Thermal fatigue tests of a prototype beryllium limiter for jet

Abstract

Beryllium is an attractive alternative to graphite for use as armor material for plasma interactive components in fusion devices because of its low atomic number, high strength, and good compatibility with hydrogen. However, beryllium is susceptible to damage from cyclic thermal stresses because of its high elastic modulus and thermal expansion coefficient. We have performed 2-D elastic-plastic finite element stress analyses of prototype beryllium limiter tiles for the JET project that are exposed to a surface heat flux of 250 W/cm² for 15 second pulses. Plastic deformation was predicted to occur at the heated surface during both the heating and cooling phases of the cycle, thereby causing cyclic plastic strain. We also performed thermal fatigue tests using a rastered electron beam to apply the heat load to prototype limiter specimens. After 10 000 thermal fatigue cycles, the only damage of the beryllium tile was microcracking of the heated surface. The depth of this microcracking, 4 mm, corresponds closely to the calculated depth of cyclic plastic strain. These favorable results show that the operating conditions for the JET limiter design can be extended into the regime of cyclic plastic deformation without causing overall structural failure, despite the formation of thermal fatigue cracks.