Impurity radiation during ‘breathing’-like oscillations in LHD discharges using a wall limiter

By changing the distribution of the coil currents in the helical coils of the large helical device (LHD) the plasma could be scraped off on the stainless steel inboard wall at the last closed flux surface, eliminating the ergodic edge region and helical divertor (HD) plasmas. During such NBI-heated discharges slow oscillations (1–2.5 Hz) have been observed in the major global parameters. These oscillations exhibit many similarities to the ‘breathing’ relaxation phenomenon observed during long pulse experiments in LHD when the divertor material was stainless steel. These similarities include a large core radiation fraction, significant radiation from the metallic impurities and dependence of the oscillation frequency on density. In contrast, discharges using the graphite HD show hollow radiation profiles and reduced levels of radiation from heavy impurities. Modeling shows that differences in the density dependence of the oscillation frequency between the wall limiter and HD cases can be attributed to differences in the impurity diffusion between the core and ergodic edge regions.