Plasma Wall Interactions : linking Sheaths, Dusts and Turbulence. – I-WISHDOTS

The present project deals with the transition from a magnetized plasma to a material wall. More precisely we want to characterize the influence of several parameters, such as the inclination and magnitude of the magnetic field, the presence of dusts, plasma turbulence, etc… on the wall erosion. This aims at obtaining a more realistic description of the wall erosion processes in fusion reactors, as well as a better understanding of the growth of carbonaceous dusts and of their transport, especially in the vicinity of the divertor. Indeed, the latter (made of carbon tiles for its major part) faces a significant part of the heat loads during operation. These issues are of primary importance with respect to the fusion research programme, and for the success of the large international ITER reactor. Dealing with topics as various as turbulence or carbonaceous dusts growth is quite challenging and requires gathering specialists of the different fields. For sake of clarity our project is composed of four parts. The first part consists in the measurement of the ion velocity distribution function (IVDF) in the sheaths between plasma and a material wall. The IVDF is fundamental for plasma-wall studies, since the erosion processes at the wall depend on the energy and on the angle of the impacting ions. The IVDF will be measured using Laser Induced Fluorescence, in several reactors located at the LPMIA. Effects of the inclination of the magnetic field will be studied in detail, and in particular the reorientation of the ions in the magnetic presheath. Indeed, magnetic field lines are grazing the wall in tokamaks, and dusts growth is influenced by their inclination. Experimental measurements will be compared to numerical studies performed at the LPMIA, based on a Vlasov simulation code. The second part aims at characterizing experimentally turbulence and its consequences on the intermittent transport of particles and energy towards the walls. This study will be carried out on the Mirabelle device, where the same kind of electrostatic turbulence that exists in the Scrape-Off-Layer (SOL) can be produced. SOL is defined in fusion reactors as the region of the intersections between open magnetic field lines and the wall. Optical diagnostics such as an ultrafast camera, as well as probes will be used in order to obtain accurate measurements. The third part of the project will address the issue of dust particle formation. In-situ diagnostics will be used, such as FTIR (Fourier Transform Infrared absorption Spectroscopy) for the detection of carbonaceous radicals, OEM (Optical Emission spectroscopy) for the detection of precursors (C2), and light scattering for dusts characterization. Modelling based on Mie theory and on statistical means (Monte Carlo simulations) will help interpreting laser scattering experiments. Inverse modelling should lead to the size distribution and density of dusts. Measurements will be made using CCDs sensors as well as the ultrafast camera, in order to study the spatial distribution, growth and transport of dusts. These experiments will be performed under various plasma conditions, in a RF discharge and in an ECR reactor. The fourth and last part aims at coupling the previous studies, in particula by studying by LIF the effects on sheaths properties of 1) turbulence and 2) dusts.