Laser cleaning of diagnostic mirrors from tungsten–oxygen tokamak-like contaminants

Abstract

This paper presents a laboratory-scale experimental investigation about the laser cleaning of diagnostic first mirrors from tokamak-like contaminants, made of oxidized tungsten compounds with different properties and morphology. The re-deposition of contaminants sputtered from a tokamak first wall onto first mirrors’ surfaces could dramatically decrease their reflectivity in an unacceptable way for the proper functioning of plasma diagnostic systems. The laser cleaning technique has been proposed as a solution to tackle this issue. In this work, pulsed laser deposition was exploited to produce rhodium films functional as first mirrors and to deposit onto them contaminants designed to be realistic in reproducing materials expected to be re-deposited on first mirrors in a tokamak environment. The same laser system was also used to perform laser cleaning experiments, exploiting a sample handling procedure that allows one to clean some cm2 in a few minutes. Cleaning effectiveness was evaluated in terms of specular reflectance recovery and mirror surface integrity. The effect of different laser wavelengths ( λ = 1064 , 266 nm) on the cleaning process was also addressed, as well as the impact of multiple contamination/cleaning cycles on the process outcome. A satisfactory recovery of pristine mirror reflectance (⩾90%) was obtained in the vis–NIR spectral range, avoiding at the same time mirror damaging. The results here presented show the potential of the laser cleaning technique as an attractive solution for the cleaning of diagnostic first mirrors.

Helium load on W-O coatings grown by pulsed laser deposition

“…The role of different PLD parameters in the deposition procedure, such as the wavelength of the laser beam used to ablate the W source (532 nm in this case), the laser fluence per pulse (close to 15 J/cm 2 ), the targetto-substrate distance (7 cm) or the pressure and composition of the used background gas are detailed elsewhere [4,5]. PLD parameters are properly tailored to produce porous-W and compact-W coating that mimic W-O re-deposited microstructures observed in tokamaks [2,3] after O absorption [5].…”

“…The compact morphology is only characterized by the growth of nanograins spread out along the entire films structure. The gathering of O through the W lattice occurs afterwards by exposing the as-deposited samples to atmospheric air [4,5]. The coatings were deposited on stainless steel (SS) 316L substrates.…”

“…An earlier investigation has shown a structural transition from a crystalline to a highly compact morphology growth in the deposits at about 30 Pa by varying the He background pressure in the range from 10 -3 Pa to 200 Pa [5], and by raising the He pressure the deposits appear to be formed by even Nevertheless, the final O content (15%-20%) is always lower compared to the case of porous deposits (∼50%) [5]. High surface-to-volume ratio of these microstructures, enhanced by a columnar growth, induces the spontaneous gathering of O after deposition in the coatings during exposition to ambient air [4]. The O content, as quantified by EDS in the surface of the porous-W films using in the present experiment was about 55 at.% [4].…”

Imaging Feature Analysis-Based Intelligent Laser Cleaning Using Metal Color Difference and Dynamic Weight Dispatch Corrosion Texture

“…Its calculation method is shown in Equation (4). The judgment method of the cleaning performance is shown in Equation (5). In this paper, the CIEDE2000 model is utilized to calculate the color difference feature between our rusty image of the workpiece and the standard image, and then the initial evaluation of whether the cleaning performance is qualified can be determined.…”

“…Laser cleaning is a green and environmentally friendly technology that has been widely used in the preservation of cultural heritage, nuclear fusion research, ship repair and industrial manufacturing [1][2][3][4][5]. Laser cleaning has many advantages, including good cleaning quality, high cleaning efficiency, and stable cleaning precision.…”