Temperature dependence of retarded recrystallisation in helium plasma-exposed tungsten

Abstract

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