Suppression of multiple neoclassical tearing modes by two successively switched helical driven currents in tokamak plasmas

The effect of two successively switched helical externally driven currents imposed on the m/n = 2/1 and 3/2 neoclassical tearing modes (NTMs) in tokamak plasmas is investigated by using the Ci-Liu-Ti code. The results show that under carefully designed timing control, imposing a helical driven current on the m/n = 2/1 NTM first and then on the m/n = 3/2 NTM can effectively suppress these two modes. In the case without considering plasma rotation, continuous application of helical driven current in either the m/n = 2/1 or 3/2 NTM will cause “phase flip,” stopping or switching the driven current before “phase flip” can simply and effectively avoid flipping. In the plasma equilibrium configuration studied in the case, the separately excited m/n = 3/2 NTM can reach saturation without exciting the m/n = 2/1 NTM. On the contrary, the separately excited m/n = 2/1 NTM can ultimately excite m/n = 3/2 NTM, leading to accelerated instability growth after coupling between multiple modes. Therefore, it is necessary to effectively suppress the m/n = 2/1 NTM first. The simulation results have important reference significance for using helical driven current to suppress multiple NTMs under reasonable timing control in experiments.