High-temperature radiation damage studies of Reduced Activation Ferritic/Martensitic (RAFM) steel at fusion relevant He/dpa ratio using positron beam based Doppler broadening spectroscopy

High-temperature irradiation damage studies of Indian Reduced Activation Ferritic/Martensitic (INRAFM) steel have been carried out at doses relevant to fusion reactor first-wall irradiation conditions. The samples were irradiated up to 70 dpa with 1.1 MeV Fe ions at various temperatures viz., room temperature (RT), 473 K, 673 K and 773 K, with and without pre-injected helium. Depth-wise distribution of irradiation-induced defects and their thermal stability has been studied using positron beam based Doppler broadening spectroscopy. Samples without pre-injected helium show an overall reduction in S-parameter with respect to the irradiation temperature and no growth of voids has occurred, and complete defect recovery is observed at 673 K. In samples that were pre-injected with helium, a decrease in S-parameter due to annealing of vacancies and the formation of He_mV_n clusters is observed from RT to 673 K, and an increase in S-parameter associated with the growth of helium bubbles is observed at 773 K. Thus, helium is found to be necessary and crucial in stabilization and growth of the bubble cavities.