Thermal shock tests on some proposed limiter or first wall materials for fusion reactors

The limiter and the first wall in a fusion reactor will endure severe thermal loads due to interactions with the plasma. This can cause damage either in the form of mechanical effects (fissure, fracture, spallation ...), or in the form of thermal effects (melting, evaporation ...). Due to these effects, the impurity concentration in the plasma will increase causing increased radiation losses. By using low-Z materials for the first wall these losses can be reduced. Enumerated in order of increasing resistance against thermal shock, the following materials were tested: Al2O3, Si3N4, B4C, vitreous carbon, SiC, very fine-grain graphite and Mo. The test method and procedure used is described and it is shown that fairly reproducible results are obtained. Use was made of an electron gun with an accelerating voltage of 5.5 kV. The intensity of the beam can be pulsated and can be adjusted continuously. Various impulse- and rest durations and numbers of impulses were applied. Different beam profiles were obtained by changing the gun configuration and the supply voltages. Results are discussed and interpreted in the light of theoretical considerations