Monitoring of tritium and impurities in the first wall of fusion devices using a LIBS based diagnostic

Laser-Induced Breakdown Spectroscopy (LIBS) is one of the most promising methods for quantitative in-situ determination of fuel retention in Plasma-Facing Components (PFCs) of fusion devices. The current state of understanding in LIBS development for fusion applications will be presented, based on a complete review of existing results and complemented with newly obtained data. The work has been performed as part of a research programme, set up in the EUROfusion Consortium, to address the main requirements for ITER: a) quantification of fuel from relevant surfaces with high sensitivity, b) the technical demonstration to perform LIBS with a remote handling system and c) accurate detection of fuel at ambient pressures relevant for ITER. The elemental composition of ITER-like deposits, including deuterium (D: as substitute for tritium (T)) or helium (He) containing W-Be, W, W-Al and Be-O-C coatings, was successfully determined: D surface densities below 1016 D/cm2 could be measured with an accuracy of ~30% (depth resolution 50-250 nm). A remote handling application was demonstrated inside the Frascati-Tokamak-Upgrade (FTU), where a compact, remotely controlled LIBS system was mounted on a multipurpose deployer providing an in-vessel retention monitor system. LIBS was performed at atmospheric pressure for measuring the composition and fuel content of different area of the FTU first wall and toroidal limiter. Concerning the capabilities of LIBS at pressure conditions relevant for ITER, quantitative determination of the composition of PFC materials at ambient pressures up to 100 mbar of N2, the D content could be determined with an accuracy of 25% (50% at 1 bar using single-pulse lasers). To improve the LIBS performance in atmospheric pressure conditions, a novel approach, based on an alternative LIBS detection timing scheme, is proposed. The application of double pulse LIBS at atmospheric pressure improved the distinguishability of H isotope lines significantly, but further research is required.