Tungsten limiter start-up experiments on ASDEX Upgrade and WEST in different boronization states in support of ITER

Le résumé fourni par la source

Abstract Understanding the performance of limiter plasmas in the ITER start-up phase is important for the whole pulse and a possible challenge if performed on tungsten, as will now be the case following the switch to a W first wall in the new 2024 ITER Baseline. Experiments were performed on ASDEX Upgrade and WEST to characterise limiter plasmas using boronizations with different degree of boron surface coverage and toroidal asymmetries. Non-boronized start-up is shown to be slow and laborious, and, on both machines, was aborted in favor of a non-homogeneous boronization, performed also in support of the ITER re-baseline to study the impact of spatially non-uniform boron coatings. This allows normal start-up and short (few 100 m s − 1 , ITER will run ≈ 10 s long limiter phases) limiter phases can be run without problems. Even with a full boronization, the limiter can de-condition and long limiter plasmas of several seconds suffer from high densities and radiation. Short limiter plasmas for plasma current ramp-up remain possible. The conditioning effect on limiter plasmas is documented, but also how the start-up is affected by an ageing boronization.

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