Effect of ELMS on the SOL plasma in DIII-D

Abstract

We have studied the evolution of the edge plasma in VH-mode discharges in DIII-D, by following the changes in the radial profiles of the density and temperature, in the core plasma near the separatrix and in the scrape-off-layer (SOL) plasma outside the separatrix. The electron density and temperature profiles in the SOL do not show any significant difference between the ELM-free H-mode and VH-mode phases of the discharge. In the ELMing phase, the density profile broadens during an ELM, forming a high-density plateau that extends out into the SOL to the limit of the measurement. This plateau persists between the ELMs, although the density in the SOL does relax somewhat between the ELMs, with a characteristic time that can be greater than 10 ms, much longer than the sonic particle flow time to the divertor plates. The density scale length increases with the ELM background, as measured by the photodiode viewing the divertor floor nearest to, but outside, the outer strike point. The electron temperature profile in the SOL also broadens during an ELM, but the broad profile does not persist between ELMs.