Experimental simulation of plasma disruption with an electron beam

Abstract

The behaviour of several materials during a disruption in a fusion reactor was investigated using an electron beam. The thermal effects of the beam are strongly dependent on the density of energy deposited on the material. Calibration measurements were therefore performed with a particular regard to: — the power incident on the sample: for a given voltage of the accelerated electrons, the incident current must be measured; — the fraction of that power deposited in the material. In fact, not all of the incident electrons deposit their energy in the material, many of them being reflected and others giving rise to different phenomena; — the distribution of the density of the incident power or, indeed, of the density of the electric current of the beam. The difficulties found in the calibration methods used for the electron beam and the results were discussed. After calibration, several experiments to simulate the thermal shock onto limiters/divertor in fusion devices were performed. The following materials were investigated: — aluminium — copper — molybdenum — AISI 316L steel. The following quantities were measured during simulated disruptions: — the depth of the melted zone per pulse — the quantity of metal evaporated as a result of a large number of pulses — the fractional evaporation of Cr and Mn in AISI 316L stainless steel. The steel specimens were seriously damaged by thermal shocks; apart from the melting and evaporation, the formation of cracks was also observed. The motion of the melted metal due to convection or to surface tension gradients may play an important role in the melting.