Deposition mitigation and in-vessel optics recovery in ITER

According to recent experiments in plasma fusion machines, the contamination of optical elements under deposition-dominated conditions in ITER will result in fast degradation of their optical characteristics. The development of deposition-mitigation techniques is an important part of R&D program of ITER optical diagnostics. The paper describes the approaches to be used for the recovery of first optical elements of the Divertor Thomson scattering (DTS) diagnostics involving plasma cleaning and laser ablation. The long-term laser cleaning efficiency under continuous deposition of Al and W was studied for fused silica and alumina windows. The laser-induced damage threshold was expected to decrease by about the factor of three in highly contaminated areas and up to six times in the case of contamination with dust particles as compared to clean windows. The implementation of the laser cleaning technique for DTS diagnostics in ITER is discussed. Another technique, shown to be efficient for removal of metal and metal oxide deposits, is plasma treatment. The physical aspects of plasma cleaning based on capacitively coupled (CC) RF discharge are considered with the focus on the ITER-specific requirements to the system. Ion energy distribution and flux density in capacitively coupled RF discharge were measured as function of RF frequency and power. The grounded sheath voltage was found to be nearly constant at different discharge frequencies and absorbed power. The frequency dependence of effective sputtering yield of Be and Mo is presented for Ne and He plasma discharges.