Impact of Edge Magnetic Island on Thermal Instability, Equilibrium, and Detachment Dynamics Induced by Impurity Radiation in LHD

Abstract The impact of edge magnetic islands on divertor detachment in LHD is investigated, with emphasis on thermal instability and thermal equilibrium. During a density ramp-up, as the edge plasma temperature is reduced, radiation is enhanced in cases with an edge magnetic island compared to those without one, and the detached plasma state remains stable. In contrast, in the absence of the island, the increase in radiation becomes uncontrollable, ultimately leading to a radiation collapse of plasma. Analysis of thermal instabilities indicates that the X- and O-points of the island are particularly susceptible to thermal instability due to their distinct magnetic topologies. Impurity radiation measurements reveal that, during density ramp-up, radiation initially emerges around the island’s X-point. Following the detachment transition, the location of peak radiation shifts to the O-point, where signatures of volume recombination are observed. Numerical simulations of edge plasma transport reproduce these dynamic trends, which reinforces the experimental interpretation. Thermal instability growth rates estimated from experimental data indicate that, within the island’s O-point, the growth rates decrease as detachment deepens; in contrast, in the absence of the island, the growth rates continue to increase, approaching radiation collapse. Additional aspects of thermal instability and radial thermal equilibrium are discussed to elucidate the factors contributing to detachment stabilization and to outline remaining challenges for future investigations.