Pulsed laser heating profile width and changes in total coupling with pulse length and pressure

Abstract

Thermal coupling involves the heat absorbed by a surface in air under irradiation by a laser beam. At high incident intensities plasmas are ignited and there is an enhancement of total thermal coupling. Part of this occurs as some energy is transferred to the region outside of the laser beam spot because of plasma growth with increasing energy. The diameter of disk-shaped targets was used as a parameter in one set of experiments in order to determine the width of the radial heating profile. Second, when the axial temperatures of thin targets were measured as a function of time, there was a relationship for those and the assumed radial heating profiles which allowed the use of the Laplace inversion technique to obtain their widths. When the laser pulse length was varied over the range 1–25 μsec, the thermal coupling peak and threshold showed only small changes. No real optimization was seen when an initial spike was present. Thermal coupling at 0.1 Torr air pressure was found to lack the normal 1-atm enhanced regime. No abrupt intensity or fluence threshold was seen for either aluminum or titanium and the variation of thermal coupling with fluence was material dependent. Total thermal coupling maxima at 0.1 Torr were lower than those seen at 1 atm.