LIBS investigation of metals suitable for plasma-facing components: Characteristics and comparison of picosecond and nanosecond regimes

Abstract

For ITER and other future fusion devices, tungsten (W) and its alloys are primary candidates for the first wall in divertor, and molybdenum (Mo) will be used as the background material for erosion mapping and thickness control of the deposited layers and mirror materials. Considering the advantage of on-line analysis, laser-induced breakdown spectroscopy (LIBS) is being considered as the most promising tool to be used at ITER for the monitoring of the erosion and deposition processes and the fuel retention at the first wall. In this paper, picosecond (∼30 ps) and nanosecond (8 ns) time-resolved LIBS spectra were recorded for the characterization and comparison of fusion-related high-Z pure materials (W and Mo) at atmospheric pressure in ambient air. The plasma temperature was calculated using the Boltzmann plots of both neutral and ionic species, while the electron density was evaluated from the Saha equation. In the case of ps laser pulses, lower electron temperature and density were observed. The lower temperature in the ps regime is leading to a higher neutral to ionic ratio. At shorter delay times, the difference between electron densities was smaller, but with the increased delay, the difference became significant. The obtained values of ablation rates show that the ablation rate in ps regime is approximately 4 times lower compared to the ns regime. Lower electron density, also due to the lower ablation rate in the ps regime, leads to less Stark broadening and finally narrower spectral lines in the ps regime than in the ns one. The lower energy for ps laser-plasma generation (0.3 mJ/pulse; equivalent to a fluence of 1.7 Jcm−2) allows to obtain a lower ablation rate, which leads to a better depth resolution, which is crucial for depth analysis of fusion-related materials.