Propagation and shielding effects of surface plasmas on coated and uncoated materials due to pulsed CO2-laser radiation

The thermo-mechanical coupling of pulsed laser radiation with matter is affected by surface plasmas and ablation. Studies concerning these transient effects were carried out at ISL with a pulsed CO2-laser. In thin layers, depending on the skin depths and absorptivities, the transformation or vaporisation temperatures can be reached even within a single pulse. In the initial stage the ignited plasmas are partially transparent for the laser radiation and enhance the coupling due to energy transfers from this plasma to the target. At a later time the rapidly expanding plasmas tend to shield the target. The temporal development of these plasmas and the ablation of material from the surface were recorded with a high-speed video camera. The dependencies of the shielding effects on laser parameters and material properties were investigated additionally by plasma transmission measurements.