Measurement and interpretation of stable and unstable Alfvén eigenmodes in the presence of fast ions in Alcator C-Mod

Abstract

The stability of two types of lightly damped resonant eigenmodes in the presence of energetic ions is studied in Alcator C-Mod. Global Alfven eigenmodes (GAEs) can exist at frequencies between the continua of adjacent poloidal mode numbers to avoid strong continuum damping. Toroidicity-induced Alfven eigenmodes (TAEs) exist in gaps in the frequency continuum induced by toroidal coupling. Passing and trapped fast ions can resonantly excite AEs, which saturate at significant amplitude and redistribute the ions. There is evidence that consequent losses of D-T alpha particles could degrade the fusion burn. Unstable TAEs driven by ICRF-heated fast ions are observed in Alcator C-Mod with toroidal mode numbers of n = -4 and n = 6. Positive mode numbers indicate that modes are driven by fast ions having hollow pressure profiles, in agreement with measurements using the Compact Neutral Particle Analyzer (CNPA). In the absence of sufficient fast ion drive, eigenmodes are detected by exciting them with antennas close to the plasma boundary and monitoring the plasma frequency response with an array of magnetic probes, a method called active MHD excitation. This thesis reports for the first time that TAEs of various toroidal mode numbers are excited with a wide-toroidal-spectrum antenna and observed using a fully resolved toroidal array of probes. GAEs with n = 0 and damping rates around y/wo = -1%, and TAEs with n = 1 and damping rates around /~wo = -1.5% are observed. Rigorous calibration is applied to the magnetic probes to reject the system response of the diagnostic. Measurements demonstrate that even with a wide-spectrum antenna, the range of AEs that are accessible to the diagnostic for any particular equilibrium remains quite limited, subject to the modes' proximity to the plasma edge. A composite spectrum of observed stable and unstable modes, and the stability spectrum calculated by NOVA-K, shows that for fast ions with approximately 150 keV effective temperature, the most unstable mode number tends to be around n = -5. In comparison, the simple scaling of fast ion drive for kopi ~~ 1 predicts unstable modes around n = -8, demonstrating reasonable agreement with the measurements. Local islands of stability that are observed in the toroidal mode number spectrum and the fast ion temperature could be exploited by the strong dependence of the AE spectrum on subtle changes in equilibrium parameters to stabilize AEs in burning plasmas.