Bulletin of the American Physical Society
APS April Meeting 2012
Volume 57, Number 3
Saturday–Tuesday, March 31–April 3 2012; Atlanta, Georgia
Session W16: Sherwood IV |
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Sponsoring Units: DPP Room: Hanover FG |
Tuesday, April 3, 2012 10:45AM - 11:15AM |
W16.00001: Energetic particle effects on n=1 MHD instabilities in a DIII-D hybrid discharge D.P. Brennan, M.R. Halfmoon, C.C. Kim, R.J. La Haye The $\delta f$ kinetic-MHD model in the 3-D extended MHD code NIMROD is used to perform a simulation study of energetic particle effects on the $n=1$ mode in a DIII-D hybrid discharge. The hybrid has low $q_{min} >\sim 1$ at high confinement, and is a candidate operational scenario for burning plasma experiments. However hybrid discharges are limited to moderate $\beta_N$ by the $m/n=2/1$ instability. Using realistic DIII-D equilibria, the stability of the $n=1$ mode is computed over a ($q_{min}$,$\beta_{N}$) space. Unstable modes are driven by energetic particles far into the MHD stable region in this space. The drive is associated with the fishbone mode or BAE mode, depending on $q_{min}$. The stability boundary is found near the experimental ($q_{min}$,$\beta_N$), where the unstable mode has a $m/n=1/1$ component localized near the axis. Experimentally, a $m/n=1/1$ structure is observed in agreement with the computed mode in key physical respects. At higher $q_{min}$ and $\beta_N$ a mode with a broad $m/n=2/1$ structure is unstable. This suggests that the $m/n=2/1$ mode is triggered by energetic particles in these discharges, as $\beta_N$ is increased. A group of several similar discharges shows strong agreement with this computational explanation of onset. [Preview Abstract] |
Tuesday, April 3, 2012 11:15AM - 11:45AM |
W16.00002: Oblique Plasmoid Instabilities S.D. Baalrud, A. Bhattacharjee, Y.-M. Huang, W. Daughton Plasmoids are secondary instabilities of thin current sheets that can initiate fast reconnection in astrophysical and laboratory (including fusion) plasmas. In 3D guide field geometries, a spectrum of plasmoids are excited at resonant surfaces across the layer, rather than just the null surface of the poloidal field, which is the only resonant surface in 2D or in the absence of a guide field. Modes on each resonant surface have a unique angle, $\theta = \arctan (k_z/k_y)$, with respect to the conventional 2D geometry. The spectrum of unstable modes has important consequences for particle acceleration theories by contracting magnetic islands, as well as turbulence generated by overlapping islands. We calculate the linear dispersion relation for oblique plasmoid instabilites from resistive MHD and collisionless kinetic theory for a Harris current sheet. In MHD, a broad spectrum of modes is excited spanning the current sheet. In kinetic theory, the spectrum is much narrower. Previous boundary layer theories did not capture the narrow spectrum in the kinetic regime due to neglect of large $\Delta^\prime$ effects as well as neglect of the phase of the vector potential in the inner layer. Correcting these gives results in agreement with recent PIC and linear Vlasov-Maxwell simulations. [Preview Abstract] |
Tuesday, April 3, 2012 11:45AM - 12:15PM |
W16.00003: Unveiling the kinetic mechanism for RMP penetration in diverted edge geometry C.S. Chang, G. Park, S. Ku, T. Evans, R. Moyer Kinetic understanding of the detailed RMP penetration mechanisms has been obtained for the first time from the Full-f XGC kinetic code in realistic diverted tokamak edge geometry. Kinetic ion and electron dynamics are simulated together with Coulomb collisions and Monte Carlo neutral particles. The system is flux-driven, with the heat and torque sources. It is found that not only the primary response currents, but also the secondary response currents from toroidal coupling is important in understanding the RMP penetration and stochasticity/islands generation. The self-consistent plasma profile response and transport are vitally important. In other words, the RMPs and plasma profile self-organize together. The X-transport also plays an important role. The understandings agree quite well, in qualitative sense, with the experimental findings on DIII-D in the sensitivity to the q-profile, electron collisionality, and in the pedestal profile responses by RMPs including the radial electric field, plasma rotation, and electron perpendicular flow. Stiffness of the outer H-mode confinement layer on RMPs is a natural consequence of the RMP penetration physics in diverted geometry. Turbulence effect and implications to ITER RMP physcs will also be discussed. [Preview Abstract] |
Tuesday, April 3, 2012 12:15PM - 12:45PM |
W16.00004: Theoretical and simulation studies of plasma-wall interactions Xianzhu Tang, Z. Guo, N. Krasheninnikova, G.L. Delzanno, V. Borovikov, D. Perez, A. Voter, B. Uberuaga A number of scientific issues in plasma-wall interaction are being studied at LANL, which broadly fall into three categories: (1) understanding the energy and angular distribution of plasma irradiation flux at the wall; (2) modeling the materials response to plasma and neutron irradiation; (3) understanding the wall feedback to the plasma in terms of recycling, wall potential, and dust transport. Here we focus on recent work which elucidate (1) the role of transport-driven (pre)sheath instabilities in modifying the particle energy and angular distribution of the plasma irradiation flux at the wall; (2) the role of wall boundary condition on upstream plasma profile; and (3) the controlling physics for plasma parallel flow acceleration along the open field lines that intercept the wall. A particularly interesting issue related to (2) and (3) is how low recycling at the wall affects the upstream plasma temperature through reduced collisionality, which we find to be opposite to the common view of a flattened profile of electron temperature close to the core value. A related subtlety is how wall recycling, through its effect on collisionality, changes the wall potential, which is a critical boundary condition in traditional tokamak edge modeling. [Preview Abstract] |
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