Bulletin of the American Physical Society
50th Annual Meeting of the Division of Plasma Physics
Volume 53, Number 14
Monday–Friday, November 17–21, 2008; Dallas, Texas
Session GI1: Energetic Particles and MHD |
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Chair: William Heidbrink, University of California, Irvine Room: Landmark A |
Tuesday, November 18, 2008 9:45AM - 10:15AM |
GI1.00001: Experimental studies on fast-ion transport by Alfv\'{e}n waves on NSTX Invited Speaker: Interaction of fast ions with Alfv\'{e}n eigenmodes (AEs) may redistribute fast ions in phase and real space, and degrade fusion and current drive efficiencies in devices such as ITER. A plethora of AEs naturally occurs in beam-heated NSTX plasmas, making it a suitable environment for fast-ion studies. Space and energy resolved measurements of fast-ion dynamics during AE activity are presented. The effects of toroidicity-induced AEs (TAE), energetic-particle modes (EPM) and compressional AEs (CAE) are discussed. Modest changes in the fast-ion profile, n{\_}f(R), correlate with TAEs for multiple, non-interacting modes. As the beam beta is increased above a critical threshold, TAEs interact non-linearly and disrupt in \textit{avalanches}. n{\_}f(R) decreases, but remains peaked at the magnetic axis. A depletion of the energy range $>$20keV, leading to drops of up to $\sim $20{\%} in the neutron rate over $<$500us, is observed. Good agreement is found between the radial profile and frequency of TAEs, measured by reflectometers and Mirnov coils, and the eigenmode structures calculated by the NOVA-k code. The latter are combined with the measured amplitude and frequency evolution to simulate the effect of TAEs on fast ion transport. Prompt fast-ion losses are also observed during EPMs. Slower time-scales ($\sim $10ms) are observed during CAE activity, which appears to be correlated with low-frequency kink modes. A depletion of the energy spectra above 20keV is observed for 100$<$R$<$130cm. Measurements of n{\_}f(R,t) by Fast-Ion D-Alpha (FIDA) spectroscopy are complemented by data from neutral particle analyzers, a scintillator-based fast-ion loss diagnostic and neutron detectors. Data are validated from studies of MHD quiescent discharges, and benchmarked against the results of simulation codes. [Preview Abstract] |
Tuesday, November 18, 2008 10:15AM - 10:45AM |
GI1.00002: Beta-induced Alfv\'{e}n-Acoustic Eigenmodes in NSTX and DIII-D Driven by Beam Ions Invited Speaker: Understanding and predicting energetic ion driven instabilities is essential for planning self-sustained burning plasma experiments where they can strongly affect fusion alpha confinement and plasma performance. We report on a new class of beam driven plasma oscillations called Beta-induced Alfv\'{e}n-Acoustic Eigenmodes (BAAEs), which results from a coupling, between the Alfv\'{e}nic and acoustic MHD branches. Because of such coupling a proper theoretical treatment of BAAEs is at the forefront of ideal MHD and kinetic theories and is an ideal application for next-generation gyrokinetic codes in a regime relevant to future burning plasma experiments. The BAAE frequency is located in the Alfv\'{e}nic acoustic gap below the GAM frequency. In NSTX with high beta ($\sim$20\%) BAAEs were observed with frequencies up to 30 kHz, whereas in DIII-D due to lower plasma pressure (beta~2\%) the mode frequencies are in the range 10-20 kHz. BAAEs observed so far are localized near the low shear region in plasmas with reversed shear. Their mode structure and frequency from the NOVA code are in good agreement with internal reflectometer and soft X-ray measurements on NSTX. On DIII-D simultaneous radially resolved measurements of density and temperature fluctuations for BAAEs with toroidal mode numbers in the range n=1-15 were made using BES and ECE diagnostics. In NSTX, multiple BAAEs induce strong beam ion losses which correlate with up to 20\% drops in neutron signal. BAAEs are useful for diagnosing the internal safety factor profile, as well as the plasma itself. MSE measurements of the minimum q and radial location in NSTX and DIII-D agree with the values inferred from the BAAE theory over a wide range of plasma conditions. [Preview Abstract] |
Tuesday, November 18, 2008 10:45AM - 11:15AM |
GI1.00003: Observation of Revered Shear Alfv{\'e}n Eigenmodes During the Sawtooth Cycle in Alcator C-Mod Invited Speaker: Reversed shear Alfv\'en eigenmodes (RSAEs) have been identified in Alcator C-Mod plasmas between sawtooth crashes at ITER relevant densities of $n_{e0} \leq 1.5 \; 10^{20} \; m^{-3}$ with 2-5 MW of ICRH power absorbed by H minority heating. The dependence of this phenomenon on $n_e$, $I_p$ and L versus H- mode confinement will be presented. The ideal MHD code NOVA [1] is used to model the RSAE spectra measured by the phase contrast imaging diagnostic and Mirnov coils and shows that $q_ {min}$ can relax to approximately 0.92 prior to the sawtooth crash. Furthermore, these calculations predict that the RSAEs can be unstable with even a slightly reversed shear q profile in the presence of an ICRF generated fast ion (H minority) tail. The formation of a reversed shear q profile during the sawtooth cycle may be explained using a resistive diffusion model starting with current density and temperature profiles that are centrally flattened during the sawtooth crash. Results of multiple resistivity models will be compared in light of the experimental observations. We postulate that the formation of a reversed shear q profile during the sawtooth cycle is similar to that of the current ramp where the parallel current peaks off axis [2]. The implications of these results will be discussed in relation to the observation that Alfv{\'e}n eigenmodes enhance transport [3] and that reversed shear q profiles are expected to be detrimental to core fast ion confinement.