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Description
The plasma facing components of next step fusion devices will handle unprecedented heat flux and particle fluence. The WEST tokamak, equipped with an actively cooled tungsten ITER grade divertor, aims to assess the divertor performance under tokamak conditions. A first high fluence campaign was performed in WEST, in 2023, on the new actively cooled tungsten divertor based on the ITER-grade monoblock concept. The campaign consisted of the repetition of a 60 s long Deuterium L-mode pulse in attached divertor conditions, cumulating over 3 hours of plasma exposure. A deuterium fluence of approximately 5×1026 m−2 was reached in the outer strike point region, representative of a few high performance ITER pulses. This attached condition with electron temperature in the range of 20-40 eV leads to intensive tungsten erosion and to the formation of tungsten deposits prone to the release of flakes that penetrate into the plasma and can trigger disruptions.
After laser cleaning of the whole lower divertor, a second high fluence campaign was performed in WEST, in 2025, with cold edge plasma (Te<5eV) using the X-point radiator regime. The campaign consisted of 4 weeks of repetition of 75s long L-mode pulses (Ip = 370 kA, PLHCD = 3.8 MW, nle= 4×1019 m-2) with a controlled XPR phase of ~70s obtained with nitrogen seeding. Each XPR pulse cumulated a particle fluence of about 3.5×1024 m−2, 25% lower than the pure deuterium attached plasma pulses, while the heat load was divided by factor of 4. Core confinement was improved through a combination of ion dilution effects and reduced tungsten contamination with the reduction of the tungsten sources in the divertor (factor 10 reduction). This scenario was alternated with ohmic discharges of 10s in limiter or divertor configuration to reduce nitrogen legacy and ensure good repeatability of the pulses over the day. After 3 weeks among 4, already ~110 repetitive XPR pulses were performed cumulating ~3 hours of plasma operation, ~25 GJ injected and ~3.8×1026 m−2 of particle fluence, on the right track to cumulate a deuterium fluence equivalent to the previous high fluence campaign. The infrared measurements during the campaign show some divertor surface evolution but at this stage not a single UFO has been observed during the entire campaign. A comparative analysis of the two high fluence campaigns will be discussed as well as the first observations from the visual inspection and post-mortem measurements after the campaign.