Time-resolved study of Bremsstrahlung emission and spectra at the early stage in a nanosecond laser ablated tungsten plasma

Abstract

Laser-Induced Breakdown Spectroscopy (LIBS) has been considered as a crucial tool for element analysis in plasma wall interaction (PWI) of fusion devices. In this work, a one-dimensional gas-dynamic numerical model has been optimized to simulate the Bremsstrahlung emission and the spectral lines of W and W+ ranging from 180 nm to 450 nm at the early stage of plasma in a nanosecond laser ablation. In the literature, most of the researchers pay more attentions on the spectral lines but the Bremsstrahlung emission (continue background) is neglected. In our study, the relationship between plasma temperature and the continue Bremsstrahlung background is found. The numerical results show that the plasma Bremsstrahlung emission dominated in the early time of laser ablation but exponentially decreases from 10 ns to 160 ns. The intensity of spectral lines of W increase while that of W+ increase firstly, then decrease gradually from 10 ns to 160 ns. In addition, a Q-switch Nd:YAG pulsed laser with 8 ns FWHM and 1064 nm wavelength is used to experimentally study the plasma emission processes. The results indicate that the plasma Bremsstrahlung domains during the early stage of laser ablation, which is agreement with numerical prediction. The spatial integrated intensity of Bremsstrahlung, W-atomic line emission and W-ionic line emission were investigated. We hope that the results would provide a good guidance for optimizing LIBS application for PWI diagnosis of fusion devices.