Bulletin of the American Physical Society
51st Annual Meeting of the APS Division of Plasma Physics
Volume 54, Number 15
Monday–Friday, November 2–6, 2009; Atlanta, Georgia
Session XI3: Energetic Particles |
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Chair: Nikolai Gorelenkov, Princeton Plasma Physics Laboratory Room: Centennial II |
Friday, November 6, 2009 9:30AM - 10:00AM |
XI3.00001: Marginal Stability Dynamics for Energetic Particles Invited Speaker: Marginal stability in plasmas characteristically sets a stiff limit to the range of that can be achieved. Below this limit, the system is governed by classical. Near marginal stability, however, plasmas may be subject to rapid processes, resulting in a system that hovers near marginality. This scenario emerged from nonlinear studies of energetic particle relaxation and may be to more general plasma transport. We describe results from several such which include. [1] Avalanches---Near marginal stability, an important point is whether an instability driven by resonant particles where the distribution function has ``free energy'' will cause global radial diffusion. For that,modes need to overlap. This process can be continuous or bursty, the latter having been recently observed in NSTX and DIII-D. [2] Frequency chirping---Recent simulations by Vann showed that marginal stability can be sustained when there is only one unstable linear mode, due to the mechanism of spontaneous frequency sweeping. Although a single mode near stability should not cause dramatic relaxation, nevertheless in the Vann simulations, the achievement of marginal stability induced a continual chirping of that had removed energy from the bulk of the region where the external beam to deposit free energy. The distribution was then found to hover near stability. This mechanism may apply to the n=0 GAM where frequency sweeping might be a mechanism for extracting energy from alpha particles in a burning plasma, thereby reducing the stored alpha particle pressure. One way to implement this is to have the n=0 geodesic acoustic modes (GAM) be preferentially excited, since energy rather than momentum (leading to spatial diffusion) is then primarily extracted from alpha particles. [Preview Abstract] |
Friday, November 6, 2009 10:00AM - 10:30AM |
XI3.00002: Gyrokinetic Simulations of Enhanced Alpha Transport by De-stabilized Alfv\`en Turbulence Invited Speaker: Alfv\`en turbulence, destabilized by fusion-produced
$\alpha$-particles,
is expected to greatly enhance transport of these hot fusion
products. Previously, the gyrokinetic code GYRO [1] was used to
simulate the convective transport of fusion alpha particles by
electrostatic ($\beta=0$) ITG/TEM turbulence driven at low $k$
($0 |
Friday, November 6, 2009 10:30AM - 11:00AM |
XI3.00003: Energetic Particle Transport by Microturbulence Invited Speaker: The confinement of energetic particles is a critical issue in ITER, since ignition relies on self-heating by the energetic alpha particles. Recent DIII-D experiments [1] show significant transport of fast ions produced by neutral beam injection (NBI) in the absence of appreciable MHD activity. We study the diffusion of energetic particles by microscopic ion temperature gradient turbulence in large-scale simulations using a global gyrokinetic toroidal code (GTC). The ion radial excursion is found to be a diffusive process and thus the diffusivity as a function of the energy and pitch angle can be calculated using the random walk model. We find that the diffusivity decreases drastically for high-energy particles due to the averaging effects of the large gyroradius and orbit width, and the fast wave-particle decorrelation [2]. The NBI ion diffusivity is found to decrease rapidly for birth energies up to ten times the plasma temperature and more gradually to a very low level for higher birth energy. Results from GTC simulations explain many features of the fast-ion transport in beam-heated DIII-D plasmas. As predicted by the simulations, the profiles and spectra of fast ions (measured by fast ion $D_\alpha$ diagnostic) deviate more strongly from neoclassical predictions when the plasma temperature is high and when the fast-ion energy is low. TRANSP simulations that use fast-ion diffusion coefficients derived from the GTC simulations improve the agreement with experimental measurements, indicating that the predicted transport by the microturbulence is the correct size to explain the observations.\par \vskip8pt \noindent [1] W.W.\ Heidbrink, {\em et al.}, submitted to Phys.\ Rev.Lett.\ (2009).\par [2] W.~Zhang, {\em et al.}, Phys.\ Rev.\ Lett.\ {\bf 101}, 095001 (2008). [Preview Abstract] |
