Observation of electrostatic fluctuations driven by runaway electrons in EAST disruptions

Electrostatic fluctuations driven by runaway electrons (REs) have been observed following a thermal quench during Experimental Advanced Superconducting Tokamak-intended disruptions, which are triggered by massive gas injection. Electrostatic fluctuations are clearly detected using several radiation-related diagnostics and in two distinct frequency bands: 10-20 kHz and 30-40 kHz. The appearances of fluctuations are directly correlated with REs. Fluctuations observed during argon injection and neon injection have significantly different evolution with time, whereas no fluctuations can be found with helium injection. The measured frequency scales with different amounts of injected gases finally tend to be saturated. A clear phase difference is detected, and a mode structure of (m, n) = (1, 0) is identified in the soft x-ray detector array. Here, m and n are the poloidal and toroidal mode numbers, respectively. The geodesic acoustic mode proposed as a candidate instability is further discussed, and the barely trapped/passing electrons can contribute to drive the mode. Fluctuations are also correlated with significant RE loss, which supports the possibility of kinetic instability for RE mitigation in a tokamak reactor.