Large-spot thermal coupling of CO2 laser radiation to metallic surfaces

Abstract

Thermal coupling of pulsed 10.6-μm laser radiation to aluminum and titanium targets was measured as a function of incident fluence, focal-spot size, and ambient pressure. Thermal coupling coefficients were measured with both calorimetric and fast-response surface-thermocouple techniques. Thermal coupling coefficients of over 0.3 were observed with the onset of a well-developed plasma at the target surface. The thermal coupling was observed to increase slightly with increasing irradiated spot size and to decrease monotonically with increasing laser fluence. Under conditions of low ambient pressure (∼0.5 Torr) the breakdown threshold was increased by a factor of 5 and at high incident fluences the thermal coupling was roughly a factor of 2 higher than at atmospheric pressure.

Time-resolved optical emission spectroscopic measurements of He plasma induced by a high-power CO2 pulsed laser

“…In this case, practically all the laser irradiance is absorbed by the gas plasma [18]. It is expected that, in some medium range pressures, both gas and target plasmas are produced.…”

Time-resolved optical emission spectroscopy of laser-produced air plasma

“…On the other hand, it is well known that a gas plasma, due to the gas breakdown process, is produced when a TEA CO 2 laser is focused onto a metal sample at a gas pressure of around 1 atm, in which case practically all the laser irradiance is absorbed in the gas plasma. 27,28 It is expected that, in some medium range pressures between 0.01 torr and 1 atm, both gas and target plasmas are produced. In such a case, some interaction inevitable takes place between the gas and the target laser-induced plasmas.…”

Pulse compression techniques for laser generated ultrasound

“…The laser beam is collimated with an aspheric lens to a 5mm diameter and then focused to a small spot on the sample using a 25.4mm diameter, 75mm focal length lens. Assuming the collimated beam has a Gaussian distribution, we calculate that the spot radius at the focus is approximately 8mm and the surface power density is of the order of 10 5 W/cm 2 , which is well below the ablation limit of 2x10 6 W/cm 2 for aluminum [17]. Positioning the sample slightly off the focus of the 75mm focal length lens increased the spot size and reduced the power density far below material damage.…”