Desorption of helium from austenitic stainless steel heavily bombarded by low energy He ions

To discuss helium in the fusion reactor environment, we need more information at much higher doses where the accumulation of helium atoms by implantation in the wall material and the loss by sputtering erosion are at least in balance. In the present work, the behavior of implanted helium in 304SS at high dose was studied by means of in situ microstructure observation and thermal desorption spectroscopy (TDS). TDS experiments showed that helium desorption below about 500 K increased drastically and the total retention saturated at the fluence above 1 × 1021 He/m2. At this condition a remarkable amount of helium was released even at room temperature with a decay time of 1.7 × 104 s. The microscopic observations lead to the conclusion that helium desorbed in the low-temperature region are detrapped from rather weak trapping sites such as the heavily distorted lattice in the vicinity of highly pressurized bubbles, and other lattice defects, but not from the bubbles themselves.