Speaker
Description
Hydrogen (H) isotopes retention in plasma-facing materials is a critical issue for nuclear safety in fusion devices operating with a deuterium-tritium mixture and tungsten plasma-facing components. Its reliable detection is of great importance for both safety and material characterisation after plasma exposure. However, we observed that in vacuum conditions, the Balmer alpha spectra (Ha) measured by LIBS exhibit complex Doppler shifts, leading to significant spectral distortions.
In this work, we report the first systematic study of Doppler shifts in hydrogen spectra obtained by picosecond Laser Induced Breakdown Spectroscopy (ps-LIBS, pulse width: 35ps, max energy: 25 mJ, wavelength: 355 nm) on tungsten samples loaded with hydrogen. Our results reveal not only Doppler shifts associated with single-velocity H atoms, but also, at different time delays, distinct dual-wavelength shifts arising from the simultaneous presence of fast and slow H atoms in the LIBS plasma. The fast component exhibits a fluence-dependent shift, whereas the slow component shows the opposite behavior. Fast and slow H atoms can be temporally separated due to their different velocities. Moreover, it is demonstrated that, in the near surface layer, fast hydrogen corresponds to surface-adsorbed H, while slow H possibly corresponds to H present in the W lattice. Building on these findings, we demonstrate a new approach for tuning the Hα wavelength over a range of up to nm level (depends on laser fluence), making it possible to temporally isolate fast and slow hydrogen atoms and thereby suppress spectral interference. The Doppler characteristics additionally offer a potential spectroscopic tool using LIBS for distinguishing between hydrogen present at the surface and in the tungsten lattice. Finally, the dependence of the Doppler shift on fluence suggests that it may also serve as a proxy for in-operando monitoring of the laser fluence during LIBS measurements.
This work not only resolves a fundamental spectral distortion mechanism, but also provides guidelines for improving the accuracy of H isotopes detection in fusion-relevant plasma-facing materials (PFMs) in LIBS measurements. Importantly, neglecting this Doppler-induced effects may cause misinterpretation in H isotope analysis and absolute isotope-resolved quantification, since the Doppler-induced wavelength displacements can mislead isotope signatures.
This work was supported by the BMFTR project “SyrVBreTT” (Grant No: 13F1011G).