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

Blister formation and D retention in W have been investigated for low energy (similar to 55 +/- 15 eV), high flux (similar to 10(22) m(-2) s(-1)), high fluence (<= 4.5 x 10(26) m(-2)) ion bombardment at moderate temperature (similar to 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 similar to 1 mu m) 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 x 10(19) D m(-2) corresponding to about 2 x 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 x 10(21) D m(-2).