Temperature dependence of retarded recrystallisation in helium plasma-exposed tungsten

Recrystallisation has been identified as a critical issue for the ITER divertor but currently the synergistic effects of heat and particle bombardment on tungsten recrystallisation are still unclear. In this study, tungsten was exposed to helium plasma under a range of sample temperatures (573 K-1073 K) and ion fluences (10²⁴-10²⁵ m^-2) followed by annealing at a range of temperatures (1373 K-1673 K). It is quantitatively found that larger helium nano-bubbles form at higher sample temperatures during plasma exposure due to increased nano-bubble mobility and coalescence. The retardation of recrystallisation showed the opposite trend, with less recrystallisation occurring in samples exposed at lower temperatures and with smaller nano-bubbles. This result is attributed to the Zener drag force from the helium nano-bubbles opposing grain boundary movements during annealing with the effect of the force being inversely proportional to bubble size. Ion fluence is not found to affect the extent of recrystallisation at any temperature. The presence of the sub-surface helium nano-bubbles increased the microstrain in the tungsten which was eliminated during subsequent annealing as a clear sign of the recovery stage proceeding.