Bernhard Unterberg

Powder injection molding (PIM) has been used to produce nearly net-shaped samples of tungsten-based alloys. These alloys have been previously shown to have favorable characteristics when compared with standard ITER-grade tungsten. Six different alloys were produced with this method: W-1TiC, W-2Y₂O₃, W-3Re-1TiC, W-3Re-2Y₂O₃, W-1HfC and W-1La₂O₃-1TiC. These were tested alongside ITER-grade tungsten in the PSI-2 linear plasma device under ITER-relevant plasma and heat loads to assess their suitability for use in a fusion reactor. All materials showed good behavior when exposed to the lower pulse number tests (≤1000 ELM-like pulses), although standard tungsten performed slightly better, with no observable difference in surface roughness. High-power shots, namely one laser pulse of 1.6 GWm⁻², revealed that samples containing yttria are more prone to melting and droplet ejection. After high pulse number tests (10,000 and 100,000 pulses), with and without plasma, the reference tungsten showed the most cracking and highest surface roughness of all materials, while the PIM samples seemed to have a higher resistance to cracking. This can be attributed to the higher ductility of these alloys, particularly those containing rhenium. This means that tungsten-based alloys, whether produced via PIM or other methods, could potentially be used in certain areas of a fusion reactor.

Plasma-facing materials (PFMs) in the ITER divertor will be exposed to severe conditions, including exposure to transient heat loads from edge-localized modes (ELMs) and to plasma particles and neutrons. Tungsten is the material chosen as PFM for the ITER divertor. In previous tests, bubble formation in ITER-grade tungsten was detected when exposed to fusion relevant conditions. For this study, ITER-grade tungsten was exposed to simultaneous ELM-like transient heat loads and D/He (6%) plasma in the linear plasma device PSI-2. Bubble formation was then investigated via SEM micrographs and FIB cuts. It was found that for exposure to 100.000 laser pulses of 0.6 GWm⁻² absorbed power density (P_abs), only small bubbles in the nanometer range were formed close to the surface. After increasing P_abs to 0.8 and 1.0 GWm⁻², the size of the bubbles went up to about 1 µm in size and were deeper below the surface. Increasing the plasma fluence had an even larger effect, more than doubling bubble density and increasing bubble size to up to 2 µm in diameter. When using deuterium-only plasma, the samples showed no bubble formation and reduced cracking, showing such bubble formation is caused by exposure to helium plasma.

Once ITER commences full power operation, the ITER divertor will be exposed to high thermal and particle loads. Tungsten was chosen as plasma facing material for the ITER divertor. It is, therefore, of the utmost importance to understand the behavior of ITER-grade tungsten under conditions similar to those it will have to withstand inside the reactor. In this study, ITER-grade tungsten samples were exposed to stationary D/He(6%) plasma and ELM-like transient heat loads in the linear plasma device PSI-2. The effects of each kind of load was first studied separately, and the synergistic effects obtained when exposed to both loads simultaneously were then analyzed. Additionally, the hardness of a recrystallized tungsten sample after exposure to simultaneous loads was tested via nanoindentation. The results indicate that hydrogen and helium embrittlement worsens the cracking behavior of the material when exposed to the simultaneous loads compared to only heat loads. Additionally, bubbles of up to 1 μ m are formed under the surface due to the synergistic effects at the highest heat load. The nanoindentation tests showed that plasma and heat loads increase the hardness of the material by 39%, but only plasma loads appeared to have no effect on it.

The influence of helium and argon impurities on the deuterium retention in tungsten is investigated by a numerical diffusion model, which treats diffusing depth profiles for deuterium and helium or argon in tungsten, taking into account the suggested effects of helium or argon. With helium, a helium nanobubble layer builds up at the surface of the sample, with depths higher than the penetration depth of the incident helium and deuterium ions. The nanobubbles form a porous network, which allows the release of trapped deuterium by surface recombination and diffusion through the pores to the surface. For argon, only a shallow layer of argon-induced defects exists, which also act as trapping sites for deuterium. A number of experiments with tungsten samples were conducted at the linear plasma device PSI-2 in support of the model. Helium and argon were admixed to deuterium plasma in ratios of up to 8% for otherwise similar exposure conditions. In addition, a variation of ion fluences was performed for investigation of the onset and evolution of the effects of impurities. The model shows that the influence on the deuterium retention both for helium nanobubbles as well as for argon-induced defects depends strongly on the ratio between the thickness of the helium- or argon-affected layer and the penetration depth of deuterium ions.