Laser heating of metallic surfaces

Abstract : Thermal coupling of pulsed 10.6 micrometer laser radiation to aluminum, copper, and titanium targets has been measured as a function of incident fluence focal spot size, and ambient pressure, using both calorimetric and fast-response surface-thermocouple techniques. A peak enhancement in thermal coupling of approximately a factor of ten was observed to occur at the onset of a well-developed plasma at the surfaces of the copper and aluminum targets. After passing through a maximum, the enhanced coupling decreased with increasing fluence and approached CW values at high incident laser fluences. For small spot sizes (area approximately equal to, or less than 0.03 sq cm), most of the enhanced absorption occurred outside the focal spot. The fraction of energy coupled to the target within the focal spot increased with increasing spot size. Under conditions of low ambient pressure (approximately 0.5 torr), the breakdown threshold was increased by a factor of 5, and at high incident fluences the thermal coupling for aluminum was roughly a factor of 2 higher than at atmospheric pressure.