Experimental study of tungsten interactions with atomic, neutral, and charged species as a function of material fabrication processes. Applications to divertor target and first wall materials in ITER.

Context

This research program aims to study the interactions between neutral species (¹H, ²H) and charged species (¹H₂⁺, ²H₂⁺), originating from a plasma (¹H₂, ²H₂), and tungsten (W) surfaces fabricated using different processes (cathodic sputtering, spray-coating, rolling, etc.). These manufacturing methods impart varied crystallographic morphologies to tungsten (99.97% pure W), influencing its retention, adsorption, and chemisorption properties with respect to incident species [1]. These interactions occur over a wide energy range, from a few hundred kelvins for neutral species (¹H, ²H) to several tens of electron volts for ions (¹H⁺, ²H⁺, ¹H₂⁺, ²H₂⁺).
A commercial species source (Surfatron S-Wave, marketed by SAIREM) will be implemented on the SCHEME-III experimental setup, specifically designed for this study. This setup will allow the exposure of small tungsten (W) samples (15 mm diameter disks) at precisely controlled temperature and potential. The small sample size offers several advantages: it facilitates access to various fabrication and post-processing methods (notably vacuum annealing) and enables in-depth surface analyses (AFM, electron microscopy, XRD, XPS spectroscopy) without altering the sample.
During the exposure of these samples to species fluxes, non-intrusive diagnostics such as emission and absorption spectroscopy will be employed to characterize both the incident species—primarily monoatomic (Balmer series) and molecular triplet Q(vʹ,vʹʹ): d³Πᵤ → a³Σg⁺—as well as those produced by surface recombination (¹H₂/²H₂ X¹Σg⁺(v’’, J’’)).
The latter will be quantified in absolute terms using VUV absorption spectroscopy at the SOLEIL synchrotron, via a diagnostic developed by LPSC on the DESIRS beamline over the past ten years [2, 3]. Additionally, intrusive diagnostics such as Langmuir probes and laser photodetachment will complement the characterization of the plasma environment (¹H⁺, ²H⁺, ¹H₂⁺, ²H₂⁺, ¹H⁻, ²H⁻) in which the tungsten samples will be immersed.

Objective

Comparing surface analyses before and after exposure to particle fluxes will help identify and quantify potential morphological modifications. Indeed, ¹H and ²H are known for their ability to integrate into the crystalline lattice, causing irreversible alterations that may modify the surface properties of materials concerning incident species.
This research project aims to deepen the understanding of interactions between tungsten (W) materials with different morphologies and fluxes of species representative in type and energy of those present near the tokamak wall (edge plasma) and ITER divertor targets. Ultimately, this enhanced understanding and the associated experimental data will contribute to a more realistic modeling of interactions between fusion plasma species and exposed surfaces.

This project stands out for:
1. A precise and absolute characterization of incident atomic fluxes and produced molecular species
The combination of complementary diagnostics will enable a rigorous quantification of the incident atomic flux as well as the molecular species generated by recombination at the material surfaces.
2. A detailed monitoring of the morphological evolution and damage of tungsten materials
In-depth surface analyses will be conducted before and after exposure to particle fluxes using advanced techniques such as AFM, electron microscopy, XRD, and XPS spectroscopy to assess the impact of atom-surface interactions.
3. Exposure conditions representative of real environments
The SCHEME-III experimental setup allows for the reproduction of experimental conditions similar to those of the edge plasma in a tokamak (~0.3 MW/m³), ensuring reliable exploitation of the results for modeling the behavior of surfaces exposed to a flux of neutral and charged particles.