Effect of background gases at reduced pressures on the laser treated surface morphology, spectral emission and characteristics parameters of laser produced Mo plasmas

Molybdenum owing to its high melting point and high reflectivity over a broad range of wavelengths has been used as first mirrors in EAST and HL-2A tokomaks and is also a candidate material for ITER mirrors. The effect of atmospheric conditions of Ar, N₂ and He at reduced pressure (2.0 × 10^-4 to 5 mbar) on the physical processes of the atomic and background emission, mass ablation, electron density and temperature of a laser produced Mo plasma has been investigated in order to adapt Laser Induced Breakdown Spectroscopy (LIBS) for in situ diagnostic and cleaning of the first mirrors. The obtained results strongly indicate that pressure and physical properties of the background gases play a crucial role to interpret the emission (line and continuum background) and characteristic parameters (electron density and temperature) of the plasma. The plasma shielding effect was found less pronounced for He environment and it leads to enhancement of the material evaporation from the target surface.