The high reactivity of oxygen with carbon films has driven the development of techniques aimed at the removal of tokamak co-deposits by exposing them to either molecular oxygen at elevated temperatures (thermo-oxidation) or to oxygen containing plasmas, created by different methods (1). However, the deleterious side effects of introducing oxygen in a fusion device represent a serious draw-back for the use of such techniques in a reactor. Nitrogen plasmas are especially appealing as a low Z alternative (2). First, nitrogen atoms show a high reactivity with carbon, comparable to that of oxygen. Second, nitrogen doest not react with metals at low temperatures, thus preventing the non-carbon first wall materials from chemical modification. However, a systematic use of glow discharges in nitrogen will not be possible until other possible side effects are ruled out. Experiments performed at Ciemat have shown that erosion rates in H2/N2 GD plasmas above 4 nm/min at room temperature can be achieved. However, thermal desorption of the films after exposure to the erosion plasmas indicates that some modification of the binding states of H takes place, increasing the temperature of desorption. The studies reported in the present work include the characterization of the plasma chemistry by Cryotrap-Assisted Mass Spectrometry (3), of the H removal yield by laser Induced Desorption, erosion rates by laser interferometry and characterization of the binding states by thermal desorption spectroscopy, among others. Optimization of the plasma parameters trough mass spectrometry and erosion rate measurements has also been addressed. Finally, a model is proposed to account for all the observations, which can be used for the extrapolation of the technique to larger systems.
1. G. Counsell et al. Plasma Phys. Control Fusion 48(2006) B189-B199
2. F.L Tabarés et al. Plasma Phys. Control Fusion. 46 (2004) B381-B395
3. J.A. Ferreira and F.L. Tabarés. J. Vac.Sci.Technol. in press
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