Characterization of Energetic Particle-Induced Geodesic Acoustic Modes on DIII-D

The energetic particle-induced geodesic acoustic mode (EGAM) is predominantly an electrostatic n=0, m=0 mode which can cause losses of injected beam ions. On DIII-D, the low frequency mode often appears with the injection of the counter-current, on-axis neutral beam between 20-40 kHz. Mode frequency and amplitude are studied using a database of DIII-D shots containing EGAM activity during the current ramp stage of the discharge. Initial observations show that the EGAM frequency scales with plasma current and density while the amplitude scales inversely with them. Additionally, a single discharge containing EGAM activity, with no other significant modes, is examined in more detail. While the EGAM density fluctuation is dominantly an m=1 mode, vertical line integrated measurements from the CO₂ interferometer suggests that contributions from other poloidal harmonics are significant. Measurements from the magnetic probes show that the average period between each successive EGAM burst is found to slightly increase as the discharge evolves during the current ramp.