Tungsten: an option for divertor and main chamber plasma facing components in future fusion devices

Abstract

The tungsten programme in ASDEX Upgrade is pursued towards a full high-Z device. The spectroscopic diagnostic and the cooling factor of W have been extended and refined. The W-coated surfaces represent now a fraction of 65 % (24.8 m 2). The only two major components which are not yet coated are the strikepoint region of the lower divertor as well as the limiters at the low field side. While extending the W surfaces, the W concentration and the discharge behaviour have changed gradually pointing to critical issues when operating with a W wall: anomalous transport in the plasma centre should not be too low, otherwise neoclassical accumulation can occur. A very successful remedy is the addition of central RF heating at the 20-30% level. Regimes with low ELM activity show increased impurity concentration over the whole plasma radius. These discharges can be cured by increasing the ELM frequency through pellet ELM pacemaking or by higher heating power. Moderate gas puffing also mitigates the impurity influx and penetration, however at the expense of lower confinement. The erosion yield at the low field side guard limiter can be as high as 10 3 and fast particle losses from NBI were identified to contribute a significant part to the W sputtering. Discharges run in the upper, W coated divertor do not show higher W concentrations than comparable discharges in the lower C-based divertor.

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

“…In extreme cases of peaking -called accumulation -the central W concentration can reach values which are more than 50 times larger than the edge density [48]. Such a strong accumulation can cause very high central energy losses and lead to hollow temperature profiles and a subsequent collapse of the discharge, where the confinement is strongly degraded.…”

“…It could be shown early in the W programme that central deposition of heating power is very beneficial in reducing the W-peaking [50,48]. This heating can be provided either by ICRH or ECRH and even the deposition profile of beams using different acceleration voltage [4] or injection geometry plays a role.…”

Operational conditions in a W-clad tokamak

“…The sequential procedure allowed integrating the construction work into the annual repair and upgrading works. Simultaneously, the influence of certain PFCs on the plasma performance (for example operating with C or W divertor [13]) and mixed material effects could be studied [5]. However, carbon is still present in the machine and in the plasma discharges, which could influence the results of the operation with tungsten due to surface layers, edge cooling and W sputtering by impact of C ions.…”

“…Comparing the W concentrations measured in the edge and in the centre of the plasma shows that the concentrations profiles can be strongly peaked (up to a factor of 60, see [13]) pointing to the dominant effect of central transport. Additionally, a strong suppression of the edge transport (as in ELM-free H-Mode) can lead to an increased W density over the whole plasma radius [13,16]. The central accumulation can be suppressed by increasing the anomalous transport using central wave heating.…”

Final steps to an all tungsten divertor tokamak

“…Simultaneously, the influence of certain PFCs on the plasma performance (for example operating with a C or a W divertor [14]) and mixed material effects could be studied [6]. Fig.1 shows a view into the ASDEX Upgrade vacuum vessel.…”