<title>Contribution to the beam plasma material interactions during material processing with TEA CO2 laser radiation</title>

Abstract

The TEA-C02-laser (ransverse1y xcited atmospheric pressure) is a tool for the pulsed processing of materials with peak power densities up to 1O" W/cm2 and a FWHM of 70 ns. The interaction between the laser beam, the surface of the work piece and the surrounding atmosphere as well as gas pressure and the formation of an induced plasma influences the response of the target. It was found that depending on the power density and the atmosphere the response can take two forms. (1) No target modification due to optical break through of the atmosphere and therefore shielding of the target (air pressure above 10 mbar, depending on the material).(2) Processing of materials (air pressure below 10 mbar, depending on the material) with melting of metallic surfaces (power density above 0.5 lO W/cm2), hole formation (power density of 5 iO W/cm2) and shock hardening (power density of 3 .5 lO W/cm2). All those phenomena are usually linked with the occurance of laser upported combustion waves (LSC) and laser upported detonation waves (LSD), respectively for which the mechanism is still not completely understood. The present paper shows how short time photography and spatial and temporal resolved spectroscopy can be used to better understand the various processes that occur during laser beam interaction. The spectra of titanium and aluminum are observed and correlated with the modification of the target. If the power density is high enough and the gas pressure above a material and gas composition specific threshold, the plasma radiation shows only spetral lines of the background atmosphere. If the gas pressure is below this threshold, a modification of the target surface (melting, evaporation and solid state transformation) with TEA-C02-laser pulses is possible and the material specific spectra is observed. In some cases spatial and temporal resolved spectroscopy of a plasma allows the calculation of electron temperatures by comparison of two spectral lines.