Dynamics of prompt electrons, ions, and neutrals of nanosecond laser ablation of tungsten investigated using optical emission

摘要

A Nd:YAG laser with a power density of 15GW/cm(2), operating at a wavelength of 1064 nm, was used to ablate a tungsten (W) target placed in an ambient atmosphere of air at a pressure of 10 mbar. A systematic study of plasma properties employing fast photography and time and space resolved optical emission spectroscopy was carried out. The results showed that there were continuum, neutral, and singly ionized W atom optical emission in the laser-induced plasma; the spectra of ambient gas N-2 and N-2(+) were aslo observed which appeared at very early times ( 30 ns). The spectra of N-2(+) presented a temporal profile with a narrow width (similar to 20 ns) and a steep rising edge (similar to 10 ns) that was very close to the laser pulse duration, which revealed that the prompt electrons ejected from theW target collisionally excited and ionized ambient N-2 gas molecules. The approximate timescales from nanoseconds to microseconds of four laser-produced W plasma processes including prompt electrons, continuum radiation, ions, and neutral emission were given according to time-resolved optical emission spectroscopy. Fast imaging showed that the prompt electrons were expanding hemispherically. The optical time of flight dynamics of N-2(+) corresponding to the prompt electrons at different positions showed that the kinetic energy of prompt electrons was up to 22 eV. These prompt electrons with high energy result in the separation from the core plasma which indicated the existence of the transient dynamic plasma sheath during the laser ablation process and interpreted the acceleration of ions. Published under license by AIP Publishing.