Ex-situ LIBS study for the determination of boron content in WEST divertor tiles after the 2019 campaign

Laser Induced Breakdown Spectroscopy (LIBS) is a chemical analysis method that has been used in various remote handling applications and is currently being developed for the remote composition analysis of layers deposited on the first walls and divertor components of fusion reactors. The technique applies laser pulses to ablate a small amount of material, which forms a plasma plume and emits a spectrum characterizing the elements originating from the investigated material. The use of consecutive laser pulses at the same spot allows us to obtain elemental depth profiles with a resolution depending on the laser ablation rate. The aim of the present study was to investigate the feasibility of the LIBS method to analyze the boron (B) content in the surface layer of WEST divertor marker tiles removed after the 2019 experimental campaign (C4) when they were subjected to 16 boronizations, 3 during the C3 campaign and 13 during the C4 campaign. Boronizations are used in tungsten wall tokamaks to reduce the W contamination in core plasma, but the formed boron layers retain their effect only for a limited time due to erosion and redeposition. The six studied samples originated from different poloidal locations along the divertor including erosion and deposition-dominated regions. The LIBS spectra contained the emission lines of tungsten (W), molybdenum (Mo) and carbon (C), which were the main elements of the marker tile layers, and B as one of the species in the surface layers deposited during the experimental campaigns. The depth profiles showing the LIBS line intensities as a function of the applied laser pulse number at the same spot were generally consistent with the Glow Discharge Optical Emission Spectroscopy and Secondary Ion Mass Spectrometry depth profiles obtained from nearby tile positions. The depth profiles corresponded to the expected deposition and erosion regions. The depth of the B-containing layer varied from tens of nanometers to several micrometers. The ablation rates of the deposit layers were generally 50–100 nm per laser shot, comparable to the rates of bulk W and Mo layers. The rate was considerably higher in the thickest deposits, which had stratified structures. The study shows that the LIBS method is sufficiently sensitive and has adequate depth resolution to study the B composition in the deposited layers.