Can gas puff be used for cleaning of the diagnostic first mirrors in ITER?

Abstract

Erosion from the first mirrors of optical diagnostics can be enhanced by injecting neutral particles into plasma in the vicinity of the diagnostic entrance aperture. In the present paper feasibility of this method for cleaning from deposits is investigated numerically for conditions of the ITER main chamber diagnostics. Besides erosion on the mirror the local gas puff also increases fluxes of impurities due to sputtering from the neighboring surfaces. Calculations have shown that for external impurities coming from the main chamber (Be) the enhancement of erosion – cleaning effect – by far exceeds the extra deposition. However, two further problems have been revealed which may prevent practical use of the proposed technique. First, the gas puffing rate required for effective cleaning may be too large. Second is the contamination by “internal impurities” – sputtered material of the channels where the mirrors are installed.