Impact of a resonant magnetic perturbation field on impurity radiation, divertor footprint, and core plasma transport in attached and detached plasmas in the Large Helical Device

Abstract

The effects of resonant magnetic perturbation (RMP) field on impurity radiation, divertor footprint distribution, and core plasma transport are investigated in the detachment discharges of the Large Helical Device (LHD). The RMP with m/n = 1/1 mode creates an edge magnetic island in the stochastic layer, which enhances the impurity emission from low charge states, C2+ and C3+, and then triggers a detachment transition. Emission from the higher charge states, C4+ and C5+, implies enhanced penetration of impurities during the detachment phase with RMP. The toroidal divertor particle flux distribution exhibits n = 1 mode structure in both the attached and detached phases, but with a large toroidal phase shift between the two phases. The distribution in the attached phase is well correlated with the magnetic footprint of field line connection length calculated by the vacuum approximation. During the detached phase, however, the phase shift is not well explained by the vacuum approximation, where a significant plasma response to the external RMP is observed. The energy confinement time becomes systematically shorter with RMP application due to the shrinkage of plasma volume caused by the edge magnetic island. On the other hand, the pressure profile during detachment with RMP is found to be more peaked than without RMP. The analysis using the core transport code TASK3D, considering the heating profiles of neutral beam injection, shows no significant transport degradation during detachment with RMP application, even with the enhanced radiation, reduced divertor flux, and possible impurity penetration.