Experiments on helium pumping of pebble divertor

The pebble divertor concept was proposed for compact and high power density fusion reactors. In this concept, fuel gas and impurities including the helium ash might be pumped out by using gas gettering material at the surface layer of the pebbles. In this study, the helium pumping performance of the pebble divertor is experimentally studied. In particular, the transient release from graphite just after the high particle flux irradiation of 5x1020 He/m2s (He+, 2 keV) have been measured with keeping the sample temperature at the maximum temperature in the irradiation. Then the residual retained helium in the sample has been measured by heating up the sample at 873 K. The amount of transiently retained helium is not appreciably different with that of the residual retained helium. From this result in the helium pumping performance, the pebble temperature in the irradiation must be lower than 730 K to satisfy the requirement in steady state operation of A-SSTR