Laser processing of tungsten interacting with helium ions – LETHE

We acquired a high power laser (500W, 1080 nm, model Laser Qube 500W) and an in-situ spectroscopic ellipsometer (400-1000 nm, model Woolam iSe). The two instruments have been installed in two different experimental setups to perform on the one hand laser-induced thermo-desorption (LID) measurements on W samples implanted with He ions and, on the other hand, the study of the morphological modification and optical properties of W in a plasma environment.

The development of the LID technique was the main objective of WP2a (Workpackage Development of the LID) and allowed to reproduce the thermodesorption experiments performed with classical methods (radiative heating with a tungsten filament). The adaptability of the LID technique allowed us to progress faster than expected on task WP1a, the objective of this task being to characterize the He+-W interaction and to study the kinetics of blister formation. We have been able to perform thermo-desorption experiments, from room temperature up to about 2000 K, a temperature difficult to reach in short times (< 60 secs) with radiative heating. These experiments represent one of the first in situ investigations of He depletion implanted in the W (a W(110) single crystal) and allowed the measurement of He desorption temperatures. These temperatures can be associated (using the so-called «Redhead« analysis) with binding energies of the different «species« (He cluster, He - gap cluster) formed during the He trapping and diffusion processes in the W. The comparison of the energies obtained experimentally with the energies from theoretical models (e.g. DFT) is in good agreement and may allow to better constrain the development of He retention and blister growth models based on ab initio quantities. The processes of trapping and blister formation will then be studied by changing the surface termination (specifically using polycrystalline samples) and ion energy. These studies will complement WP1a. In addition, the possibility to install the ellipsometer and the laser in two different experimental setups allowed to start in parallel the WP1b task. The objective of this task was to study the formation of He plasma induced blisters correlating it to the optical response (in particular using an in-situ spectroscopic ellipsometer in the 400-1000 nm range) of the W during plasma exposure. This task was prepared by conducting preliminary studies on the optical properties of W, the role of surface roughness and chemical composition of the samples. The optical properties of five W samples with different roughness values (20-100 nm) were measured, with an optical reflectometer, during laser annealing (temperature ramp from 300 to 1100 K). We observed an increase in reflectivity after annealing and demonstrated that it was due to a change in the chemical composition of the surface, in particular a reduction of the native oxide thickness. By highlighting the role played by surface roughness and impurities (e.g., oxide), these studies will provide a better understanding of the ellipsometric measurements we have acquired by exposing different W samples to D and He plasmas.

In the initial version of the project, the systematic study of the D/W plasma interaction was not foreseen, nevertheless calibration experiments performed with a D plasma revealed the possibility to study the D outgassing kinetics thanks to in-situ ellipsometric measurements. Therefore, we decided to devote three months to a more detailed study of this interaction. In addition, we have received funding from ISFIN (Institut Sciences de la Fusion et de l'Instrumentation en environnements nucléaires) to finance a mission to the Jozef Stefan Institute in Slovenia and to carry out, in 2023, complementary experiments using the Nuclear Reaction Analysis (NRA) technique on our W samples implanted with D and He ions.

1. Pappalardo et al., Optical properties of tungsten: a parametric study to characterize the role of roughness, surface composition, and temperature, Optics 2022, 3, 216–224. doi.org/10.3390/opt3030021
2. Dunand et al., Flux dependence of helium retention in clean W(110): experimental evidence for He self-trapping, under review in Nuclear Materials and Energy

In a fusion plasma, ions escape from the plasma core and hit the reactor's walls where they remain implanted. During operation, the walls are hot (~900 K) and while absorbing hydrogen isotopes and helium they also release them. This implantation/degassing process is called recycling. The recycling process affects mainly the tungsten divertor which receives the highest power fluxes (up to 40 MW/m²). The interaction of intense particle fluxes with walls can induce changes in the surface condition and thermo-mechanical properties of plasma facing components and thus affecting the proper functioning of the reactor.

For these reasons, in the framework of the LETHE project, we will experimentally study changes in the recycling process induced by He/wall and light/wall interactions. Such studies are necessary to predict how the walls will behave during plasma operation in tokamaks. The experiments will be carried out using an ultra-high vacuum device allowing to characterize the atomic composition of sample surfaces, to implant helium with ion beams or plasma, and to quantify the species trapped in the volume of the materials by using the temperature-programmed desorption technique.
Three are the main objectives of the LETHE project:
1. Understanding the physical mechanisms underlying the degradation of materials (e.g. blister formation) and the change in their physico-chemical properties after He implantation/thermo-desorption cycles. Moreover, an in situ spectroscopic ellipsometer, installed in the framework of the LETHE project, will allow to probe the degradation of the surface of materials during ion-surface interaction.
2. The study of the influence of thermal loads in the recycling process and, consequently, on the parameters of the edge plasma. Thermal loads, simulated by a high-power laser, reaching pre-implanted samples, will induce sudden desorption of trapped species which, consequently, will perturb the plasma. The properties of the plasma (e.g. temperature and density) will be measured by a Langmuir probe.
3. Development of an optical method to prevent surface degradation, e.g. blistering, which can lead to plasma disruption.