jnurnalof N nuclear materials ELSEVIER Journal of NuclearMaterials 245 (1997) 66-71 Oxidation and hydrogen isotope exchange in amorphous, deuterated carbon films Wenmin Wang b, W. Jacob a j. Roth a, * a Max-Planck-lnstitutfiJr Plasmaphysik, EURATOM Association, D-85748 Garching, Germany b Shanghai Institute of Nuclear Research, Academia Sinica, Shanghai 201800, People's Republic of China Received 12 November 1996; accepted 5 December 1996 Abstract The hydrogen desorption and air oxidation of amorphous deuterated carbon (a-C:D) films deposited on Si are investigated. The results are compared with a hydrogenated carbon layer deposited on a silicon collector sample mounted on the wall of tokamak ASDEX Upgrade during 6 months' plasma operation. The a-C:D films, deposited on a silicon substrate by the RF glow-discharge method, are exposed to air at room temperature and between 500 and 800 K. The total amounts and depth profiles of both deuterium and carbon are measured by ERD (elastic recoil detection) and PES (proton enhanced scattering). It is found that the surface deuterium loss is accompanied by oxygen uptake and hydrogen isotope exchange in the film until deuterium is completely released. Further annealing leads to the removal of the whole carbon layer. In accordance with previous oxygen implantation studies the erosion is modeled to be due to the chemical reaction of carbon and deuterium with oxygen from air. The removal of the codeposited layer from the ASDEX-Upgrade sample proceeds much faster than that found for a-C:D films under identical treatment conditions. The removal rate strongly depends on film structure. 1. Introduction Graphite was used for limiters and protective tiles in almost all fusion devices, and was selected as the reference material for plasma-facing components in the future fusion experiment ITER. However, a problem emerges because of its erosion and hydrogen retention behavior when exposed to high fluxes of plasma particles. The eroded carbon atoms are redeposited on less exposed surfaces together with hydrogen isotopes to form an amorphous hydro- genated carbon (a-C:H) layer with an atomic ratio of hydrogen isotopes to carbon of about 0.4. The trapping of large amounts of hydrogen isotopes in graphite tiles may result in accidental recycling of the fuel and problems of density control. Furthermore, significant amounts of tri- tium will be retained in the codeposited layer on the wall surface during D-T operation, resulting in a potential * Corresponding author. environmental risk during accidental vacuum loss because the codeposited films are not stable in air at elevated temperatures. It is, therefore, important to know the release rate of the trapped hydrogen isotopes from the codeposited layer when it is exposed to air. In order to keep the total tritium inventory below an acceptable level it is desirable to find an effective method of removing the codeposited layer during the intervals between every two plasma cam- paigns. Therefore, it is necessary to study the hydrogen release from samples of graphite implanted with deuterium or amorphous carbonized layers (a-C:D films) in labora- tory. The removal of hydrogen isotopes from hydrogenated carbon layers was investigated by different methods [1-4]. The chemical release of implanted deuterium in graphite was investigated by Chiu and Haasz applying laser thermal desorption [1]. They found that oxygen exposure is most effective in removing the implanted deuterium from graphite. An amorphous, deuterated carbon (a-C:D) film was studied by Haasz et al. [2]. D20, CO 2 and CO were found to be the predominantly released reaction products 0022-3115/97/$17.00 Copyright © 1997 Elsevier ScienceB.V. All rights reserved. PII S0022-3 1 15(96)00745-3