Laser cleaning of diagnostic mirrors from tokamak-like carbon contaminants

Abstract

This paper presents a laboratory-scale experimental investigation of laser cleaning of diagnostic First Mirrors (FMs). Redeposition of contaminants sputtered from tokamak first wall onto FMs surface could dramatically decrease their reflectivity in an unacceptable way for the functioning of the plasma diagnostic systems. Laser cleaning is a promising solution to tackle this issue. In this work, pulsed laser deposition was exploited to produce rhodium films functional as FMs and to deposit onto them carbon contaminants with tailored features, resembling those found in tokamaks. The same laser system was also used to perform laser cleaning experiments by means of a sample handling procedure that allows to clean some cm 2 in few minutes. The cleaning effectiveness was evaluated in terms of specular reflectivity recovery and mirror surface integrity. The effect of different laser wavelengths (λ = 1064, 266 nm) on the cleaning process is also addressed.

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

“…For this reason we decided to use UV laser light for all the cleaning experiments here reported. Such a sharp variation of the cleaning efficacy depending on the laser wavelength was not observed in the case of carbon contamination [16]. It can be explained considering that W-based contaminants are (to different extents) oxidized, and thus they present an optical band gap which inhibits the absorption of NIR photons (the band gap for crystalline WO 3 is about 2.8 eV, while the photon energy for λ = 1064 nm is 1.17 eV).…”

“…The laser fluence (defined as the energy per pulse divided by the laser beam size) for each wavelength must be chosen according to the strict requirement of leaving the mirror's surface undamaged during the laser cleaning treatment. In [15] and [16], we found that the damage threshold for bare highly-oriented Rh coatings deposited on Si was higher than 350 mJ cm −2 for NIR and 150 mJ cm −2 for UV pulses, using the irradiation procedure described above. Due to some concerns about the stability of the laser system during UV operation, the laser fluence for the UV cleaning trials described hereafter was set to 110 mJ cm −2 .…”

“…Laser cleaning was performed in high vacuum ( × − 3 10 3 Pa) [16]. The laser beam hit the sample at 45°, and the laser spot on the sample had an elliptical shape with a major axis of ∼13 mm and a minor axis of ∼9 mm.…”

“…With the aim of cleaning areas of some cm 2 to ensure the uniform irradiation of the sample, the latter was moved during the cleaning. In [16], we described a sample handling procedure that allows fast and uniform irradiation and is readily scalable to samples of larger size.…”

“…Choosing ∆ = x 6 mm and v y = 4 mm s −1 each point of the sample received approximately 45 pulses per scan, and the time required to clean a × 2.6 2.6 cm 2 mirror was around 1 min. This choice proved to be a good compromise among cleaning effectiveness, time requirement and substrate integrity [16]. Multiple scans can be repeated on the same sample if required; in order to mitigate the effects of possible laser non-uniformity, the sample holder was rotated by 90° between one scan and another.…”