Plasma formation resulting from the interaction of a laser beam with a solid metal target in an ambient gas

The formation and role of a plasma resulting from the interaction between a laser beam and a metal target in an ambient gas are reviewed in this paper. The plasma is initiated after the production of primary electrons by multiphoton absorption and thermionic photoemission mechanisms. Vaporization of the surface then occurs at a lower threshold than theoretically deduced, due to surface defects and impurities. Vapour ionization is first a thermal process, primary electrons gaining energy by inverse Bremsstrahlung, than electron cascade growth occurs. The plasma propagates with absorption waves: laser-supported combustion or detonation waves depending on the laser irradiance regime. The ablation process is favoured with the shortest wavelengths, whereas ambient gas breakdown occurs with the largest wavelengths. The nature of the ambient gas must be adapted for the chosen application. Plasma development can be an inconvenience with target shielding effect when the ionization degree is too high and with laser-supported detonation wave generation occurring in this case. However, transparent plasmas provide optimum laser machining, as good surface coupling with plasma formation occurs which is useful for drilling or welding.