Processes with neutral hydrogen and deuterium molecules relevant to edge plasma in tokamaks

Abstract

Neutral hydrogen molecule is an important constituent of edge plasma in tokamak reactors. These molecules are continuously produced by ion neutralisation and recombination on plasma walls and are subsequently reemitted in the plasma. We initiated an experimental program to study processes in which vibrationally excited hydrogen molecules are involved. Two specific experimental techniques have been developed, one for vibrational spectroscopy of hydrogen molecules (H 2 and D 2) and another one for H and D depth profiling in materials. New information has been acquired on hydrogen recombination on tungsten and also on dissociative electron attachment in hydrogen, both relevant to edge plasma. Experimental set-ups are shortly presented as well as some of the results on above processes. 1. Introduction Detailed understanding and characterization of plasma-wall interaction and edge plasma in present tokamaks and future fusion reactors is becoming more and more important due to the ITER project. Different atomic and molecular collision processes decisively determine various processes of primary importance for stable and reliable operation of fusion reactor. Such processes are for instance, the physical and chemical sputtering of the wall material, fuel retention in the wall and its recycling, plasma detachment in tokamak divertor, photon emission from the edge plasma, mitigation of transient phenomena etc. An extensive overview of different aspects of plasma-wall interaction is given in [1]. Neutral hydrogen atoms and molecules are present in the edge plasma. They are continuously generated by ion recombination on the wall of the fusion reactor and on other plasma facing components and then reemitted to the plasma. They can be created also by volume recombination of ions and electrons in the relatively cold edge plasma. The interaction of excited neutral molecules with the walls and their importance for the edge plasma is still not well understood since there is not many experimental studies of relevant processes. Optical emission spectroscopy of edge plasma, especially from limiter and divertor regions, brought evidence of abundant presence of molecules and their involvement in volume processes. Direct correlation of the observed phenomena to the processes with neutrals is mainly possible by numerical simulations and very important activity exists in this field. As experimental results accumulate and numerical methods become more sophisticated also more processes have to be included in the models. For adequate data bases completion more experimental and theoretical works are needed. We have constructed a set-up for vibrational spectroscopy of hydrogen molecules (H 2 and D 2) that is based on the properties of the dissociative electron attachment in hydrogen in order to facilitate dedicated experimental studies of processes with hydrogen molecules. For the same purpose we also