [1] on the anomalous ion thermal conduction due to ion temperature gradient instabilities in two different gases (hydrogen and deuterium) closely confirms the tokamak results. Another series of experiments designed to explore the physics basis of this scaling appears to lead to a new model for this scaling based on 3-wave coupling of two ion temperature gradient radial harmonics and an ion acoustic wave. The resulting isotopic scaling of transport is $\sim A_i^{-0.5}$ dictated primarily by the ion acoustic damping. This basic physics is deemed to be extrapolatable to tokamaks resolving the paradox [2]. Secondly, the much discussed theoretical role of zonal flows in transport regulation is critically examined by another set of experiments [3]. Direct detection of zonal flows in tokamaks has been nearly impossible rendering its widely believed role experimentally unverified. A novel diagnostic has been developed by the observation that the effect of zonal flow can be seen as the FM modulation (at zonal flow frequency) of the carrier frequency of the large equilibrium Doppler shift frequency of ITG modes both in tokamaks and CLM. The experimental results indicate zonal flow levels roughly of the order of our model prediction. However, the experimental shear level is much lower than that predicted by the present theories for transport regulation. This research was supported by U.S. Department of Energy Grant No. DE-FG02-98ER-54464.\\ References\\ 1. V.Sokolov and A.K.Sen, Experimental Study of Isotope Scaling of Ion Thermal Transport, Phys. Rev. Lett. 89, 095001 (2002).\\ 2. V.Sokolov and A.K.Sen, New Paradigm for the Isotope Scaling of Plasma Transport Paradox, Phys. Rev. Lett. 92, 165002 (2004).\\ 3. V.Sokolov, X.Wei and A.K.Sen, APS DPP meeting, Savannah, '04. [Preview Abstract] |
Wednesday, October 26, 2005 10:30AM - 11:00AM |
KI1.00003: Neoclassical and Turbulent Transport in Shaped Toroidal plasmas Invited Speaker: The nonlocal physics associated with turbulent and neoclassical transport in tokamaks has been investigated. When using the global neoclassical particle code GTC-Neo to realistically assess the irreducible minimum level of transport in NSTX plasma, the non-local effects in the collisional relaxation of the ions are clearly observed when the ion orbit size is large compared to either the plasma gradient scale length or the local minor radius. This generally brings the simulated ion thermal transport closer to the experimental measurements. When compared to the radial force balance relation with the standard neoclassical poloidal flow, the radial electric field from these simulations also shows significant differences in the region of the internal transport barrier in NSTX plasmas. Applications of a new general geometry version of the GTC code for gyrokinetic simulation of shaped plasmas have demonstrated that ion temperature gradient (ITG) driven turbulence, which grows initially in the linearly unstable region, spreads in both the inward and outward radial directions into the stable regions, leading to radially global turbulence and transport nonlocality. The global phenomenon of turbulence spreading appears quite generic, independent of the presence of zonal flow. The zonal flow, however, may significantly change the nonlinear dynamics of the spreading process. In the presence of self-generated zonal flow, an early spreading with substantial growth in turbulence intensity is observed before saturation of ITG instability in the unstable region. The nonlinearly driven turbulence in the stable region grows even faster than the initial linear instability. The evolution of the turbulence spectra in both the linearly unstable and stable regions, and the associated nonlinear dynamics of energy cascading to the longer wavelength (low-n) modes, will be presented and compared to a single-n nonlinear simulation. Work collaborated with SciDAC GPS Center and NSTX Experiment, and supported by US Department of Energy. [Preview Abstract] |
Wednesday, October 26, 2005 11:00AM - 11:30AM |
KI1.00004: Gyrokinetic simulations of ETG Turbulence* Invited Speaker: Recent gyrokinetic simulations of electron temperature gradient (ETG) turbulence [1,2] produced different results despite similar plasma parameters. Ref.[1] differs from Ref.[2] in that [1] eliminates magnetically trapped particles ($ r/R=0 $), while [2] retains magnetically trapped particles ($ r/R \approx 0.18 $). Differences between [1] and [2] have been attributed to insufficient phase-space resolution and novel physics associated with toroidicity and/or global simulations[2]. We have reproduced the results reported in [2] using a flux-tube, particle-in-cell (PIC) code, PG3EQ[3], thereby eliminating global effects as the cause of the discrepancy. We observe late-time decay of ETG turbulence and the steady-state heat transport in agreement with [2], and show this results from discrete particle noise. Discrete particle noise is a numerical artifact, so both the PG3EQ simulations reported here and those reported in Ref.