W, H and He fundamental Studies in a Complete and Integrated approach – WHeSCI

For the first objective, we performed experiments in vacuum chambers similar to those of reactors, in order to test the effect of the presence of natural impurities (oxygen) on the trapping of fuel at the surface and in the volume of the walls. Models combining atomic scale quantum chemistry and reaction rate calculations in/on the walls reproduce the experimental observations. For the second objective, a model describing the atomic processes of helium bubble growth has been created to calculate the size and distribution of these bubbles under the wall surface. These results were compared to electron microscopy measurements made on wall elements that underwent experimental conditions similar to those in a fusion reactor. In the third objective, we combined the techniques of the first objective with those of the second objective to quantify the effect of helium bubbles on fusion fuel trapping. Finally, we set up a laser heating experiment in a vacuum chamber to test methods for fuel and ash outgassing.

We demonstrated that oxygen affects fuel trapping in walls differently at their surface and in their volume. A helium bubble growth model reproduces the size and distribution of bubbles below the wall surface and we have shown experimentally that heating the bubbles changes their shape. Then, we measured an increase in fuel trapping in the vicinity of these bubbles. Finally, we succeeded in inducing the outgassing of fusion fuel and very small helium bubbles by laser heating techniques.

The integrated experimental/modeling approach adopted by the WHeSCI project has been validated by the numerous results obtained. New research paths are open to improve the accuracy of fusion reactor wall models. In particular, and without being exhaustive, we can mention the study of the formation of helium bubbles, their mobility as a function of their size and their interaction with the fusion fuel. Pursuing these research paths will allow us to propose versatile laser outgassing scenarios that could be integrated in future fusion reactors.

The WHeSCI project has led to the publication of about 20 scientific articles in the best international journals and about 50 contributions in major international conferences. One of the fuel trapping modeling codes (FESTIM) has been deposited on the GitHub platform and is available for open access. Presentations to the general public are planned during Festiv'On R organized by Aix-Marseille University in spring 2024 in Marseille (France).

WHeSCI is proposed in the context of the comprehension of Plasma-Wall Interactions in tokamaks. In current fusion reactor models, the wall is described usually by the ratio of the wall outgas to the plasma outflow, the so-called recycling coefficient. In order to prepare the deuterium-tritium phase of the ITER reactor, we propose to provide a better description using Macroscopic Rate Equations (MRE) models stringently derived and tested on well-controlled theoretical and experimental data sets. Multi-scale theoretical methods will be used together with multi-scale experimental techniques. Theoretical studies will focus on both the bulk and the surface of tungsten materials. The atomistic scale properties will be investigated at the level of Density Functional Theory (DFT) while the understanding of mesoscopic behavior will rely on Object Kinetic Monte Carlo (OKMC). Studies with three dimensional finite element methods (3D-FEM) will enable to better define the parameters needed to construct 1D MRE models. The uniqueness of our experimental approach is to exploit an all in situ ultra-high vacuum setup able: 1. to prepare and characterize sample with atomically controlled-surface; 2. to perform deuterium and helium implantation with beams and/or plasma; 3. to quantify retained species and to determine outgassing kinetic parameters (activation energy and prefactor) with thermo-desorption from 120 to ~ 2400 K; 4. to heat samples with lasers up to power densities relevant for ITER operation. In addition, tungsten samples will be damaged in a controlled manner thanks to accelerator-based techniques. Using such a methodology, the behavior of tungsten under high flux of deuterium, tritium, helium and neutron will be better understood and analysis of samples from the WEST tokamak, the testbed of ITER plasma-facing components, will be eased. Kinetic characterization by thermo-desorption will be complemented by structural characterization of defects and by the measurement of deuterium and helium implantation profile while maintaining ultra-high-vacuum conditions. Finally, tritium experiments will be realized to investigate isotope effects. With this experiment + theory integrated approach, we will push the limits of the state-of-the-art and we will shed light on synergistic interactions between deuterium/tritium, helium and tungsten.