Effects of the plasma-facing materials on the negative ion H⁻ density in an ECR (2.45 GHz) plasma

International audienceWithin the framework of fundamental research, the present work focuses on the role of surface material in the production of H− negative ion, with a potential application of designing cesium-free H− negative ion sources oriented to fusion application. It is widely accepted that the main reaction leading to H− production, in the plasma volume, is the dissociative attachment of low-energy electrons (T e ≤ 1 eV) on highly ro-vibrationally excited hydrogen molecules. In parallel with other mechanisms, the density of these excited molecules may be enhanced by means of the recombinative desorption, i.e. the interaction between surface absorbed atoms with other atoms (surface adsorbed or not) through the path Hads + Hgas/ads --> H2(v,J)gas + ΔE. Accordingly, a systematic study on the role played by the surface in this reaction, with respect to the production of H− ion in the plasma volume, is here performed. Thus, tantalum and tungsten (already known as H− enhancers) and quartz (inert surface) materials are employed as inner surfaces of a test bench chamber. The plasma inside the chamber is produced by electron cyclotron resonance (ECR) driving and it is characterized with conventional electrostatic probes, laser photodetachment, and emission and absorption spectroscopy. Two different positions (close to and away from the ECR driving zone) are investigated under various conditions of pressure and power. The experimental results are supported by numerical data generated by a 1D model. The latter couples continuity and electron energy balance equations in the presence of magnetic field, and incorporates vibrational kinetics, H2 molecular reactions, H electronically excited states and ground-state species kinetics. In the light of this study, recombinative desorption has been evidenced as the most probable mechanism, among others, responsible for an enhancement by a factor of about 3.4, at 1.6 Pa and 175 W of microwave power, in the case of tantalum