Laser Cleaning of Candidate Diagnostic Mirrors for ITER

Abstract

Practical methods to clean ITER’s diagnostic mirrors will be essential to ITER’s plasma operations. We report on laser cleaning of candidate ITER single-crystal molybdenum mirrors that were plasma coated with either carbon or beryllium films 150 to 420 nm thick. A pulsed Nd laser beam was focused to 1 to 2 J/cm2 and scanned at various speeds across the surface of a mirror. The cleaning effect was measured with a novel method that combined microscopic imaging and reflectivity measurements in the red, green, and blue spectral regions and at the H-alpha and H-beta wavelengths. No damage of the molybdenum mirror substrates was observed at the range of laser intensities used. For carbon-coated mirrors, complete removal of the film and restoration of the reflectivity were measured in some conditions. For the beryllium-coated mirrors, restoration of reflectivity has so far been incomplete. Heat transfer calculations suggest a shorter, ∼5-ns laser pulse would be optimal.