Energetic Particle-Induced Geodesic Acoustic Modes on DIII-D

Various properties of the energetic particle-induced geodesic acoustic mode (EGAM) are explored in this large database analysis of DIII-D experimental data. EGAMs are n=0 modes with m=0 electrostatic potential fluctuations (where n/m = toroidal/poloidal mode number), m=1 density fluctuations and m=2 magnetic fluctuations. The fundamental frequency (∼20-40 kHz) of the mode is typically observed to be around half of the traditional geodesic acoustic mode (GAM) frequency. They are most easily destabilized by beams in the counter plasma current (counter-I_p) direction as compared to co-Ip and off-axis beams. During counter beam injection, the mode frequency is found to have the strongest linear correlation with the safety factor (q) with a value of r=−0.712. The stability of the mode in the space of q and poloidal beta (β_p) shows a clear boundary for the mode stability. The stability of the mode is found to be largely determined by the damping rate rather than the drive.