TUNGSTEN MELT LOSSES UNDER QSPA KH-50 PLASMA EXPOSURES SIMULATING ITER ELMs AND DISRUPTIONS

Experimental simulations of ITER transient events with surface heat load parameters relevant to edge-localized-mode (ELM) impacts and disruptions have been performed with a quasi-stationary plasma accelerator Kh-50. In the ELM simulation experiments with heat loads exceeding the tungsten melting threshold, both droplet splashing and solid dust ejection are observed. The erosion products emitted from the exposed tungsten surfaces in the form of droplets and solid dust have been clearly distinguished by variation of impacting heat load with performed analysis of particle ejection start time, their velocities, and changes in the luminosity of the particle traces in front of the target surface recorded with a charge-coupled device. Droplets are emitted during plasma exposure, and dust generation dominates after the end of the plasma pulse, at the time of the following material cooling. The contributions of Kelvin-Helmholtz instabilities to droplet splashing from the melt layer are discussed. Decrease of droplet velocity with increasing surface heat load is observed. This decrease could be attributed to the growing size of the droplets for higher energy loads.