Femtosecond Laser-Induced Ablation – Quadrupole Mass Spectrometry (fs-LIA-QMS) experiment for the detection of trapped hydrogen isotopes and helium in nuclear fusion relevant materials

Abstract

We present a novel instrumentation using ultrashort pulsed laser induced ablation in combination with a Quadrupole Mass Spectrometer (QMS) for the detection of gaseous species retained in tungsten (W) and other metallic substrates. In particular, the potential of this technique for the investigation of hydrogen isotopes retention in plasma-facing components of fusion devices is demonstrated. Our fs-LIA-QMS experiment is capable of detecting deuterium (D) and helium (He) in W samples with high sensitivity down to $0.005\mathrm{at.\%}$, depth resolution in the order of $15\mathrm{nm/pulse}$, and high lateral resolution better than $30\mu m$. The study highlights the importance of using sub-picosecond laser pulses for an accurate depth-resolved detection of gaseous species trapped in metallic substrates. The use of ultra-short, near UV laser pulses ($400\mathrm{fs},343\mathrm{nm}$) reduces heat affection compared with nanosecond and picosecond ablation processes. Our results are in good agreement with other diagnostics applied in this context, such as 3-He Nuclear Reaction Analysis (NRA) for the D depth profiling, and numerical simulations of the He distribution with SDTrimSP. We also discuss the limitations of the technique, including the influence of heat diffusion initiated by the fs-laser and the characterization of loss-channels in the system that determine the limit of detection.