Sabina Markelj

This paper gives an overview of erosion and migration studies of tungsten (W) in the WEST tokamak during its Phase 1 (2016-2021) and Phase 2 (from 2022) experimental campaigns with a focus on plasma-facing components (PFCs) at the divertor. In Phase 1, gross erosion of PFCs is in line with observations from other major fusion devices and attributed to low-Z impurities in the plasma. In addition, a strong asymmetry is observed between the high- (inner) and low-field (outer) side divertor targets, in favour of the inner side. Net erosion at rates of <0.5 nm/s is measured around the strike points while the remaining areas are dominated by net deposition. The thickest deposited layers (up to 50 m) with the most complex structures result from a cumulated plasma exposure of ~7 h. The overall erosion-deposition pattern is further influenced by the strong magnetic ripple of WEST, which can result in almost an order of magnitude difference between the maxima and minima of the ripple. In Phase 2, increasing plasma fluence leads to the deposits growing to hundreds of micrometres in thickness. At the same time, erosion proceeds at a constant rate and can reach values up to 30 m in ~18 h of plasma time. In the main chamber, erosion is weaker than at the divertor but especially at low densities it can result in notable transport of W into the core. In addition, upon switching on the ICRF antennas, W sputtering on the close-by limiter structures can increase by a factor of more than 10. Modelling is able to catch many of the observed phenomena in Phase 1, with the exception of the inner-outer asymmetry and the formation of the thick deposits. In contrast, the patterns during the high-fluence operations in Phase 2 require more work to be reproduced.

The application of hydrogen flooding was recently shown to be a simple and effective approach for improved layer differentiation and interface determination during secondary ion mass spectrometry (SIMS) depth profiling of thin films, as well as an approach with potential in the field of quantitative SIMS analyses. To study the effects of hydrogen further, flooding of H₂ molecules was compared to reactions with atomic H on samples of pure metals and their alloys. H₂ was introduced into the analytical chamber via a capillary, which was heated to approximately 2200 K to achieve dissociation. Dissociation of H₂ up to 30% resulted in a significant increase in the intensity of the metal hydride cluster secondary ions originating from the metallic samples. Comparison of the time scales of possible processes provided insight into the mechanism of hydride cluster secondary ion formation. Cluster ions presumably form during the recombination of the atoms and molecules from the sample and atoms and molecules adsorbed from the gas. This process occurs on the surface or just above it during the sputtering process. These findings coincide with those of previous mechanistic and computational studies.

The application of hydrogen flooding was recently shown to be a simple and effective approach towards improved layer differentiation and interface determination during SIMS depth profiling of thin films as well as an approach with a potential in the field of quantitative SIMS analyses. To further study the effects of hydrogen, the flooding of H₂ molecules was compared to the reactions of the atomic H on the samples of pure metals and their alloys. H₂ was introduced into the analytical chamber via a capillary which was, to achieve the dissociation, heated to approximately 2200 K. Dissociation of H₂ up to 30% resulted in a significant increase in the intensity of the metal hydride cluster secondary ions originating from the metallic samples. Comparison of time scales of possible processes provided an insight into the mechanism of the hydride cluster secondary ion formation. Cluster ions presumably form during the recombination of the atoms and molecules from the sample and adsorbed atoms and molecules from the gas. This process occurs on the surface or just above it during the sputtering process. These findings coincide with the previous mechanistic and computational studies.

Retention of hydrogen isotopes (protium, deuterium, and tritium) in tungsten is one of the most severe issues in the design of fusion power plants, since significant trapping of tritium may cause exceeding radioactivity safety limits in future reactors. Hydrogen isotopes in tungsten can be detected using the nuclear reaction analysis method in the channeling mode (NRA/C). However, the information hidden within the experimental spectra is subject to interpretation. In this work, we propose the methodology to interpret the response of the experimental NRA/C spectra to the specific lattice locations of deuterium by simulations of the NRA/C spectra from atomic structures as obtained from the first-principles calculations. We show that trapping conditions, i.e., states of local crystal structures retaining deuterium, affect the lattice locations of deuterium and the change of lattice locations can be detected by the ion channeling method. By analyzing the experimental data, we are able to determine specific information on the deuterium trapping conditions, including the number of deuterium atoms trapped by one vacancy as well as the presence of impurity atoms along with deuterium in vacancies.