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The feasibility of laser-induced breakdown spectroscopy (LIBS) for measuring fuel retention was first demonstrated in 2024 in a tokamak operating with tritium. This was achieved using a remotely controlled in-situ application at JET [1]. Following the third deuterium-tritium campaign, DTE3, and the in-situ LIBS experiment selected plasma-facing components were removed from the JET vacuum vessel for post-mortem analyses. Prior to the in-situ LIBS experiment at JET the LIBS setup was tested at VTT by analysing JET divertor CFC and bulk Be samples from the main wall exposed in 2011-2016 [2] using the LIBS tool developed at ENEA. The tool consisted of the LIBS enclosure equipped with a sub-nanosecond Nd:YAG laser and focusing optics, which was connected via a 20 m optical fibre to an Echelle type spectrometer with wide spectral range 260-760 nm. The LIBS experiment at VTT focused on the co-deposited layers and on the calibration of ablation rates. The depth profiles clearly distinguished the various layers on the samples.
The LIBS experiment at VTT will be repeated in 2026 mainly because a high resolution Littrow spectrometer used at JET was not available at VTT in 2024. More importantly, the LIBS setup was optimized at FZJ for the experiment at JET after the experiment at VTT and the modifications made to the optical setup changed the focusing of the laser beam on to the sample and the transmission properties of the optical setup. In addition to the Littrow spectrometer, a high spectral resolution Double Echelle Monochromator series in Littrow configuration (DEMON) will also be tested for the separation of the alpha lines of the hydrogen isotopes. The resolution of the DEMON spectrometer is 3-10 times better than that of the Littrow spectrometer at FZJ. The same samples as in 2024 as well as the first DTE3 samples, including from JET limiters and divertor, will be characterised for calibration-free LIBS and for calibration of the ablation rates. This work also presents updated results from depth profiling and CF analyses of the JET divertor obtained during the experiments at VTT in 2024.
[1] J. Likonen et al, First Demonstration of Laser Induced Breakdown Spectroscopy using Remote Handling for In-vessel Analysis of JET Components, Nuclear Materials and Energy 45 (2025) 102021.
[2] J. Ristkok et al., Preparing LIBS for in-situ measurements in JET tokamak: system overview and co-deposited layer thicknesses, Nuclear Materials and Energy 44 (2025) 101968.