Plasma-wall interaction and plasma behaviour in the non-boronised all tungsten ASDEX Upgrade

Abstract

ASDEX Upgrade has successfully started the second experimental campaign with a full tungsten coverage of the plasma facing components and without using a boronisation for machine conditioning. The tungsten erosion at all relevant positions in the main chamber and the divertor was investigated. The outer divertor is by far the strongest source region, especially in discharges with high divertor temperature in-between ELMs. In the main chamber, the central column is usually the first limiting structure and produces then larger W erosion fluxes than the outboard limiters. Nevertheless, the tungsten influx from the outboard limiters has a much stronger effect on the tungsten content in the confined plasma. An increase of the available power from the fly-wheel generator allowed for improved H-mode operation at 1 MA, and H factors in the range of 1.2 could be achieved at acceptable W concentrations of about 2¢10 .

Overview on plasma operation with a full tungsten wall in ASDEX Upgrade

“…The divertor W source depends strongly on the divertor plasma temperature and the erosion during ELMs accounts for a significant fraction (above 80%) of the total W eroded, for low temperature divertor conditions. The erosion is found consistent with sputtering by low-Z impurities -either intrinsic or as a result of impurity seeding [34]. During type III ELMy discharges, achieved by very high [32,34].…”

“…The erosion is found consistent with sputtering by low-Z impurities -either intrinsic or as a result of impurity seeding [34]. During type III ELMy discharges, achieved by very high [32,34]. The main chamber W influx mainly depends on the proximity to the W surfaces and a compromise between high field side (HFS) sources and low field side sources has to be found.…”

“…The time resolved W influx is obtained spectroscopically by measuring the WI line radiation at 400.9 nm using about 40 lines-of-sight covering the outboard divertor, some low field side guard and ICRH antenna limiters as well as the central column [34]. The measured photon flux density is transformed into an eroded tungsten flux density using the inverse photon efficiency, i.e.…”

“…The main chamber W influx mainly depends on the proximity to the W surfaces and a compromise between high field side (HFS) sources and low field side sources has to be found. Typically, the integrated fluxes from the LFS and HFS are similar in size for similar distances, but it is more beneficial to have the LFS gap 1-2 centimetres larger than the HFS gap to reach the lowest W content [34]. Obviously, the low field side penetration of W is higher as also seen in Alcator C-Mod for molybdenum [36].…”

“…Obviously, the low field side penetration of W is higher as also seen in Alcator C-Mod for molybdenum [36]. Comparing the absolute value of main chamber and divertor sources the time averaged divertor source is at least one order of magnitude larger in type I ELMy H-mode discharges without using ICRH [34,10]. During ICRH operation the limiter W source can approach the size of that in the divertor.…”