Blister formation and D retention in W have been investigated for low energy (∼55 ± 15 eV), high flux (∼1022 m−2 s−1), high fluence (⩽4.5 × 1026 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 × 1019 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 × 1021 D m−2.

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W targets are exposed at fixed temperature in the range ∼420–1100 K, to either pure D2, D2–δHe (0.1 < δ < 0.25), or D2–δHe–γAr (γ = 0.03) mixture plasma, or He pretreatment plasma followed by exposure to D2 plasma. A strong reduction in D retention is found for exposure temperature above 450 K and incident He-ion fluence exceeding ∼1024 m−2. Reduced D retention values lie well below that measured on D2 plasma-exposed reference targets, and the scatter in retention values reported in the literature. A small level of Ar admixture to D2–0.1He plasma, leading to an Ar ion density fraction of ∼3%, is found to have minimal effect on the D inventory reduction caused by He. In targets with reduced inventory, nuclear-reaction analysis reveals shallow D trapping (<50 nm), in the same locale as nanometre-sized bubbles observed using transmission electron microscopy. It is suggested that near-surface bubbles grow and interconnect, forming pathways leading back to the plasma–material interaction surface, thereby interrupting transport to the bulk and reducing D retention.

The JET 2019-2020 scientific and technological programme exploited the results of years of concerted scientific and engineering work, including the ITER-like wall (ILW: Be wall and W divertor) installed in 2010, improved diagnostic capabilities now fully available, a major Neutral Beam Injection (NBI) upgrade providing record power in 2019-2020, and tested the technical & procedural preparation for safe operation with tritium. Research along three complementary axes yielded a wealth of new results. Firstly, the JET plasma programme delivered scenarios suitable for high fusion power and alpha particle physics in the coming D-T campaign (DTE2), with record sustained neutron rates, as well as plasmas for clarifying the impact of isotope mass on plasma core, edge and plasma-wall interactions, and for ITER pre-fusion power operation. The efficacy of the newly installed Shattered Pellet Injector for mitigating disruption forces and runaway electrons was demonstrated. Secondly, research on the consequences of long-term exposure to JET-ILW plasma was completed, with emphasis on wall damage and fuel retention, and with analyses of wall materials and dust particles that will help validate assumptions and codes for design & operation of ITER and DEMO. Thirdly, the nuclear technology programme aiming to deliver maximum technological return from operations in D, T and D-T benefited from the highest D-D neutron yield in years, securing results for validating radiation transport and activation codes, and nuclear data for ITER.

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