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Ablation of materials (Cu is presented in this report) in air at an ambient gas pressure of 1 bar with a KrF excimer laser (3-47 J cm-2) leads to gasdynamic processes above the target surface which affect the processing result, the efficiency of the treatment and the debris in the environment of the irradiated area. These laser-induced processes have been diagnosed using fast schlieren photography and shadowgraphy. Five discontinuities have been discerned and their propagation mechanisms have been detected. A physical interpretation of the discontinuities is given along the lines of existing theories and plausible reasoning. The locally most advanced discontinuity can be explained by the classical Sedov-Taylor blast wave theory, and conclusions on the energy content in the shock wave, the pressure distribution and the surface pressure evolution will be presented. The results show that, at high energy densities (3-47 J cm-2), about 80% of the available laser pulse energy is deposited in the shock wave. A reduction in the energy density leads to a decrease in the fraction of the energy that is deposited in the shock wave. Close behind the first discontinuity follows a second one that is interpreted as the ionization front. The contact front, which separates shocked air and ablated material vapour, has been observed within the laser pulse duration. The complex structure of the contact front is interpreted in terms of gas flow phenomena inside the two outer discontinuities.

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This paper deals 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.