Friday, November 6, 2009 11:00AM - 11:30AM |
XI3.00004: Experiments in DIII-D Toward Achieving Rapid Shutdown with Runaway Electron Suppression Invited Speaker: For safe discharge shutdown in future large tokamaks in the event of an unavoidable disruption, it is important to develop rapid ($\sim$ several ms)shutdown methods to avoid large runaway electron currents, which pose a serious threat to plasma facing components. Prevention of runaway current formation has been proposed by either increasing electron-electron collisionality with massive particle injection, or magnetically by using externally applied non-axisymmetric fields to increase radial diffusive losses of a runaway seed population. Experiments studying both approaches have been pursued in the DIII-D tokamak. For collisional suppression, three different rapid shutdown methods are being investigated: massive gas injection, massive shattered cryogenic pellet injection, and polystyrene shell pellet injection. First-of-kind demonstrations of fast shutdowns were produced by 3000 Torr-l (0.8-g) shattered $D_2$ pellets and large, 10-mm diameter, 0.3-g polystyrene shell pellets filled with boron powder. The application of external magnetic perturbations shows promising preliminary results in suppressing seed runaway electrons, although lack of repeatability in the runaway seed term made these results challenging to interpret. Experiments have been performed to help understand how runaways form and are transported during rapid shutdown. These experiments confirm that the commonly used 0D loop voltage + Dreicer evaporation picture of runaway seed formation is not applicable here, with relativistic $E > 0.5\,$MeV electrons forming before any external loop voltage appears. Present applications of 0D, 1D, and 2D models to the rapid shutdown and runaway confinement experiments, as well as preliminary extrapolations to ITER, will be discussed. [Preview Abstract] |
Friday, November 6, 2009 11:30AM - 12:00PM |
XI3.00005: Beam particle distribution modification by low amplitude modes Invited Speaker: Modification of a high energy particle distribution by a spectrum of low amplitude modes is investigated using a guiding center code. Only through resonance are modes effective in modifying the distribution. Diagnostics are investigated to illustrate the mode-particle interaction. Effects of pitch angle scattering and drag are included, as well as time dependence of the equilibrium and mode frequencies. A specific example of changes observed in a DIIID neutral beam distribution in the presence of low amplitude Toroidal Alfv\`{e}n (TAE) eigenmodes and Reversed Shear Alfv\`{e}n (RSAE) eigenmodes is examined in detail. Comparison with experimental data shows that low amplitude modes, properly including sufficient mode harmonics and the polarization potentials, in conjunction with pitch angle scattering and mode frequency chirping, can account for significant modification of high energy beam particle distributions. The existence of these effects make necessary the inclusion of a phase space dependent term for beam evolution in deposition codes, which can be larger than neoclassical diffusion terms. It is found that there is a stochastic threshold for beam profile modification, and that the experimental amplitudes are only slightly above this threshold. \\[4pt] Collaborators: N. Gorlenkov, PPPL, W. Heidbrink and M. Van Zeeland, General Atomics [Preview Abstract] |
Friday, November 6, 2009 12:00PM - 12:30PM |
XI3.00006: Observation of a nonaxisymmetric MHD self-organized state Invited Speaker: A nonaxisymmetric stable magnetohydrodynamic (MHD) equilibrium within a prolate cylindrical conducting boundary has been produced experimentally at SSX (Swarthmore Spheromak Experiment). It has $m=1$ toroidal symmetry, helical distortion, and flat $\lambda$ profile. Each of these observed characteristics are in agreement with the magnetically relaxed minimum magnetic energy Taylor state. The Taylor state is computed using the methods described by A. Bondeson $\emph{et al.}$, Phys. Fluids $\bf{24}$, 1682 (1981) and by J. M. Finn $\emph{et al.}$, Phys. Fluids $\bf{24}$, 1336 (1981) and is compared in detail to the measured internal magnetic structure. The lifetime of this nonaxisymmetric CT is comparable to or greater than that of the axisymmetric CTs produced at SSX, thus suggesting good confinement. Despite varied initial conditions determined by two helicity injectors on the SSX device, this same equilibrium consistently emerges as the final state. These results therefore describe a new example of self-organization in an MHD plasma. [Preview Abstract] |
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