Speaker
Description
A compact laser-induced breakdown spectroscopy (LIBS) set-up recently operated on a remote handling arm in the JET tokamak after end of operation in a radioactive environment. The picosecond-LIBS set-up has successfully measured the fuel retention and surface composition on different plasma-facing components in-situ at 840 positions [1]. This LIBS set-up uses a spectroscopic plasma observation, which is co-linear with the laser beam. Two identical fused-silica lenses with focal length of 75.3 mm were used to collect and focus the LIBS plasma emission light into a 20 m long 1.5 mm diameter fused-silica fiber. The fiber is terminating into a 7-branch fiber bundle, which distributes the plasma emission light between an Echelle spectrometer, a large etendue Littrow spectrometer, both of which had equipped with ICCD cameras, a compact spectrometer, a compact monochromator equipped with silicon photomultiplier (SiPM) and a filterscope having 3 SiPMs equipped with different bandwidth H$\alpha$ interference filter. The use of lenses provides the necessary compactness of the optical system 340x136x(328+89)mm$^3$. The light collecting lens is also used to focus the laser beam (1064 nm, 800 ps, 10 mJ) on the PFC surface.
Quantitative analysis of LIBS spectra requires calibration of the sensitivity of all available spectral channels. The standard calibration procedure using an extended calibrated light source at the target position is not applicable due to the substantial chromatic aberration of the lens optical system. To account for this effect, the optical transmission of LIBS collecting optics was calculated using ZEMAX software for different light source diameters. Calibrations of the LIBS system along with the 20m long optical fiber were performed with help of available calibration types (absolutely calibrated integrating sphere with halogen light source, deuterium lamp, H$_2$/D$_2$ capillary discharge lamp Hg low pressure discharge lamp, Ne glow discharge
lamp) prior to its installation on the remote handling arm. In addition, several types of samples were used to determine crater size, ablation rate, and conversion factor of Balmer alpha line radiation to deuterium content in tungsten, JET divertor material. After the LIBS set-up was
jacketed and introduced into the tokamak, periodic calibrations were performed by connecting the 1.5 mm diameter fiber bundle tip to the calibration source. The complete calibration procedure and selected quantifications for a divertor and main chamber position with one spectrometer is presented as an example for the successful calibration and quantification. The calibration is applied in adjacent presentations for plasma-wall interaction studies.
[1] https://doi.org/10.1016/j.nme.2025