Modelling of tungsten contamination and screening in WEST plasma discharges

Abstract

The WEST experiment is currently operating with tungsten plasmafacing components and testing ITER-like divertor monoblocks. In order to support WEST experiments interpretation, numerical analyses were carried out. Starting from WEST experimental data, realistic background plasma conditions were reproduced through SolEdge-EIRENE and used as input for ERO2.0 simulations to investigate tungsten migration. Tungsten contamination due to the different plasmafacing components was modelled under different plasma conditions, highlighting a non-negligible contribution of tungsten coming from the tokamak main chamber. Tungsten penetration factor was computed and used as an indication for tungsten screening by the background plasma at the different tokamak plasma-facing components. Simulations showed the main chamber components to be very weakly screened. Light impurities charge was showed to influence not only tungsten sputtering, but also its probability to enter the confined plasma. Simulations results indicated that even when the tungsten source is not heavily influenced by self-sputtering, contamination of the confined plasma can be strongly impacted by it in low density 1 background plasma conditions. Finally, a one-to-one comparison between tungsten visible spectroscopy at the lower divertor from experimental data and from synthetic diagnostics was performed, showing that it is possible to reproduce a realistic lower divertor signal following experimental evidence on light impurities asymmetry between the targets.

Interpretive modelling of boron transport in the boundary plasma of WEST experiments with the impurity powder dropper

“…The antennae have a toroidal length of approximately 30 • , so in order to have a compromise in terms of the recycling flux and cooling effect on the plasma, the portion of the wall contour that represents the antennae is placed at major radius R LHCD = 3.1 m, further away from the separatrix with respect to its actual radial position in the modelled experiment. This working assumption is consistent with previous WEST modelling efforts [22][23][24]. The inner boundary of the simulation domain was placed far from the separatrix in the confined plasma to model B transport in edge plasma, since there was evidence from the VUV spectroscopy (increase of 24.3 nm B-V line intensity) that B may be transported into the confined plasma [6].…”

Developing high performance RF heating scenarios on the WEST tokamak

“…The eroded W flux of the upper divertor and baffle is expected to be stronger than that of the RF antennas but taking into account that the baffle is fully in the shadow of the antennas and the divertor is protuding by less than 1 cm, the tungsten flux should not exceed that from the RF antennas by more than one order of magnitude. However, modelling of the W transport from the emission at the wall to the confined plasma shows that the screening factor is at least two orders of magnitude larger for the upper divertor and baffle than for the RF antennas [36]. The contribution of the upper divertor and baffle to the plasma tungsten contamination can be estimated by comparing P rad-Bulk of discharges having same power, plasma current, electron density and distance of the antennas to the separatrix (D ap ).…”

Characterizing W sources in the all-W wall, all-RF WEST tokamak environment * , **

“…Hence, the significance of these main chamber sources on core contamination can be underestimated. In particular, the core W contamination can be greatly enhanced due to less screening at main chamber locations, where a lower plasma density results in increased ionization mean free path length compared to the divertor [7][8][9]. This is illustrated in figure 1.…”