Diagnostics of first wall materials in a magnetically confined fusion device by polarization-resolved laser-induced breakdown spectroscopy

Laser-induced breakdown spectroscopy (LIBS) is a powerful analytical tool for real-time diagnostics and detection of multiple elements deposited at the first wall of magnetically confined plasma fusion devices. Recently, we have tested LIBS in our laboratory for application to in situ real-time diagnostics in the fusion device EAST. In this study, we applied polarization-resolved LIBS (PR-LIBS) to reduce the background continuum and enhance the resolution and sensitivity of LIBS. We used aluminium (Al) (as a substitute for Be) and the first wall materials tungsten (W) and molybdenum (Mo) to investigate polarized continuum emission and signal-to-background ratio (SBR). A Nd:YAG laser with first, second and third harmonics was used to produce plasma. The effects of the laser polarization plane, environmental pressure and polarizer detection angle were investigated. The spectra obtained without using a polarizer (i.e. LIBS) were compared with those obtained with a polarizer (PR-LIBS). Distribution of emission spectral intensity was observed to follow Malus' law with respect to variation in the angle of detection of the polarizer. The spectra obtained by PR-LIBS had a higher SBR and greater stability than those obtained by LIBS, thereby enhancing the reliability of LIBS for quantitative analyses. A comparison of Al, Mo and W showed that W exhibited a higher continuum with stronger polarization than the low-Z elements.