Progress in Development of Deposition Prevention and Cleaning Techniques of In-vessel Optics in ITER

The lifetime of optical components unprotected from reactor grade plasmas may be very short due to contamination with carbon and beryllium-based materials eroded by plasma from beryllium walls and carbon tiles. Deposits result in a significant reduction of optical transmission. In addition, even rather thin and transparent deposits can dramatically change the shape of reflectance spectra owing to interference of reflected beams, especially for mirrors with rather low reflectivity, like W or Mo. Development of optics-cleaning and deposition-mitigating techniques is a key factor in the construction and operation of optical diagnostics in ITER. The most severe problem faces optical elements positioned in the divertor region. The latest achievements in protection of in-vessel optics are presented by example of deposition prevention/cleaning techniques for inmachine components of a Thomson scattering system in divertor. Careful consideration of well-known and novel protection approaches shows that neither of them provides guaranteed survivability of the first in-vessel optics in divertor. Only a set of mutually complementing prevention/cleaning techniques, that include special materials for mirrors and inhibition additives for plasma, is able to manage the challenging task. The essential issue, which needs to be addressed in the nearest future, is an extensive development of introduced techniques under experimental conditions (exposure time and contamination fluxes) similar to those expected in ITER.