Source-dependent kinetic separation of H in tungsten by Doppler-shift-tuned LIBS

This is the authors' abstract. We don't add key points for this paper.

Retention of Hydrogen (H) isotopes in plasma-facing materials is a critical issue for nuclear safety and self-sufficiency in fusion devices operating with Deuterium-Tritium (DT) fuel mixture and tungsten (W) plasma-facing components. Its reliable detection, either in-situ or ex-situ, is of great importance for nuclear licensing and material characterisation after plasma exposure. However, we observed that in vacuum conditions, the Balmer-alpha line emission of Hydrogen (Hα) measured by Laser-Induced Breakdown Spectroscopy (LIBS) exhibit complex Doppler shifts in the laser-induced plasma (LIP), leading to significant spectral distortions. In this work, we report the first systematic study of Doppler shifts in Balmer-alpha line emission of hydrogen obtained by picosecond LIBS on W samples. Our results reveal not only Doppler shifts associated with single-velocity H atoms, but also, between delay time from 0 to 300 ns, distinct dual-wavelength shifts of the emission lines arising from the simultaneous presence of fast and slow H atoms in the LIP. The fast component exhibits a laser fluence-dependent shift, whereas the slow component shows the opposite behavior. Fast and slow H atoms can be temporally separated during detection due to their different velocities. Moreover, we infer, that in the near surface layer, fast H atom corresponds to surface-adsorbed H, while slow H atom may correspond to H present in the W bulk material. Building on these findings, we demonstrate a new approach for tuning the Balmer-alpha wavelength as function of laser fluence, making it possible to temporally isolate fast and slow H atoms and thereby suppress spectral interference. The Doppler characteristics additionally offer a potential spectroscopic tool using LIBS for distinguishing between H present at the surface and subsurface of the W bulk material. Utilizing this method, we successfully observed the dynamic adsorption kinetics of H on the W surface at a pressure level as low as 10 −7 mbar. This work not only resolves a fundamental spectral distortion mechanism, but also provides guidelines for improving the accuracy of depth-resolved H isotopes detection in fusion-relevant plasma-facing materials (PFMs) utilizing LIBS measurements.