The excitation of an initially stable neoclassical tearing mode (NTM) is investigated in Experimental Advanced Superconducting Tokamak (EAST) low-βp H-mode plasmas (βp is defined as the ratio of the thermal pressure to the poloidal magnetic pressure). Using similar plasma parameters, n = 1 resonant magnetic perturbation (RMP) cannot always successfully excite the m/n = 2/1 NTM with the same RMP coil current setup (n and m are the toroidal and poloidal mode numbers, respectively). Data from a gas electron multiplier camera shows that NTM destabilization is related to RMP-induced crashes at the q = 1 resonant surface during the RMP ramp-up phase. The second RMP-induced crash amplitude decays exponentially as βp increases. There is a critical value above which the crash amplitude (or seed island width) is too small (below the critical island width) to trigger an NTM. Observation and analysis indicate that the m/n = 2/1 NTM is not forcibly driven by the n = 1 RMP (such as the m/n = 2/1 component), but is probably due to electron heat transport between the q = 1 and the q = 2 resonant surfaces. This paper describes experimental observations of NTM excitation which also have implications for further investigations of NTM locking and disruptions.
Physics of penetration of resonant magnetic perturbations used for Type I edge localized modes suppression in tokamaks