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Almost all optical diagnostic systems in ITER will require the implementation of mirror recovery and protection systems. Plasma cleaning is considered to be the most promising technique for the removal of metal deposits from optical surfaces. The engineering and physical aspects of RF discharge application for continuous or periodic plasma treatment are discussed with a focus on implementation under ITER conditions. The ion flux parameters obtained in capacitively coupled (CC) RF discharge were measured in the mock-up of a plasma cleaning system. The uniformity of sputtering in CC RF discharge with and without a magnetic field was studied experimentally for the cylindrical discharge reactor geometry and compared with numerical simulations. The sharp increase in the sputtering rate resulting from the non-uniform radial distribution of the ion flux was observed near the electrode edges. The longitudinal magnetic field improves sputtering uniformity. It was demonstrated that Al/Al 2 O 3 deposits can be removed in the Ne and D 2 plasma of CC RF discharge but longterm exposition results in the degradation of the polycrystalline molybdenum mirror surface. The efficiency of Al sputtering in the atmosphere containing O 2 and N 2 fractions was studied in the D 2 /O 2 and D 2 /N 2 plasma of glow discharge. The addition of 2% of oxygen or nitrogen increases the sputtering yield by 3-4 times as compared with that in a nominally pure D 2 discharge. The impact of metal deposits on the performance of diagnostic mirrors is discussed. It was shown that an ultrathin metallic film with a thickness as low as a few nm may cause a significant degradation of diagnostic mirrors with a transparent coating.

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The influence of nitrogen injection on the formation of a-C:H films in areas far away from the plasma as well as close to the plasma boundary was studied in the PSI-2 device for collector temperatures between 310 and 350 K. The balance between deposition and erosion determines the net growth rate. Small amounts of nitrogen (Φ(N 2 ) ≈ Φ(CH 4 ) ≈ 2% Φ(H 2 )) strongly reduce the net growth rate in H 2 /CH 4 mixtures. While N 2 injection does not influence erosion far away from the plasma it increases erosion close to the plasma boundary. The experiments show clear evidence for a scavenger effect due to the injected nitrogen. The conversion from N 2 to active species takes place in the hot plasma region.

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