Stability of sub-cyclotron Alfvén eigenmodes in mixed species plasmas*

Sub-cyclotron compressional (CAE) and global (GAE) Alfvén eigenmodes are routinely excited by fast ions in spherical tokamaks, and have more recently been observed in conventional aspect ratio tokamaks such as DIII-D and ASDEX Upgrade. Similar instabilities are also responsible for electromagnetic ion cyclotron (EMIC) waves in space plasmas, where the presence of multiple ion species explains the observed frequency bands of propagation and evanescence. In order to aid interpretation of high frequency magnetic measurements in recent DIII-D isotope experiments (H/D/³He), an expression for the local fast ion drive which accounts for multiple ion species has been derived for neutral-beam-driven CAEs and GAEs. Approximate analytic instability criteria are found which constrain the unstable mode frequencies as a function of fast ion properties (beam injection geometry and velocity). The dependence of the linear growth rate on relevant parameters is explored numerically and compared with experimentally measured trends in mode activity. The described stability analysis represents a complementary approach for experimental mode identification and characterization of the resonant subpopulation of fast ions.