Theory and observations of high frequency Alfven eigenmodes in low aspect ratio plasmas

摘要

New observations of sub-cyclotron frequency instability in low aspect ratio plasmas in national spherical torus experiments are reported. The frequencies of observed instabilities correlate with the characteristic Alfven velocity of the plasma. A theory of localized compressional Alfven eigenmodes (CAE) and global shear Alfven eigenmodes (GAE) in low aspect ratio plasmas is presented to explain the observed high frequency instabilities. CAEs/GAEs are driven by the velocity space gradient of energetic super-Alfvenic beam ions via Doppler shifted cyclotron resonances. One of the main damping mechanisms of GAEs, the continuum damping, is treated perturbatively within the framework of ideal MHD. Properties of these cyclotron instability ions are presented.