Laser damage thresholds of ITER mirror materials and first results on in situ laser cleaning of stainless steel mirrors

TL;DR

This study investigates laser damage thresholds of various mirror materials relevant to ITER and demonstrates effective in situ laser cleaning of stainless steel mirrors, showing potential for improved maintenance of fusion reactor optics.

Contribution

It provides new damage threshold data across multiple wavelengths and introduces a successful in situ laser cleaning method for stainless steel mirrors in vacuum conditions.

Findings

Damage threshold increases at wavelengths below 400 nm.

Nitrogen atmosphere reduces damage threshold by 2-3 times.

Laser cleaning effectively removes coatings and restores reflectivity.

Abstract

A laser ablation system has been constructed and used to determine the damage threshold of stainless steel, rhodium and single-, poly- and nanocrystalline molybdenum in vacuum, at a number of wavelengths between 220 and 1064 nm using 5 ns pulses. All materials show an increase of the damage threshold with decreasing wavelength below 400 nm. Tests in a nitrogen atmosphere showed a decrease of the damage threshold by a factor of two to three. Cleaning tests have been performed in vacuum on stainless steel samples after applying mixed Al/W/C/D coatings using magnetron sputtering. In situ XPS analysis during the cleaning process as well ex situ reflectivity measurements demonstrate near complete removal of the coating and a substantial recovery of the reflectivity. The first results also show that the reflectivity obtained through cleaning at 532 nm may be further increased by additional exposure to UV light, in this case 230 nm, an effect which is attributed to the removal of tungsten dust from the surface.