Suppression of “Edge MHD Instability of LHD” by External RMP

Abstract

We experimentally investigate the effect of external resonant magnetic perturbation (RMP) on“Edge MHD instability”, which is observed in a discharge with relatively low magnetic Reynolds number and middle beta in the Large Helical Device (LHD) and leads to minor collapse (rapid degradation of volum-averaged beta value by 10%). When the external RMP is small, the instability is continuously observed in a discharge, and the fluctuation amplitude decreases a little with the increment of the RMP. When the RMP is large enough, the appearance of the instability becomes intermittent, and the fluctuation amplitude rapidly decreases with the increment of the RMP. Moreover, the confinement performance degraded by the instability also recovers when the intermittent instability appears.