[2] have little to say about steady-state ETG turbulence and the associated anomalous electron heat transport. Our attempts to benchmark PIC and continuum[4] codes at the plasma parameters used in Ref.[2] produced very large, intermittent transport. We will present an alternate benchmark point for ETG turbulence, where several codes reproduce the same transport levels. Parameter scans about this new benchmark point will be used to investigate the parameter dependence of ETG transport and to elucidate saturation mechanisms proposed in Refs.[1,2] and elsewhere[5-7].\\ \noindent *In collaboration with A. Dimits (LLNL), J. Candy, C. Estrada-Mila (GA), W. Dorland (U of MD), F. Jenko, T. Dannert (Max-Planck Institut), and G. Hammett (PPPL). Work at LLNL performed for US DOE under Contract W7405-ENG-48.\\ \noindent [1]~F. Jenko and W. Dorland, PRL {\bf89}, 225001 (2002).\\ \noindent [2]~Z. Lin et al, 2004 Sherwood Mtg.; 2004 TTF Mtg.; Fusion Energy 2004 (IAEA, Vienna, 2005); Bull. Am. Phys. Soc. (November, 2004); 2005 TTF Mtg.; 2005 Sherwood Mtg.; Z. Lin, et al, Phys. Plasmas {\bf12}, 056125 (2005). \\ \noindent [3]~A.M. Dimits, et al, Phys. Rev. Letters {\bf77}, 71 (1996).\\ \noindent [4]~J. Candy, and R.E. Waltz, JCP {\bf186}, 5445 (2003); F. Jenko, et al, Phys. Plasmas {\bf7}, 1905 (2000).\\ \noindent [5]~S.C. Cowley, et al, Phys. Fluids B {\bf3}, 2767 (1991).\\ \noindent [6]~C. Holland and P. Diamond, submitted to Physics Letters A (2005).\\ \noindent [7]~F. Jenko, Theory of Fusion Plasmas (2002). [Preview Abstract] |
Wednesday, October 26, 2005 11:30AM - 12:00PM |
KI1.00005: Global particle-in-cell simulations of microturbulence with kinetic electrons Invited Speaker: A systematic approach (called the splitting scheme [1]) to accurately model electron kinetic effects in gyrokinetic PIC simulation is presented. The 1D application of the scheme shows that the linear properties of the simulated plasma are more accurate [1]than the conventional perturbative delta-f scheme and that the nonlinear properties are considerably improved [2] It will be shown that an accurate loading of the initial distribution function based on a neural network algorithm [3] and a noise-free collisional operator [4]for PIC codes allow for even longer simulations with good momentum and energy conservation properties. The toroidal version of the scheme, which relies on a global finite-element elliptic solver [5] is used in the GTC code [6]; preliminary results on trapped- electron mode modified ITG turbulence will be reported.\newline \newline [1]J.L.V. Lewandowski, Phys. Plasmas 8, 3204 (2003)\newline [2]J.L.V. Lewandowski, PPCF, 45, L49 (2003)\newline [3]J.L.V. Lewandowski, Phys. Lett. A, 291 (2003)\newline [4] J.L.V. Lewandowski, Phys. Plasmas 12, 2322 (2005)\newline [5] Y. Nishimura et al, submitted to JCP (2004) \newline [6] Z. Lin et al, Phys. Plasmas 6, 1857 (2000) [supported by Scidac Center for GPS project] [Preview Abstract] |
Wednesday, October 26, 2005 12:00PM - 12:30PM |
KI1.00006: Tokamak Pellet Fueling Simulations using 3D Adaptive Mesh Refinement Invited Speaker: We present results of fully 3D extended MHD simulations of fueling pellets injected into tokamaks. The physical processes of pellet injection in high temperature tokamaks span several decades of space-time scales, which has prevented effective simulations of these events in the past. The large disparity between pellet size and device size, the large density differences between the pellet ablation cloud and the ambient plasma, and the non-local electron heat transport all pose severe numerical challenges. Block structured local adaptive mesh refinement (as in Ref.[1]), extended to use the equilibrium magnetic coordinates, is employed to mitigate the problems due to the large range of spatial scales. Generalized upwinding techniques are employed to deal with sharp gradients. A critical component is the modeling of the highly anisotropic energy transfer from the background hot plasma to the pellet ablation cloud via long mean-free-path electrons along magnetic field lines. The modeling includes a semi-analytical kinetic treatment of the transport of electron energy flux through the ablation cloud[2]. The ablation process is included using an analytical model[3]. We discuss the phenomenology of the mass redistribution processes involving the density equilibrating along field lines and transport across surfaces (in the large-major-radius direction) due to interchange instabilities caused by the large local pressure at the pellet. The clear benefit of high-field-side injection relative to low-field-side injection is demonstrated and explained. Experimental comparisons are discussed along with an assessment of applying these techniques to ITER. \newline \newline [1] R. Samtaney et al. Comp. Phys. Comm., 164:220--228, 2004. \newline [2] R. Ishizaki et al. Phys. Plasmas, 11:4064--4080, 2004. \newline [3] B. V. Kuteev. Nucl. Fusion, 35:431--453, 1995. [Preview Abstract] |
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