Beam Ion Driven Instabilities in NSTX
Authors: N.N. Gorelenkov, E. Belova, H.L. Berk, C.Z. Cheng, E. Fredrickson, W. Heidbrink, S. Kaye, and G. Kramer
Date of PPPL Report: November 2003
Published in: Physics of Plasmas 2:5 (May 2004) 2586-2593.
A low-field, low-aspect-ratio device such as the NSTX (National Spherical Torus Experiment) is an excellent testbed to study the ITER-relevant physics of fast-particle confinement that is of major importance for burning plasmas. The low Alfvén speed in NSTX offers a window to the super-Alfvénic regime expected in ITER. Effects such as the large finite Larmor radius (FLR), orbit width, strong shaping, and high thermal and fast-ion betas make this effort a greater challenge. We report on the linear stability of different Alfvén eigenmode (AE) branches and compare theory with experimental data.
Low-frequency magnetohydrodynamic (MHD) activities, ~100 kHz on NSTX, are often observed and identified as the toroidicity-induced AEs (TAE) driven by beam ions. Sometimes they are accompanied by beam ion losses in the high-confinement mode (H-mode), high q(0) plasmas. Numerical analysis using the NOVA-K code shows good agreement with the experimental data. The TAE instability was compared in experiments on NSTX and DIII-D. With similar plasma conditions, we tested the theoretical prediction that the toroidal mode number of the most unstable TAEs scales with the machine minor radius, n ~ a. In NSTX, TAEs are observed with n = 1-2, whereas in DIII-D n = 2-6. The confirmation of n scaling validates the predictive capabilities of theoretical tools (NOVA-K) for studying ITER plasmas.
In the high-frequency range, recent observations of rich sub-ion cyclotron frequency MHD activities in NSTX suggest that new instabilities are excited, which we identify as Global shear AEs (GAEs). Similar to the compressional AEs (CAEs), GAEs are destabilized by the Doppler-shifted cyclotron resonance in the presence of 80 keV neutral-beam injection. To simulate GAE/CAEs in realistic NSTX plasma conditions, we have developed a nonlinear hybrid kinetic-MHD code, HYM, which is capable of computing the mode structure, saturation, and energetic particle transport.