LASER DAMAGE INVESTIGATIONS OF OPTICAL ELEMENTS FOR ITER DIVERTOR THOMSON SCATTERING SYSTEM

The metal mirrors for deflecting laser radiation in the divertor Thomson scattering system of ITER are found to be inapplicable due to high density of laser radiation on a mirror. This is caused by the short distance from the laser focus to the mirror surface. We report on investigations of the possibility to use dielectric mirrors and protective sapphire plates in a rotary unit. We study the laser damage thresholds of optical elements, which are supposed to be used in the laser input channel. These optical elements are the dielectric mirrors deposited on different substrates (quartz, sapphire, and single-crystal molybdenum) and the protecting sapphire plate. A number of sapphire samples were irradiated by a neutron flux up to 10(19) n/cm(2) (E > 100 keV) and annealed. Laser damage thresholds of the elements were measured at room temperature and at the ITER operating temperature of 150 degrees C. A YAG:Nd laser operating with 10-Hz repetition rate was used in the experiments. The laser pulse parameters were 1064-nm wavelength, 16-ns duration, and 250 to 300 mJ of energy. The experiments have not identified any dependence of optical element damage threshold versus the number of laser pulses. No damage was observed after 105 laser pulses with energy density just 10% below the damage threshold level. The applicability of these optical elements in the divertor Thomson scattering system of ITER had been proven.