Numerical Study of Instabilities Driven by Energetic Neutral Beam Ions in NSTX
Authors: E.V. Belova, N.N. Gorelenkov, C.Z. Cheng, and E.D. Fredrickson
Date of PPPL Report: July 2003
Presented at: the Thirtieth EPS Conference on Controlled Fusion and Plasma Physics, 7-11 July 2003, St. Petersburg, Russian Federation, Russia.
Recent experimental observations from NSTX {National Spherical Torus Experiment} suggest that many modes in a subcyclotron frequency range are excited during neutral-beam injection (NBI). These modes have been identified as Compressional Alfvén Eigenmodes (CAEs) and Global Alfvén Eigenmodes (GAEs), which are driven unstable through the Doppler-shifted cyclotron resonance with the beam ions. The injection velocities of the NBI ions in NSTX are large compared to Alfvén velocity, V0 > 3VA, and a strong anisotropy in the fast-ion pitch-angle distribution provides the energy source for the instabilities. Recent interest in the excitation of Alfvén Eigenmodes in the frequency range w is less than or approximately equal to wci, where wci is the ion cyclotron frequency, is related to the possibility that these modes can provide a mechanism for direct energy transfer from super-Alfvénic beam ions to thermal ions. Numerical simulations are required in order to find a self-consistent mode structure, and to include the effects of finite Larmor radius (FLR), the nonlinear effects, and the thermal plasma kinetic effects.