Surface plasma influence on nanosecond laser ablation

The comparison of laser ablation and plasma evolution has been carried out for a molten steel sample in the absence and in the presence of surface plasma. A continuous wave (cw) laser beam was utilized for local melting of a steel (Fe>99  wt.%) sample, but it also induced a surface plasma according to optical emission spectroscopy. The cw laser was switched off for a few milliseconds to dissipate the surface plasma, but the surface temperature did not change according to optical pyrometer measurements. Molten metal was ablated by a nanosecond Nd:YAG laser pulse during cw laser operation and when it was switched off for 5 milliseconds. Comparison of laser ablation and plasma evolution in the presence and in the absence of the near-surface plasma induced by the cw laser beam has been carried out. Time-integrated plasma imaging detected slightly greater emissivity of the plasma induced during cw laser operation. The cw laser operation resulted in a twofold enhancement of the intensity of atomic lines in the spectra as well as slower decay of plasma emission. Plume temperature and electron density were slightly greater at early stages of plume expansion in surface plasma.