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.