Analysis of compressional Alfvén eigenmode stability, frequency, and toroidal mode number on DIII-D

Beam-driven Doppler-shifted cyclotron resonant compressional Alfvén eigenmodes (CAEs), with frequencies and toroidal mode numbers sensitive to the fast-ion phase space distribution, are studied in an experiment to validate and advance theoretical understanding of these modes. This experiment explores a wide range of plasma parameters and exploits the flexibility and variety of neutral beams on DIII-D, allowing for analysis of mode characteristics and its separate dependence on beam pitch angle, density, and power. These modes, observed with frequencies around f ≈ 0.57f_c, are measured with a pair of toroidally separated magnetic field sensing loops, permitting novel analysis of CAE amplitude, toroidal mode number, and frequency. The on-axis co-beams, which inject in the same direction as plasma current, were found to excite co-propagating modes, with 4 ≤ n ≤ 10, while the off-axis co-beam and counter-beam were found to excite counter-propagating modes, with -10 ≤ n ≤ -1. The possibility of mode numbers aliasing is under evaluation. A beam current threshold at constant voltage was also observed, suggesting a beam density threshold for the instability.