Study of spectral intensity of the laser ablated tungsten plasma and ablation mass at various laser spot sizes and laser fluence in vacuum environment

The ablation features such as spot size and crater volume are essential parameters for the in-situ laser-induced breakdown spectroscopy (LIBS) technique to be well applied in the quantitative diagnosis of the plasma-wall interaction (PWI) processes on the first wall or divertor in tokamak devices. The spectroscopic characteristics and the ablation features as well as the plume dynamics of laser-induced tungsten plasma in vacuum environment at various spot sizes were investigated in this work. A nano-second laser pulse (5 ns, 1064 nm) with a maximum energy of 200 mJ was focused on the tungsten target with different spot sizes (from 0.42 mm to 2.71 mm) by adjusting the distance from focal point to sample surface. A paraxially collective configuration (15°) and time-integrated spectral collection mode were applied, similar to the present in-situ LIBS system on Experimental Advanced Superconducting Tokamak (EAST). It was observed that the spot size affects the spectral intensity significantly, and the optimal spot size is 1.16 mm both for the observed tungsten ionic and atomic lines at the laser energy of 200 mJ. The electron temperature (Te), electron density (ne) and ablation mass also reach maxima at the optimal spot size. Temporally spectral dynamic investigation shows the tungsten line emission time at the optimal spot size lasts longer and decays more slowly than other sizes, and the fast photography investigation also verified this. It was found that the central intensity profile of the plasma plume image from the target surface along with the opposite direction of the laser incident at the early stage (< 8 ns, during the laser pulse) is stronger at the smaller spot size while it decays faster. This was attributed to the stronger plasma shielding effect leading to the relatively less total ablation mass. Moreover, the optimal spot size was found to increase with the laser energy in the energy range from 65 mJ to 200 mJ, and the corresponding optimal laser fluence almost keeps the constant value of ∼18 J/cm2, which provides valuable information for optimizing the current in-situ LIBS for quantitative diagnosis of the PWI processes such as fuel retention and impurity deposition in EAST.