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Description
Laser-induced Breakdown Spectroscopy (LIBS) is a method for the analysis of material composition and is used for Plasma-Wall Interaction studies. Especially for the detection of implanted fuel, deuterium (D) and tritium (T), in plasma-facing components (PFCs) in a fusion reactor, its in-situ capabilities are promising. In the case of tungsten (W) PFCs, the expected low levels of implanted fuel will pose challenges to the technique in early operation phases, when neutron damage and associated increased retention is low.
Detection relies on the line emission of excited atoms in laser-induced plasma (LIP) plumes after ablation. Under vacuum conditions, a major contribution to the limit of detection (LOD) is essentially the total energy present in the LIP. A vacuum is required e.g. to study in operando in the vessel. Therefore, schemes need to be applied to enhance the energy content in the LIP. A laser pulse energy increase of the LIBS setup increases line emission as a larger volume is ablated, but it also reduces the depth-resolution of the LIBS measurement, thereby shifting its balance.
Another way to increase the total energy in the LIP is to absorb laser light from a second laser pulse in the established LIP [1]. In this double-pulse configuration, a second laser pulse is focussed into the LIP with a given delay relative to the first laser pulse, which initiates the LIP. Differences in LIP species mass cause differences in expansion speed in vacuum [2]. This difference leads to a different optimum Inter-Pulse Delay (IPD) for enhancement of hydrogen or tungsten line emission.
The IPD is set by an adjustable optical delay stage that delays the second laser pulse by varying the optical beam path length (20-160ns) as both pulses are generated simultaneously by a Nd:YAG laser (τ=35ps; 1st pulse: 355nm, 21mJ ;2nd pulse: 1064nm, 5.3mJ). The first laser pulse impacts normal to the sample surface (self-damaged tungsten, plasma loaded with 1 at.% D [2]) in a vacuum chamber (p=2*10-7mbar). The second laser pulse has an incident angle of 5°. A custom-made Littrow spectrometer measures Balmer-α line emission of the LIP and is thus used to detect hydrogen isotopes. A signal enhancement factor of up to 1.95 was measured at an IPD of 92ns. This optimum hydrogen IPD is different from the optimum of tungsten line emission.
This work is part of the project SyrVBreTT, funded by the BMFTR under grant no. 13F1011G
[1] https://doi.org/10.1016/j.sab.2006.09.003
[2] https://doi.org/10.1063/5.0211493