Survey of tungsten gross erosion from main plasma facing components in WEST during a L-mode high fluence campaign

An initial high fluence campaign was performed in WEST, in 2023, on the newly installed actively cooled tungsten divertor composed of ITER-grade monoblocks. The campaign consisted in the repetition of a 60 s long Deuterium L-mode pulse in attached divertor conditions, cumulating over 10000s of plasma exposure. A maximum deuterium fluence of approximately 5 ⋅ 1 0 26 m −2 was reached in the outer strike point region, representative of a few high performance ITER pulses. Gross tungsten erosion inferred from visible spectroscopy shows that the most eroded plasma facing component is the inner divertor target with rates ten times larger than on the outer divertor target. The outer midplane tungsten bumpers, located a few centimeters from the plasma, show gross erosion rates two times lower than at the outer divertor. We conclude that the outer midplane bumpers have a negligible contribution to the long range tungsten migration and deposition onto the lower divertor. The cumulated gross erosion rate on the inner divertor translates in an effective gross erosion thickness of about 20 μ m , while it is about 2 μ m for the outer divertor. Strikingly, these orderings coincide with the thickness of deposits found locally on the divertor : the exposed surfaces of high field side monoblocks are covered with several tens of μ m tungsten deposits, while on the lower field side, few μ m thin tungsten deposits are only found on the magnetically shadowed parts of monoblocks. The strong impact of those deposits on WEST operation, namely perturbation of surface temperature measurement with infra-red thermography, and the emission of flakes causing radiative perturbation of the confined plasma, calls for anticipating similar issues in ITER. In particular, the start of research operation shall consider the definition of a divertor erosion budget in order to anticipate the formation of deleterious deposits. • Focus on plasma material interaction on main plasma facing components of WEST during the repetitive L-mode discharges of a Deuterium high fluence campaign. • Overview of main plasma properties along these plasma facing components. • Characterisation of gross tungsten sources from visible spectroscopy. Results are put in regards of micron thick tungsten deposits found on the divertor components. • Contribution from outer midplane limiter erosion is disregarded as the primary source location for deposit formation in the divertor. • The coincidence between gross tungsten erosion rates and local deposit thickness is not yet understood, but calls for the definition of a gross erosion budget in ITER at the start of research operation, in order to anticipate the formation of tungsten deposits on the divertor monoblocks.