In the ITER first wall and divertor scenario presently foreseen, the simultaneous presence of tungsten, beryllium and carbon as plasma facing components will lead to the formation of complex co-deposits, of particular concern in terms of tritium retention and material degradation. A periodic cleaning strategy of such co-deposits is to be developed. One of the most appealing candidates to date is the use of oxygen-containing glow discharges, either as ECR or ICR plasmas, potentially compatible with the presence of the high magnetic field in ITER, or as DC glow discharges. Although a high cleaning rate of pure carbon/H films can be achieved by these techniques for re-deposits existing in areas fully exposed to the plasma, the incorporation of metals or other atomic species into the deposits can dramatically change their reactivity against the cleaning discharge. Examples of that are the lower erosion rates observed in tungsten-contaminated carbon films (1) and in the presence of boron (2). Of especial relevance for ITER is the Be/C /H system. The strong gettering activity of Be, used for the sake of oxygen-free plasma production, can represent an efficient scavenger for the active species involved in the carbon oxidation process. In the present work, the effect that strong metallic getters has in the carbon removal rate by He/O2 glow discharges is addressed. Due to the stringent conditions required for the use of Be in laboratory experiments, Li and Mg have been tested as O getters. Samples of C/Metal mixtures are produced by introducing the metallic evaporator into the glow discharge deposition chamber. Hydrogen methane mixtures are used as precursors of hard a-C:H films in a DC glow discharge apparatus. The relative content of metal is simply regulated by controlling the evaporation rate, and values in the few % range have been tested. Removal rates up to 12 nm/min are obtained in the absence of metals. Surface Analysis and Laser Induced Desorption techniques have been applied for the mixture characterization prior and after the exposure to the oxidizing plasma. The results are tentatively extrapolated to the conditions prevailing in ITER.
1. J.W.Davis et al. J. Nucl. Mater 305(2002) 66
2. J.A. Ferreira et al doi:10.1016/j.jnucmat.2007.01.109
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