Depth-resolved deuterium retention profiles in displacement-damaged tungsten measured via picosecond-laser-induced ablation quadrupole mass spectrometry

Tungsten (W) is the most promising plasma-facing material candidate for future deuterium–tritium (D–T) fusion reactors due to its favorable properties, such as low sputtering yield, low chemical reactivity, high melting point, and low intrinsic fuel retention. However, highly energetic neutrons from DT fusion reactions can cause displacement damage in the W lattice and enhance fuel retention. This affects the tritium cycle requirements and nuclear safety, as a tritium inventory builds up in the vessel. Therefore, diagnostics are required to quantify the D and T content in-situ in the plasma-facing and structural materials. Laser-induced Ablation Quadrupole Mass Spectrometry (LIA-QMS) is a promising method for quantifying fuel content with good spatial and depth resolution. LIA-QMS can be simultaneously applied with Laser-induced Breakdown Spectroscopy (LIBS). Combining both techniques provides the high depth resolution of LIBS with the quantification capabilities of LIA-QMS. This study compares D depth profiles recorded with pico-second LIA-QMS with Nuclear Reaction Analysis (NRA) with 3He beam on a displacement-damaged W sample. The comparison reveals the depth profiling capabilities, strengths, and weaknesses of LIA-QMS using picosecond lasers. A set of similarly self-damaged (10.8 MeV W3+ irradiated) ITER-grade W samples from PLANSEE was gently loaded with D in a low-temperature plasma at 370 K. The D concentration was varied by subsequent annealing of the samples at different temperatures in a vacuum after the D decoration. The ratio between D2 and HD, both contributing to the total D content, increases from 1:1 to 1:5, starting at the surface and extending to 4μm, with increasing depth. LIA-QMS shows a similarly high sensitivity (<0.05 at% D at a 15 nm average ablation rate (AAR)) as NRA (around 150-400 nm resolution). ps-LIA-QMS can be calibrated via a known amount of reference gas injections and deviates from the NRA results by a factor of 1.7 across all samples, which also includes non-volatile species. The laser-induced crater surface stays relatively flat for up to 4μm until surface structures start dominating the crater’s surface under the given laser parameters. μ-NRA in and around the craters shows complete removal of D inside the laser crater. Thermal effects due to the ps-pulses within the crater floor are indicated, but could not be quantified yet. In conclusion, this study shows a good agreement between ps-LIA-QMS, a potential in-situ method, and the reference ex-situ method NRA for D quantification. This paves the way for studies to investigate open questions about particle–wall interactions during the ablation process.