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Description
Picosecond-Laser-Induced Ablation Quadrupole Mass Spectrometry (ps-LIA-QMS) was employed to investigate deeply located deuterium retention in polycrystalline tungsten (W). As the foreseen primary material for fusion reactors, its high heat conductivity, low sputtering yield, and good fuel retention properties make it a promising choice as first wall material. However, high-energy particles, particularly ~14 MeV neutron fluxes, will induce defect generation that drastically enhances the saturation level of deuterium (D) and tritium (T) retention, affecting the fuel cycle. To address this challenge, ps-LIA-QMS has been developed, combining the strengths of quantification inherent to Laser-Induced Desorption Quadrupole Mass Spectrometry (LID-QMS) and depth-resolved measurements possible with Laser-Induced Breakdown Spectrometry (LIBS). Using ps-LIA-QMS enables depth profiling and quantification of retained fuel, with a compact setup which is supposed to be suitable for remote handling in future fusion reactors. Moreover, it allows simultaneous measurements with LIBS to build a complementary diagnostic setup.
This study addresses two major challenges in the application of ps-LIA-QMS: Understanding the QMS signal composition and possible probing depth. The remarkable depth profiling capabilities of QMS are showcased in 1.4 MeV, 0.05 dpa proton-irradiated and subsequently deuterium-decorated W samples, achieving a depth profile of over 15 μm. Quantification of the released deuterium yields a deuterium content of (0.7±0.2) at%, closely matching $^3$He Nuclear Reaction Analysis (3He NRA) results (1±0.15) at% within the first 4 µm depth. The QMS signal composition was analyzed for the proton-irradiated and for W$^{3+}$ self-damaged samples decorated with D.
Both profiles reveal a peak with exponential decrease at the surface, not observed in NRA measurements. Moreover, up to 50 at% of D are detected in the first 75 nm of the proton-irradiated sample. Both profiles could be attributed to strong thermal influences during the first laser pulses. Simulations of deuterium diffusion and trap annealing under transient heating in application to reference NRA measured D depth profiles qualitatively agree with ps-LIA-QMS results. These simulations suggest that the D signal is dominated by outgassing from regions below the ablated surface on time scales much larger than the laser pulse duration (up to ms).
In conclusion, this study marks a significant progress in the analysis of deuterium retention in tungsten. It showcases the unparalleled probing depth and sensitivity of ps-LIA-QMS and gives new insights about the signal creation during ps-LIA-QMS, making it a suitable candidate for fuel retention diagnostic in reactor-relevant conditions and studies of physical processes in damaged materials.