Plasma induced arcs in remote areas of ASDEX Upgrade

Le résumé fourni par la source

Introduction Arcing as a phenomenon at the first wall in fusion experiments is known for more than 30 years [1]. The occurrence of arcs was concluded from arc tracks found during in-vessel inspections and in post mortem laboratory investigations. In ASDEX Upgrade 3 groups of arcs are found: (i) arcs on components that are connected via field lines to the Scrape Off Layer (SOL) plasma, (ii) arcs in remote areas, i.e. areas that are protected by limiters or other components against the SOL plasma and (iii) arcs at the passive stabilising loop (PSL). The main focus of research is on arcs that are related to the SOL region. These arcs are triggered during normal plasma operation with magnetic confinement as can be concluded from the perpendicular orientation of the arc tracks with respect to the local magnetic field. There are only a few measurements on SOL-arcs in tokamaks with a good temporal resolution [2,3]. The contribution of SOL-arcs to the overall impurity content is small compared to erosion rates deduced from spectroscopic measurements [2,4]. But (i) arcs can dominate the local erosion when a significant part of the tungsten coating is damaged. And (ii) arc tracks are usually detected at the non-strike line modules of the divertor [2,5]. No arcs are detected at the strike line modules itself, whereas the maximum of erosion due to sputtering is highest there as measured spectroscopically [4]. In addition to this toroidally symmetric arc distribution in the divertor region, routine in-vessel inspection of ASDEX Upgrade reveals regions in remote areas that are affected by arcs too. These arcs might be single events, i.e. an arc track is found once during years of operation or they happen at certain locations for certain discharge phases. As a result, a significant amount of material can be evaporated and parts of the machine might be destroyed. Due to the relatively high energy that is stored in the PSL arcs are burning there over a longer time and can be detected by standard video systems with 40 ms time resolution. Here droplets produced by arcs are detected over 2-5 frames, i.e. about 100-200 ms. Since video data is commonly available for all discharges a statistical analysis allows to identify the discharge phase that triggers and drives arcs.

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