Nanosecond-LIBS-induced deuterium outgassing, diffusion, and surface modification in tungsten coatings

Tritium retention in plasma-facing components (PFCs) must be strictly controlled to ensure the safe operation of fusion devices. In ITER, compliance with the tritium inventory limit and assessment of fuel-removal strategies require in situ monitoring of fuel retention in PFCs. Laser-Induced Breakdown Spectroscopy (LIBS) is a diagnostic technique that can be implemented via remote handling to investigate fuel retention during maintenance shutdowns at ITER. In LIBS diagnostics, the selection of laser parameters plays a critical role in determining the ablation characteristics and thermal perturbation of the target. Used nanosecond (ns) laser pulses deliver energy on timescales that allow substantial thermal diffusion into the target, which could result in surface heating and melting. Under multi-pulse LIBS operation, these cumulative thermal effects can alter deuterium quantification, as deuterium may diffuse or outgas due to heating induced by earlier pulses. Such laser-induced perturbations can compromise accuracy of quantitative deuterium measurements if they are not properly accounted for in modelling. In this study, subsequent in situ ns-LIBS and in situ Nuclear Reaction Analysis (NRA) measurements were performed in Magnum-PSI on compact and porous tungsten coatings using a defocused, millimetre-scale laser spot (2.5×2.5mm). Deuterium retention and depth profiles were determined in and outside the LIBS crater. Under the investigated laser conditions, NRA measurements indicated a 66-79% reduction in retained deuterium within the sampled region of the LIBS crater, whereas pure material removal by ablation would account for only 21-42%, demonstrating additional laser-induced outgassing. Compact coatings further exhibited melting and intermixing with the underlying substrate. These findings highlight the importance of accounting for defocused ns-LIBS-induced perturbations in retention measurements. Further experimental and modelling studies are needed to assess the dependence of these effects on laser spot geometry and to determine whether picosecond (ps) LIBS, together with consideration of cumulative multi-pulse effects, could improve the reliability of fuel-retention diagnostics in future fusion devices.