Experimental characterization of energy transfer from large-diameter kilowatt continuous-wave laser beams to metal samples

In this work, the energy transfer from intense continuous-wave laser beams with a wavelength of 1070 nm, a power in the kilowatt range, and with diameters in the millimeter and centimeter range to metal samples is investigated. While the absorptivity of iron and steel samples is almost constant for laser intensities below 3.4 kW/cm2, a decrease in the absorptivity is observed for higher intensities which is attributed to the formation of a vapor plume in the interaction zone. The dynamics of the formation and expansion as well as the emission of light in the visible spectral range from the vapor plume are further characterized with a fixed beam diameter of 2.6 mm at a laser power of 10 kW in detail for iron and aluminum samples. The analysis of high speed video sequences yields expansion velocities of the vapor plume of 5.0 m/s for the iron and 0.29 m/s for the aluminum samples. In the spectra from the aluminum samples, emission lines from atomic aluminum as well as emission bands from molecular aluminum monoxide are identified and allow for the estimation of the basic thermodynamic parameters. A special focus is on the investigation of the effect of vapor and plasma formation on the energy transfer from the laser to the sample and on the analysis of the role of inverse bremsstrahlung in this process. The measurements indicate that the metal vapor is partially ionized and that there is a significant contribution of inverse bremsstrahlung to the absorption of laser energy in the partially ionized vapor plume.