Jila Shams-Latifi: Interactions of low-energy ions with plasma-facing materials: Fundamental and applied aspects

Abstract

Materials in ultimate proximity of a burning plasma in controlled thermonuclear fusion devices, referred to as plasma-facing materials (PFMs), are exposed to a high flux of impinging particles and extreme conditions such as high thermal loads. In consequence, these materials are modified in manifold ways due to interactions with energetic plasma species. 

In this thesis, the potential of time-of-flight low-energy ion scattering (ToF-LEIS) for fundamental and applied studies relevant to improving our understanding of plasma-materials interactions was explored. ToF-LEIS is a non-destructive method to study the near-surface composition of materials providing high depth resolution and surface sensitivity. The investigated materials are tungsten and EUROFER97, both as bulks and in the form of different thin film stacks. W is proposed for the first wall of ITER, as well as for the divertor and coating for the first wall of DEMO. EUROFER97 is a reduced activation steel to be used as a structural material for the ITER test blanket and the first wall of DEMO's blanket.

In the first part of this thesis, the potential of low-energy ions for quantitative composition depth profiling of near-surface modifications in PFMs induced by thermal loads was explored. Specifically, ToF-LEIS in combination with Auger electron spectroscopy was employed to study the influence of elevated temperatures on the near-surface composition of EUROFER97. The results demonstrated and quantified thermally induced segregation of W, Cr and S to the first nm of EUROFER97 under thermal loads. 

In the second part, this thesis focused on developing sample systems and measurement routines for assessing electronic interactions of low-energy ions with common PFMs. Developed samples were employed to experimentally deduce the electronic stopping power of W and EUROFER97 using ToF-LEIS and Monte-Carlo simulations for low-energy H, D and He ions, in the range of 0.3 – 10 keV for W and 3 – 10 keV for EUROFER97. Obtained comprehensive datasets represent the first experimental data of the electronic stopping power of W and EUROFER97 at low energies for the employed ions representing dominating plasma species. Results were compared to theoretical and semiempirical models and are essential inputs for modelling particle range or sputter yields.