Preparing LIBS for in-situ measurements in JET tokamak: system overview and co-deposited layer thicknesses

Abstract

Laser-induced breakdown spectroscopy (LIBS) is a method for elemental composition analysis that has been proposed for fusion reactor safety diagnostics. A significant milestone in this development was the LIBS campaign conducted in 2024 at the Joint European Torus (JET), using a prototype LIBS enclosure, deployed with the MASCOT tele-manipulation arm. The work presented here prepared for the JET campaign by testing the LIBS enclosure. Experiments were conducted at VTT Technical Research Centre of Finland, analyzing JET wall samples from the 2011–2016 ILW1–3 fusion campaigns, primarily from the divertor. The focus was on the analysis of co-deposited layers on the plasma-facing components containing hydrogen isotopes and elements from bulk layers: Be, W, Mo, CFC, and Inconel. Measurements were performed under atmospheric pressure air with an argon flow. Optimal experimental conditions for the use of an Echelle spectrometer in subsequent JET LIBS campaign were identified, and the depth profiles of the surface layers are presented. The LIBS depth profiles defined distinct material layers. Ablating through the co-deposited layers required 1–870 laser shots (∼0.1–90 µm) on samples from different locations, with typical variations of 10–40 % on the same sample and the largest variation spanning 15–480 shots (∼1.5–50 µm). The LIBS, Secondary Ion Mass Spectrometry (SIMS), and optical profilometry results showed good qualitative agreement. The ablation rate was ∼30–50 nm/shot for the W layers, ∼100–140 nm/shot for bulk Be limiters, and intermediate for the co-deposited layers. The insights gained in this study supported the preparation of the JET LIBS campaign.