Removal of codeposited layers by ECR discharge cleaning

Abstract

Redeposition of eroded carbon on the walls of fusion devices together with the D–T plasma forms amorphous, hydrogenated carbon films with D–T concentrations ranging for deposition at a surface temperature of 300 K from 0.3 up to 0.6, depending on the deposition conditions. The trapping of large amounts of hydrogen isotopes in these so-called codeposited layers may represent a potential safety hazard during D–T operation because large amounts of tritium will be retained in these layers. To erode carbon films and deplete the T concentration an experimental investigation was performed using O 2, D 2, H 2, H 2O, and O 2/H 2, electron cyclotron resonance low-pressure discharges. Real-time in-situ ellipsometry was applied to measure the optical response of the layers during deposition and erosion. This allows a precise determination of the optical constants and the film thickness. From these data the deposition and erosion rates are calculated. The dependence of the erosion rates on substrate temperature, applied ion energy, gas composition and discharge pressure for two different types of a-C:H films was measured. In the investigated parameter range oxygen discharges showed the highest erosion rates of all investigated source gases. The highest rates achieved at 300 K with the substrate at floating potential are 3.6 μm/h for polymer-like, soft C:H films (H/C about 1) and 1.7 μm/h for hard a-C:H films (H/C about 0.5). The erosion rates increase with increasing substrate temperature (roughly by a factor of 3–4, 300–650 K) and with increasing ion energy (by about a factor of 6–8, 0–200 eV). The paper presents the experimental results and a discussion of the parameter dependencies and the consequences for the removal of codeposited layers by low-pressure discharge cleaning in ITER.