Laser cleaning of pulsed Laser Deposited diagnostic mirrors for nuclear fusion applications

Full text: Laser cleaning technique has attracted more and more attention from scientist and engineers as a powerful tool for many high-tech applications, in which a solvent-free cleaning process, easy to automate and gentler than abrasive techniques, is required. In the field of nuclear fusion research, laser cleaning is a potential candidate for the cleaning of the delicate First Mirrors (FMs), essential components of the diagnostic systems in future fusion reactors like ITER. Due to the complex interactions with ITER harsh environment, the FMs' surface may be covered by the materials eroded from the reactor main chamber, leading to a dramatic decrease of FMs reflectivity which can have an ultimate impact on the reactor safety and economics. An effective laser cleaning process has to remove the contaminants without damaging the mirror surface nor altering its optical properties. In this work, the laser cleaning of ITER FMs is investigated in details, following an integrated approach in which a single Nd:YAG nanosecond laser source is exploited to produce rhodium (Rh) coatings suitable as FMs, deposit ITER-relevant contaminants onto mirrors' surface and study the laser cleaning process of the ITER-like diagnostic mirrors. Nanostructured, high-reflective Rh films with realistic properties as FM coatings (i.e. reflectivity, mechanical properties, behavior under reactor-relevant conditions) are produced by means of Pulsed Laser Deposition (PLD) and are used as test system for cleaning. The great versatility of the PLD technique is also exploited to deposit ITER-relevant contaminants with controlled features on mirrors' surface. By properly controlling the deposition it is possible to tune finely the properties of the contaminants. Previously, we successfully cleaned tokamak-like carbon layers deposited on Rh mirror surface. Here we discuss the cleaning of tungsten- and aluminium- (as beryllium proxy) based contaminants, with different composition, degree of oxidation, morphology (from compact dense layer to open, very porous structure) and thickness (from 100 nm up to 1 𝜇m). Laser damaging tests are performed on bare Rh coatings, in order to find a range of laser parameters (fluence, wavelength, number of pulses) safe for the cleaning experiments. The mirrors are cleaned using the fundamental Nd:YAG wavelength (NIR, λ=1064 nm) or its fourth harmonic (UV, λ=266 nm). The laser fluence per pulse varies from ~100mJ/cm² to ~400mJ/cm². A suitable remote handling procedure is developed to clean samples of some cm² in a few minutes in vacuum condition. Coatings morphology and composition are assessed by SEM, EDS and Raman spectroscopy before and after the contamination and after cleaning. The specular reflectivity (Rspec) of the mirrors is characterized using UV-Vis- NIR spectrophotometry before and after cleaning. We observe that the cleaning effectiveness depends on the contaminant composition, morphology and optical properties. Nevertheless, by properly choosing the laser parameters, a satisfactory recovery of Rspec (from 70% to 90% of the original value for λ > 400 nm) is achieved for most of the samples here considered, thus proving the robustness and the adaptability of the technique also in the context of nuclear fusion industry and research. An investigation of the structural and morphological changes experienced by the contaminant during the laser irradiation, and their impact on the efficacy of the cleaning process, is also presented. Finally, the effect of repeated cycles of cleaning-contamination-cleaning on the mirrors' performances is discussed. (author)