Response of plasma-facing materials to high transient heat loads in a tokamak

Abstract

High transient heat loads to plasma-facing components, as they occur during plasma disruptions, edge localized modes (ELMs), or vertical displacement events, can cause damage such as thermal erosion, cracking, or melting. The incidence of high heat flux from a plasma onto a material surface triggers a sequence of dynamic plasma–material interaction processes of a non-linear character, commonly termed `vapour shielding'. As a consequence, the further incident heat flux and the resulting ablation are strongly reduced. To study these effects, fast probe experiments were carried out in the TEXTOR tokamak. The materials exposed to the plasma were carbon fibre composites with and without silicon addition. The duration of the plasma exposure was 80 ms at a depth of up to 9 cm into the boundary plasma. Together with a strong decrease of the electron temperature in the boundary plasma, strongly localized emission of radiation was observed in front of the probe tip. The incident heat flux to the probe was strongly reduced, which was also found as result of numerical modelling of the local shielding processes.