Subcyclotron Instability of Alfvén Eigenmodes due to Energetic Ions in Low Aspect Ratio Plasmas
Authors: N.N. Gorelenkov, E. Fredrickson, E. Belova, C.Z. Cheng, D. Gates, and R. White
Date of PPPL Report: August 2003
Presented at: the Thirtieth European Physical Society Conference on Controlled Fusion and Plasma Physics, 7-11 July 2003, St. Petersburg, Russia Federation, Russia.
High-frequency modes with frequencies below the fundamental cyclotron frequency of thermal ions were observed in the National Spherical Torus Experiment (NSTX). Based on the measured spectrum of high-frequency modes they are identified as Compressional Alfvén Eigenmodes (CAEs) and Global Alfvén Eigenmodes (GAEs). CAEs have similar time evolution as plasma parameters change, while GAEs may intersect due to q-profile relaxation. A theory has been developed to study the properties of these modes. Both types of instabilities are driven by the tangential neutral-beam injection in NSTX. Beam ions excite CAEs/GAEs through the Doppler-shifted cyclotron resonance. The main source for the drive is the velocity space anisotropy of the beam ion distribution function. Simulations of the effect CAEs/GAEs may have on plasma ions indicate that these modes may provide a channel for efficient energy transfer from fast ions directly to thermal ions.