K.G. Watkins, University of Liverpool, Liverpool, UK;
J.F. Asmus, Institute of Pure and Applied Sciences, University of California, San Diego, CA;
G.F. Counsell, M. Forrest, UKAEA Fusion Culham Laboratory, Oxford, UK
Next generation tokamaks offer the possibility of highly efficient energy generation from the fusion of tritium and there is a large scale international effort to develop this technology. Part of the wall linings of the tokamak reactor are carbon-carbon fibre composite tiles and during operation these tiles become liberate a carbonaceous debris (‘dust’) comprising flake like particulates and more continuous ‘soft’ carbon (polymeric CH) and ‘hard’ carbon (diamond-like) layers. Since this dust contains tritium, the build up leads to an increase in the allowable tritium load if the dust is not periodically removed.
Amongst candidate removal methods, photonic based techniques employing pulsed xenon flashlamps and Q-switched Nd:YAG laser pulses have been investigated. The flashlamp based work has been carried out at UCSD and the laser based work at Liverpool using carbon-carbon fibre composite tiles coated with simulated CH (‘soft’ carbon) films of thickness 7 microns. It has been shown that at a fluence of 3.4 J/ sq cm and a pulse length of 200 microseconds, the xenon flashlamp pulses were effective in removing the coating without damaging the substrate. Similarly a 300W Nd:YAG laser giving pulses of 6ns was also effective in removing the film. Substrate condition after cleaning was observed by SEM. A simple model to account for the temperature rise produced during cleaning has been developed and possible removal mechanisms have been proposed. Details of further work in this continuing project will be presented.