RESPONSE OF PLASMA-FACING MATERIALS TO HIGH TRANSIENT HEAT LOADS IN ATOKAMAK

High transient heat loads to plasma-facing components, as they occur d uring plasma disruptions, edge localized modes (ELMs), or vertical dis placement events, can cause damage such as thermal erosion, cracking, or melting. The incidence of high heat flux from a plasma onto a mater ial surface triggers a sequence of dynamic plasma-material interaction processes of a non-linear character, commonly termed 'vapour shieldin g'. As a consequence, the further incident heat flux and the resulting ablation are strongly reduced. To study these effects, fast probe exp eriments were carried out in the TEXTOR tokamak. The materials exposed to the plasma were carbon fibre composites with and without silicon a ddition. The duration of the plasma exposure was 80 ms at a depth of u p to 9 cm into the boundary plasma. Together with a strong decrease of the electron temperature in the boundary plasma, strongly localized e mission of radiation was observed in front of the probe tip. The incid ent heat flux to the probe was strongly reduced, which was also found as result of numerical modelling of the local shielding processes. (C) 1998 Elsevier Science S.A. All rights reserved.