Characterization of laser-induced breakdown spectroscopy on tungsten at variable ablation angles using a coaxial system in a vacuum

As an in situ and remote diagnostic technique, laser-induced breakdown spectroscopy (LIBS) has been well proven to detect the elemental distribution on plasma-facing materials (PFMs) in the magnetic confinement nuclear fusion device of Experimental Advanced Superconducting Tokamak (EAST). However, due to the limited optical windows and irregular shape of the PFM surface, such as the monoblock structure of the divertor, the laser ablation angles (LAAs) of LIBS always vary with the laser spot scanning on the different regions of the PFMs. In this work, a LIBS system with a coaxial collection configuration based on the linear array fiber bundle is developed to investigate the features of laser-induced tungsten (W) plasma spectra at variable LAAs under vacuum conditions. The results show that the LIBS signal intensity, signal to background ratio, and signal to noise ratio of the W II line have different trends compared with W I under different LAAs. The intensities of the W I line decrease faster than those of W II with the increase of LAAs. Moreover, it is confirmed that the emission of the continuum background propagates mainly along the direction of the incident laser, while the plume of W I and W II propagates along the normal direction of the target. For the coaxial LIBS system, the results show that the plasma spectral collection area with a diameter of no less than 7 mm can minimize the spectral signal loss rate under different LAAs, no matter for W I or W II. This result will further improve the understanding of the spatial evolution of LIBS plasma at variable LAAs, to optimize and upgrade the current collection system of the in situ LIBS in the nuclear fusion device of EAST.