Damaging of inclined/misaligned castellated tungsten surfces exposed to a large number of repetitive QSPA plasma loads

Abstract

In fusion devices like ITER and DEMO, transient events could result in substantial damage in the form of erosion of exposed surfaces due to the extreme energy loads. Castellated tungsten mock-ups were chosen as reference design for the plasma-facing components (PFC) of fusion devices. The material response of these mock-ups is crucial for the lifetime of the PFC and needs to be evaluated for a large number of repetitive plasma impacts. Therefore, an experimental investigation of erosion processes on castellated tungsten surfaces has been performed within the powerful quasi-stationary plasma accelerator QSPA Kh-50 with conditions that simulated a failure of the edge localized mode control in ITER. The surface energy load measured with a calorimeter was 0.9 MJ m−2, i.e. above the melting (0.6 MJ m−2) and below the evaporation (1.1 MJ m−2) thresholds of tungsten. The plasma pulse duration was 0.25 ms. Intense overheating of the castellated edge, which interacted with the plasma first, was observed alongside the formation of an excrescence of shifted material during the first few tens of plasma pulses. The maximum number of ejected particles was registered after the plasma pulses, i.e. during the cooling process. Nevertheless, the amount of particles ejected during the plasma exposure increased with the plasma pulse number. The splashing of droplets and ejection of dust was suppressed on flat surfaces of castellated monoblocks, whereas the edges were identified as the main source of ejected particles.