Thermal shock behavior of beryllium limiters in tokamak fusion devices

Thermal fatigue tests were performed on prototype beryllium limiter tiles for the JET tokamak using a 30 kW, 1 amp rastered electron beam system. A heat flux of 250 W/cm2 was applied for 15 seconds to a tile 9.5 cm x 6.35 cm x 1 cm for a total of 10,000 cycles. The only damage to the tile was the formation of thermal fatigue microcracks on the heated surface. The depth of microcracking corresponds closely to the region of cyclic plasticity that was predicted by elastic-plastic finite elements stress analyses. The surface cracking did not cause gross structural failure; hence, the beryllium limiters in JET are expected to survive operation at 250 W/cm2 for at least 10,000 plasma discharges