Theoretical modelling of deuterium ICRF wall conditioning discharges

Abstract

Ion cyclotron conditioning (ICC) discharges are produced by injecting ICRF power to a gas target in the presence of magnetic fields. They have low electron temperature, moderate density and their ion energy distributions show high ion energy tails. ICC has high wall conditioning efficiency and since it is operated with energized magnetic fields, in contrast to glow discharges, they are a promising technique for first wall conditioning of superconducting reactors. Additionally, deuterium ICC has been proposed for reducing the tritium inventory of the first wall of reactors through surface isotope exchange and more recently to etch carbon deposits by adding small amounts of oxygen. Consequently ICC should be further developed as a routine wall conditioning technique for ITER and besides making further experimental progress in its first wall conditioning efficiency, it is also necessary to study the physics of ICC plasmas. This is the objective of this paper, which focuses on the atomic and molecular reactions in ICC plasmas, on the resulting ion and neutral species concentrations and on the ion and electron energy distributions. From the particle and power balance the electron and ion energy is deduced and compared with the experimental results. Estimations of the ion confinement time indicate, that energetic ions do not reach thermal equilibration, which explains the formation of suprathermal tails. The concentration of D, D+, and is calculated with a 0D model and found to be strongly dependent on the discharge conditions. Dissociative recombination of molecular ions with electrons and charge exchange reaction of ions with neutrals strongly determine the particle and power balance. The limitations and possible error sources of the results of the theoretical modelling presented here are critically discussed.