Observations of suppressed retention and blistering for tungsten exposed to deuterium–helium mixture plasmas

TLDR

The tungsten samples were irradiated with low‑energy, high‑flux D+He ions at ~573 K in the PISCES‑A simulator, and the authors propose that the resulting nano‑sized helium bubbles near the surface act as diffusion barriers that reduce deuterium uptake. Blister formation and deuterium retention were markedly suppressed in tungsten exposed to D+He plasmas, with nano‑sized helium bubbles forming as a diffusion barrier; this effect, also seen in fine‑grained W–TiC, indicates that current fusion‑device retention predictions may be overestimated.

Abstract

Blister formation and D retention in W have been investigated for low energy (∼55 ± 15 eV), high flux (∼10 22 m −2 s −1 ), high fluence (⩽4.5 × 10 26 m −2 ) ion bombardment at moderate temperature (∼573 K) in mixed species D+He plasmas in the linear divertor plasma simulator PISCES-A. The amount of D retained in W is found to decrease significantly when compared with that in W exposed to pure D plasmas, as measured with high resolution thermal desorption spectroscopy. Scanning electron microscopy observations reveal the suppression of the blisters, a surface feature known to drive up retention, in the D + He mixture plasma exposed W samples. Reduced D retention is accompanied by the formation of nano-sized high density He bubbles in the near surface, observed with a transmission electron microscope (TEM). It is believed that the nano-bubbles act as a diffusion barrier to implanted D atoms and consequently reduce the amount of uptake in the W material. This newly observed effect implies that current predictions of D retention in W, in actual fusion devices, may be overestimated, since there will be He ash in fusion plasma. Toughness enhanced, fine-grained (grain size of ∼1 µm) W–TiC samples, exposed to pure D plasma conditions, also show little or no evidence of blistering. The measured D retention in the W–TiC samples was approximately 1 × 10 19 D m −2 corresponding to about 2 × 10 −7 of the implanted D fluence, and is very low compared with the retention in pure stress relieved W, which exhibited surface blisters and had a D retention of about 1 × 10 21 D m −2 .