W-Y 2 O 3 alloy with low DBTT (ductile-brittle transition temperature) and high RCT (recrystallization temperature), processed by high energy rate forging (HERF) was exposed to ITER-like steady-state and transient hydrogen plasma in the linear plasma generator Magnum-PSI. The steady-state heat fluxes were in the range of 8.35-16.32 MW•m −2 , resulting in surface base temperatures of the samples in the range of 1271 °C to 1982 °C. The applied transient peak heat flux with a frequency of 5 Hz (a total of 1000 pulses) was about 0.50 GW•m −2 . The exposure time was ~220 s. No obvious morphological change of the exposed sample with a base temperature of 1271 °C was observed, except the preferential erosion of the W/Y 2 O 3 interface. However, cracks along grain boundaries were formed on the surface of the exposed samples with base temperatures above 1389 °C. Pronounced recrystallization and grain growth also occurred for the samples with base temperatures of 1666 °C and 1982 °C. It is desirable to find that no wide and deep crack with preferential propagation direction with regard to the sample dimension was observed, even extensive recrystallization and cracking along grain boundaries occurred. This indicates that decreasing DBTT and increasing RCT simultaneously is desirable to broaden the safe operational temperature window of W as plasma-facing material, and therefore to increase the power handling capability of the plasmafacing units of a tungsten-based divertor in future fusion reactors. However, the formation of W/Y 2 O 3 composite, melting and depletion of the Y 2 O 3 particles gradually occurred with increasing the surface temperatures. It implies that the doping of Y 2 O 3 particles complicates the plasma-material interaction, compared to the pure W case. For example, it raises concerns about the formation of complex dust which will potentially be a significant issue for the safe operation of future fusion reactors (e.g. core plasma contamination and fuel recycling).

In the direct vicinity of plasma-facing surfaces, the incident plasma particles interact with surface-recombined neutrals. Remarkably high near-surface plasma pressure losses were observed in the high-flux linear plasma generator Magnum-PSI. Combining the incoherent and coherent Thomson scattering diagnostics, we directly measured particle, momentum and energy fluxes down to 3 mm from the plasma target surface. At the surface, the particle and total heat flux were also measured, using respectively an in-target Langmuir probe and thermographic methods. The near-surface momentum and energy losses scale with density, and amount to at least 50 % and 20 %, respectively, at ne = 8•10 20 m −3 . These losses are attributed to the efficient exchange of charge, momentum and energy between incident plasma and surface-recombined neutrals. In low-temperature plasmas with sufficient density, incident particles go through several cycles of interaction and surface deposition before leaving the plasma, thereby providing an effective alternative dissipation channel to the incident plasma. Parallel plasma parameter profiles exhibit a transition with increasing plasma density. In lowdensity conditions, the plasma temperature is constant and near-surface ion acceleration is observed, attributed to the ambipolar electric field. Conversely, deceleration and plasma cooling are observed in dense conditions. These results are explained by the combined effect of ionneutral friction and electron-ion thermal equilibration in the so-called thermalized collisional pre-sheath. The energy available for ambipolar acceleration is thus reduced, as well as the upstream flow velocity. In the ITER divertor, enhanced near-surface p-n interaction is expected as well, given the overlap in plasma conditions. Including these effects in finite-element scrape-off layer models requires a near-surface resolution smaller than the neutral mean free path. This amounts to 1 mm in Magnum-PSI, and possibly an order of magnitude smaller in ITER.

It is highly desirable to understand the long term evolution of the divertor material under the extreme steady-state and transient heat and particle loads expected during ITER operation. Here the impact of ELM-like transient loading under combined high-flux plasma and transient ELM-like heat loading in Magnum-PSI was explored to determine how plasma affects the fatigue cracking threshold of tungsten due to ELMs. Mock-ups consisting of five ITER-like monoblocks in a chain were simultaneously exposed to high flux plasma and a high power pulsed laser which closely simulated the ELM impact in terms of heat flux and duration. Loading conditions were chosen to enable comparison to existing data from electron-beam loading, while the influence of surface base temperature (750 °C, 1150 °C or 1500 °C) and impurity seeding (addition of 6.5 ion% He+ and/or 8 ion% Ne+) were also investigated. The plasma loading leads to differences in surface morphology and indicates synergistic effects on the extent of the surface damage. Base temperatures at or above 1150 °C are found to lead to a significant reduction in the fatigue cracking threshold by a factor of two or more compared to at 750 °C. Cracked surfaces are found to be more than ten times rougher than the original microstructure, and additionally when seeding impurities are added surface roughness can be significantly increased by up closely factor of two compared to roughening using pure H plasma. Overall the results indicate that avoiding fatigue cracking in ITER will be very challenging, and that understanding the level to which this can therefore be tolerated is vital for anticipating divertor lifetime and reliability.

As a divertor plasma-facing material, W will experience high -flux plasma irradiation. In particular, severe surface morphology change like fuzz formation can be induced by He plasma irradiation. In this study, fuzz formation on additively manufactured W and W-Ta was investigated. Rolled W, laser -powder -bed -fused (LPBFed) W and W-Ta were exposed to high flux (∼10 23 m −2 s −1 ) He plasma in the linear plasma generator Magnum-PSI with ion energy 12-13 eV at 1273 K. The mean thickness of the fuzz at grain interiors of rolled W, LPBFed W and W-Ta was measured as 0.37 µm, 0.71 µm and 0.23 µm, respectively. The fuzz suppression in LPBFed W-Ta can be attributed to the synergetic effect of solid-solution, dislocation, and secondary -phase nanoparticles. Abnormally grown fuzz was observed near the pre-existing cracks of LPBFed W, while no such structure was found in LPBFed W-Ta. It is found that dislocations play a crucial role in inhibiting fuzz growth. This is confirmed by the difference in fuzz structure in rolled W and LPBFed W, where rolled W has a much greater dislocation density compared to LPBFed W. This work suggests that the fuzz growth kinetics may be tuned by tailoring the microstructures using the LPBF technique.

The lifetime of plasma-facing components (PFCs) will have a strong influence on the efficiency and viability of future fusion power plants. However, the PFCs suffer from thermal stresses and physical sputtering induced by edge-localized modes (ELMs). ELMs in future fusion devices are expected to occur with a high plasma density compared to current day devices such that coupling of recycling neutrals and plasma ions will be strong. Because of the scale hierarchy of future fusion devices compared to the present ones, the influence of this coupling is difficult to predict. Here, we investigate the ELM-like hydrogen plasma induced heat loads on tungsten in the linear device Magnum-PSI, producing ∼1 ms plasma pulses with electron densities up to 3.5 × 10 21 m −3 . A combination of time-resolved Thomson scattering and coherent Thomson scattering was used to acquire plasma parameters in front of the target. Moreover, a fast infrared camera coupled to finite element thermal analyses allowed to determine the deposited heat loads on the target. We found a significant inconsistency between the plasma power calculated with a conventional collisionless sheath model and the absorbed power by the target. Moreover, plasma stagnation upstream and plasma cooling downstream were observed during the pulses. The observations are explained based on ionization and elastic collisions between the recycling neutrals and plasma ions. The results highlight the impact of plasma-neutral interaction on the power deposition behavior of ELM-like hydrogen plasma on tungsten.