This paper dears with the absorption and defocusing of a CO2 laser beam by the laser-induced plasma plume in deep penetration welding. To derive the 'effective' intensity distribution in the focal plane theoretically, the laser beam propagation through the plasma plume is calculated by solving the paraxial wave equation with a finite-difference scheme. Corresponding to experimental results, documented in the literature, the properties of the plasma plume (spatial temperature distribution and shielding gas content) are pre-set within the calculation. Parametric studies demonstrate that the intensity at the focus is reduced due to the defocusing effect of the plasma plume, mainly, and only to a minor extent due to absorption within the plume. Because of refraction within the plume, the intensity distribution in the focal plane is dependent on the plasma's size, position and temperature. On studying the dependency of the optical properties on plasma temperature and shielding gas composition, it is found that, by applying a shielding gas mixture of He and Ar in the ratio 3:1, the variation of the focal diameter with plasma temperature can be significantly reduced. This shielding gas mixture, therefore, is recommended for enhancing process stability when welding with high-power CO2 lasers.
