Observation of NTM excitation by a Gas Electron Multiplier (GEM) based camera in EAST low-βp H-mode plasmas

Abstract Excitation of an initially stable neoclassical tearing mode (NTM) is investigated in Experimental Advanced Superconducting Tokamak (EAST) low-β_p H-mode plasmas (β_p define as the ratio of the thermal pressure to the poloidal magnetic pressure). Under similar plasma parameters, the n=1 resonant magnetic perturbation (RMP) can’t always successfully excite the m/n=2/1 NTM with the same RMP coil current set-up (n and m are the toroidal and poloidal mode number). Data from a Gas Electron Multiplier (GEM) camera shows the NTM destabilization related to RMP-induced crashes at the q=1 resonant surface during RMP ramping-up phase. The 2nd RMP-induced crashing amplitude exponentially decays with β_p increasing. There exists a critical value (β_p≈0.76), above which the crashing amplitude (or seed island width) is too small (below the critical island width) to trigger a NTM. Observation and analysis indicate that the m/n=2/1 NTM is not forcedly driven by the n=1 RMP (such as the m/n=2/1 component), but is probably due to the electron heat transport between the q=1 and the q=2 resonant surface. This paper provides a direct observation of NTM excitation which also has implications for further investigations of NTM locking and disruptions.