Repetitive Plasma Loads Typical for ITER Type-I ELMS: Simulation in QSPA Kh-50

Abstract

The paper presents energy characteristics of the plasma streams generated with quasi‐steady‐state plasma accelerator QSPA Kh‐50 and the main features of plasma interaction with tungsten surfaces in dependence on plasma heat loads. The samples of pure sintered tungsten of EU trademark have been exposed to hydrogen plasma streams. The experiments were performed with up to 450 pulses (pulse duration of 0.25 ms) and the surface heat loads in the range of (0.45 – 1.2) MJ/m2. A threshold character of morphological changes on the melt tungsten surface with the pulses number is demonstrated. The number of initial exposures without formation of corrugations and pits is about 200, which does not depend on the target heat load magnitude. Further increase of the exposures results in the corrugation structures becoming dominant. The melt layer disintegrates into a set of droplets remaining at the surface. Such evolution seems to be driven by the surface tension forces within the fine cells of crack network. Finally a stabilization of the surface pattern occurs. The onset of intensive evaporation at molten tungsten surface under the repetitive heat loads has also been studied. The evaporation can result in enhanced mass losses: increase of the heat load from 0.75 to 1.1 MJ/m2 rises mass losses for one order of magnitude and causes bubble structures at the surface. Due to evaporation, the erosion crater grows with the rate of 0.04 μm/pulse. It was obtained, that for ELM‐like repetitive loads the melt motion driven by plasma pressure gradient did not contribute significantly to the erosion profile.