Parameter optimization of the spectral emission of laser-induced tungsten plasma for tokamak wall diagnosis at different pressures

Abstract

Laser-induced breakdown spectroscopy (LIBS) is a promising method for the in situ online diagnosis of plasma–wall interaction processes in tokamaks. The effect of gas pressure on the plume expansion and spectral emission of nanosecond laser-induced plasma of the tungsten wall material was systematically investigated using a coaxial optical configuration. A nanosecond pulsed laser (5 ns, 1064 nm) was used to ablate a tungsten target surface to produce plasma at various air pressure levels from 10⁻³ Pa to 10⁵ Pa. It was found that the plasma size was strongly dependent on the ambient gas pressure, and the plume presented complex behaviors such as free expansion, splitting, sharpening, confinement, etc. The spectral intensity, the signal to background ratio, and signal to noise ratio were also affected significantly by the ambient pressure. The ambient pressure influence on the plasma plume expansion and spectral emission can be ignored when the gas pressure is lower than ∼1 Pa. The pressure of 10–100 Pa and delay time of 200–500 ns were the optimum conditions for the LIBS application in tungsten impurity of tokamak wall diagnosis. Moreover, the ablation-depth results indicated that the depth-resolution of fusion-related material analysis can be controlled by the ambient gas pressure. Temporally-resolved emission measurements were carried out to further investigate the continuum radiation and ionic and atomic emission behaviors. The temporal evolution of the electron density and electron temperature at different pressures was also determined. The plasma parameter change and laser ablation depth reduction were observed which are attributed to the ambient gas confinement and enhanced plasma shielding.