Basic understanding on beam - Plasma interaction in laser welding

In early development days of laser welding, most of engineers and scientists believed that the laser induced plasma was the high temperature and high pressure plasma which could reflect the incident beam by plasma due to the plasma frequency [1]. However, the necessary electron number density to reflect the incident beam is 10¹⁹ 1/cm³ for YAG laser and 10²¹ 1/cm³ for CO₂ laser. These electron number densities are unachievable in the one atmospheric pressure thermal plasma. The maximum electron number density of thermal plasma at 1 atm. is in the order of 10¹⁷ 1/cm³ which is much less than the equivalent number density of cut-off frequency for infrared lasers such as CO₂ and YAG lasers.

The paper describes the type of laser induced plasma, behaviors of plasma during welding, spectroscopic characteristics of plasma, measured plasma temperature and electron number density, and possible energy dissipation of beam energy in laser plume. It has been concluded that the plasma induced in laser welding is a low temperature and weakly ionized plume. The energy loss in plasma is interpreted due to Inverse Bremsstrahlung for longer wavelength lasers and to Rayleigh scattering by ultra-fine particles formed in and around plume for shorter wavelength lasers.