Abstract
ITER foresees applying laser induced breakdown spectroscopy (LIBS) as a tool for quantitative assessment of fuel retention in the first walls. One open problem related to LIBS application is the choice of the laser type. Here we compare two Nd/YAG lasers with different pulse durations, 0.15 and 8 ns, working at λ = 1064 nm for LIBS studies of samples with D-doped W/Al coatings of ≈ 3 μm thickness (Al is used as a proxy of Be) on Mo. Low pressure argon was used as a background gas. Experiments were done in conditions where other factors (broadening of spectral lines, signal-to-noise ratio, limited thickness of coatings etc.) did not shadow the effect of laser pulse duration. For these reasons, low pressure argon was used as the background gas and fluences were kept at comparatively low values. Spectra of laser-produced plasma were recorded as a function of the number of laser pulses.Partially overlapping lines of hydrogen isotopes were fitted with Voigt contours, intensities were fitted and depth profiles of deuterium were reconstructed. The relative standard error of curve-fitting of spectra recorded with the laser of shorter pulse duration was two times smaller than that recorded by the longer pulse laser. The electron density was found from the Stark broadening of Hα line of the laser-produced plasma and the electron temperature of plasma was found on the basis W and Mo lines.It was found that in the case of ps laser an acceptable accuracy of the detection of deuterium was possible at considerably lower values of fluence. Steps needed for comparison of ps and ns lasers in ITER-relevant conditions were discussed.
Highlights
The ITER strategy foresees applying laser induced breakdown spectroscopy (LIBS) for quantitative in situ diagnostics of fuel retention in the first walls during maintenance breaks
Quantitative information could be obtained by calibration-free LIBS, CF LIBS, where the elemental composition of a sample is determined on the basis of the accurate knowledge of the electron temperature Te and the electron density ne [1]
The fitting of LIBS profile obtained by ps laser of Φ = 5.5 Jcm−2 fluence with the GDOES profile resulted in the value of coating ablation rate ≈ 0.17 μm/shot
Summary
The ITER strategy foresees applying laser induced breakdown spectroscopy (LIBS) for quantitative in situ diagnostics of fuel retention in the first walls during maintenance breaks. Quantitative information could be obtained by calibration-free LIBS, CF LIBS, where the elemental composition of a sample is determined on the basis of the accurate knowledge of the electron temperature Te and the electron density ne [1]. To this end, reliable detection of spectral lines of hydrogen isotopes as well as those of the host material is required. Stronger lines of the target material are influenced by self-absorption and at larger values of td the intensity of lines with lower self-absorption has a low S/N ratio These are the main reasons triggering the search for alternative solutions for reliable recording of LIBS spectra. Some studies [3,4] indicate that using lasers with a shorter pulse duration would be advantageous for determining the surface characteristics and retained fuel depth profiles of the analyzed samples
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