Mr. Mykola IALOVEGA
Will publicly support his thesis work entitled :
Surface conditions of W components : Impact on hydrogen inventory
Defense scheduled for Tuesday, February 23, 2021 at 9:30 a.m.
Place : IRFM – René Gravier – Centre de Cadarache 13115 Saint paul lez Durance
Composition of the proposed jury :
- M. Thierry Angot, Aix Marseille Université, Thesis Director
- Mme Anne-Lise Thomann, Université d’Orléans, Referee
- M. Wolfgang Jacob, Max Planck Institute, Referee
- M. Christian Linsmeier, Forschungszentrum Jülich, Member of the board
- Mme Sabina Markelj, Jozef Stefan Institute, Member of the board
- M. Christian Grisolia, CEA Cadarache, Member of the board
- Mme Céline Martin, Aix Marseille Université, Member of the board
- M. Régis Bisson, Aix Marseille Université, Invited member
- Mme Elodie Bernard, CEA Cadarache/IRFM, Member of the board, thesis advisor CEA
Investigations of hydrogen isotopes and helium retention in plasma facing components (PFC) that are exposed to various plasma conditions are important for future fusion devices such as ITER and DEMO. Due to its favorable physical properties, in particular its high melting point, tungsten (W) has been chosen as the plasma-facing material of the ITER divetor. In the deuterium/tritium (D/T) phase of ITER, W PFC will be subjected to intense fluxes composed of hydrogen isotopes (HI), helium (He), impurities and neutrons.
In particular, it has been found that He significantly affects W PFC near surface, with the formation of dislocation loops, bubbles, or even W-fuzz. The presence of impurities in the edge plasma may cause redeposition or codeposition of mixed layers on the surface of the PFC W, and in the presence of residual oxygen, surface oxidation is possible due to the high temperature of the ITER divertor. Such structural modifications of W PFC may considerably modify the properties of the material, and therefore its life expectancy, as well as its hydrogen retention, which arises safety concerns as tritium is radioactive.
In this PHD thesis, we used laboratory experiments involving ion implantation and thermal desorption spectrometry (TDS) technique to investigate the fundamental retention properties of HI in W PFC due to different surface conditions: 1) Presence of an oxide layer formed on the surface of polycrystalline W (PCW) in ITER relevant conditions, 2) Presence of structural changes due to an impact of ITER relevant helium irradiation.
The TDS measurements were coupled with microscopy observations in order to characterize the modifications occurring on the surface and in the bulk of the material at different scales: scanning electron and confocal laser scanning microscopy techniques were used for surface observations from micrometer to nanometer scale; transmission electron microscopy was used for cross-sectional observations. Raman and X-ray spectroscopy techniques were used to characterize the structure and chemical composition of the samples.
TDS experiments involving low energy D ion implantation or T gas loading on oxidized PCW samples highlighted drastically different retention mechanisms of HI in tungsten oxides as compared to pristine PCW. The initial surface condition of the material (for example the presence of adsorbed water) can also significantly modify the outgassing of HI. We have also evidenced a change in the color of the oxide following HI implantation, that suggests tungsten bronze formation. The fact that the oxide grown on PCW was not modified upon D implantation and TDS experiments (the surface morphology and elemental composition were restored), raises additional concerns for the safe long-term operation of W PFC in ITER.
In the other set of experiments, we exposed PCW to helium plasma in fluence conditions relevant to ITER and WEST both at low and high flux. Surface and bulk morphology observations combined with sequences of low flux and low fluence 250 eV D ion implantations and TDS measurements up to 1200-1350 K revealed interesting effects of the thermal cycling on He-induced defects evolution and deuterium retention. D implantation and thermal cycling experiments highlighted a major impact of thermal cycling on the evolution of defects created by He and deuterium retention. D retention in the He bubbles enriched near-surface is increased 3 to 8-fold as compared to non-damaged PCW. The evolution of D retention upon thermal cycling appears to be linked with the density and size of various He bubbles. Furthermore, we show that high temperature annealing induces He bubbles faceting into a polyhedron shape as well as a faceting of holes into a rectangular shape on