\\ \\ Work supported by DOE under DE-FG02-94-ER54235 and DE-FC02-99- ER54512.\\ \\ {[1]} C.Z Chen and M.S. Chance, J. Comput. Phys. 71, 124 (1987).\\ {[2]} M. Porkolab \emph{et al.}, IEEE Transactions on Plasma Science, 34, 229 (2006).\\ {[3]} W. Heidbrink \emph{et al.}, Phys. Rev. Lett. 99, 245002 (2007). [Preview Abstract] |
Tuesday, November 18, 2008 11:15AM - 11:45AM |
GI1.00004: Nonlinear evolution of the cylindrical tearing mode and its diamagnetic stabilization Invited Speaker: The $m=n=1$ tearing mode is widely believed to be responsible for sawtooth crashes in tokamaks, and more broadly, as a paradigm of fast reconnection in collisionless plasmas. Past research has demonstrated that the resistive MHD evolution of this mode fails to capture essential features of experimental observations. While there have been several reduced two-fluid models for the nonlinear evolution of the instability, which have elucidated conditions under which the instability exhibits near-explosive growth or is suppressed, the demonstration of this entire range of dynamics in a complete Hall (or extended) MHD simulation of a tokamak has remained a formidable challenge. This challenge has been overcome by the Magnetic Reconnection Code (MRC). The computational approach to solving these equations has to confront two major challenges: the development thin current sheets and thus a wide range of spatial scales, as well as the large range of time scales caused by fast dispersive waves that are much faster than the reconnection time scale. We have employed variable grids to efficiently resolve small scales, and implicit time stepping methods to step over the fast time scales which are not of physical interest while maintaining stability of the time integration. We obtained very good performance with a Newton-direct solver method, using a code generator to calculate the sparse Jacobian matrices. We used the MRC to produce realistic simulations of sawtooth crashes which are quantitatively comparable with experimental results. Our results include a comprehensive study of the sawtooth instability in resistive and extended MHD, and cover a whole range of near-explosive sawtooth crashes and their nonlinear stabilization. In this process, we have identified new physical mechanisms for nonlinear stabilization of sawtooth crashes. We will also present our latest results for the evolution of the $m=1$ mode in a reversed field pinch configuration. [Preview Abstract] |
Tuesday, November 18, 2008 11:45AM - 12:15PM |
GI1.00005: 3D perturbed tokamak equilibria and the importance of plasma response Invited Speaker: Tokamaks are sensitive to deviations from axisymmetry as small as dB/B $\sim $ 0.01{\%}. In light of the importance of the 3D effects, the Ideal Perturbed Equilibrium Code (IPEC) [1] has been developed to study 3D perturbed tokamak equilibria. Comparisons of IPEC predictions with experiments have highlighted the importance of plasma response effects such as poloidal coupling and amplification of perturbations. These effects are essential to explain error field correction results on NSTX and DIII-D [2], and to evaluate the tolerances to error field and Locked Mode (LM) thresholds in ITER. Also, IPEC predicts that the variation of the field strength including the plasma response can make a large difference in calculating Neoclassical Toroidal Viscoscity (NTV), which has been systematically tested in tokamaks by measuring rotational damping [3]. The impact of the plasma response on NTV torque will be discussed and compared with the n=3 braking experiments on NSTX and DIII-D spanning different ranges of collisionality. Although IPEC can improve modeling of NTV torque, the result is not self-consistent since the currents associated with the torque are missing in the ideal perturbed equilibrium. The shielding by the torque is essential to explain measured amplifications of n=1 travelling waves in high beta-N NSTX plasmas, but is expected to be weak when n$>$1. The non-ambipolar transport driven by NTV is also important to understand Resonant Magnetic Perturbation (RMP) experiments for Edge Localized Mode (ELM) control. IPEC predictions for NSTX and DIII-D RMP experiments will be discussed including implications for ITER. [1] J.-K.Park, Phys. Plasmas 14, 052110 (2007) [2] J.-K.Park, Phys. Rev. Lett. 99, 195003 (2007) [3] W.Zhu, Phys. Rev. Lett. 96, 225002 (2006) [Preview Abstract] |
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