• Energy Research

Plasma-facing materials for next generation fusion devices, like ITER and DEMO, will be submitted to intense fluxes of light elements, notably He and H isotopes (HI). Our study focuses on tritium (T) retention on a wide range of W samples: first, different types of W materials were investigated to distinguish the impact of the pristine original structure on the retention, from W-coated samples to ITER-grade pure W samples submitted to various annealing and manufacturing procedures, along with monocrystalline W for reference. Then, He and He-D irradiated W samples were studied to investigate the impact on He-damages such as nano-bubbles (exposures in LHD or PSI-2) on T retention.We exposed all the samples to tritium gas-loading using a gentle technique preventing any introduction of new damage in the material. Tritium desorption is measured by Liquid Scintillation counting (LSC) at ambient and high temperatures (800 °C). The remaining T inventory is then measured by sample full dissolution and LSC. Results on T inventory on He exposed samples highlighted that in all cases, tritium desorption as a gas (HT) increases significantly due to the formation of He damages. Up to 1.8 times more T can be trapped in the material through a competition of various mechanisms, but the major part of the inventory desorbs at room temperature, and so will most likely not take part to the long-term trapped inventory for safety and operational perspectives. Unfortunately, investigation of “as received” industrial W (used for the making of plasma-facing materials) highlighted a strong impact of the pre existing defects on T retention: up to 2.5 times more T is trapped in “as received W” compared to annealed and polish W, and desorbs only at 800 °C, meaning ideal W material studies may underestimate T inventory for tokamak relevant conditions. Keywords: Tungsten, Helium, Tritium inventory, Plasma-wall interactions

The influence of pre-existing displacement damage on the early stages of helium (He) interaction with tungsten (W) and the resulting defect creation was investigated experimentally. Samples were irradiated with 20.3 MeV W ions to different damage levels of 0.004, 0.01 and 0.1 displacements per atom (dpa). Displacement-damaged and undamaged W samples were exposed to a He plasma at room temperature at He fluxes of 1.2–1.8 × 1018 He/m2s to fluences of up to 1022 He/m2. He ion energy of 100 eV was used to remain well below the threshold for displacement damage creation in the bulk. Elastic recoil detection analysis (ERDA) shows that the He retention in the damaged samples is up to one order of magnitude larger than in the undamaged sample. Compared with the W samples exposed to He with energies of 300 – 500 eV in literature, the data in this study shows substantially lower He retention. Detailed He depth distributions were derived by stepwise removal of thin surface layers (via anodic oxidation and dissolution of the oxide) and subsequent ERDA measurements of the remaining He content. Pre-damaged samples show a significantly faster decrease in He concentration with depth than the undamaged sample, indicating that He is efficiently stopped by pre-existing defects from diffusing into deeper regions beyond 34 nm. The undamaged sample exhibits a lower He concentration in the near surface region and a flatter distribution of He up to a depth of 100 nm. The characterization of samples before and after He plasma exposure by Doppler broadening spectroscopy of the positron annihilation line (DBS) gives no indication for self-trapping or trap-mutation in both the pre-damaged and the undamaged samples. From the clear influence of the initial displacement damage level on the He uptake and retention we conclude that He self-trapping mechanisms have a negligible, if any, effect on the He diffusion depth in W at fluxes of 1.2–1.8 × 1018 He/m2s.

The influence of surface roughness on the sputter yield was investigated as a function of the angle of incidence. In this work, nm-smooth and rough samples with roughnesses on the µm length scale were produced by depositing thin Fe and W films on smooth and rough Si substrates via magnetron sputtering. The surface morphology of the samples was determined by atomic force microscopy. The samples were exposed to a 6 keV D3 + ion beam (2 keV/D) under various angles of incidence ranging from 0° to 75° with respect to the surface normal to fluences of the order of 1022 D/m2 . The layer thickness was measured by Rutherford backscattering spectrometry (RBS) before and after erosion. The resulting sputter yields were compared to simulations performed with SDTrimSP (static and dynamic) and SDTrimSP-3D (static), showing good qualitative agreement in all cases, as well as agreement with literature data at normal incidence. This constitutes the first experimental benchmark of SDTrimSP-3D. A discrepancy in the value of the sputter yield for smooth W at normal incidence was observed between the SDTrimSP simulations and the experimental values obtained in this work and found in literature. Analogous experiments were performed to study the sputter yield at normal incidence of 2 keV/D on smooth Au and 6 keV He on smooth W. These sputter yields were also compared to SDTrimSP simulations and literature, showing good agreement in all cases.