Interaction of Pulsed CO2 and Pulsed HF/DF Laser Radiation with Gaseous and Solid Targets.

Abstract

Abstract : HF and DF laser radiation from an unstable resonator was used to study laser induced breakdown in air, nitrogen and argon. For air at a pressure of 760 torr a threshold of 6 x 10 to the 10th power watts/sq cm was found for HF radiation induced breakdown. This threshold is proportional to p to the -0.6 power. The breakdown threshold for pure Ar is 1.6 x 10 to the 10th power watts/sq cm at atmospheric pressure and scales to the -0.95 power with pressure. The threshold for DF laser induced breakdown of air was twice the threshold of HF laser induced breakdown. The interaction of carbon targets with pulsed CO2 laser radiation was studied. The pulse width was 5 nanoseconds and the maximum flux density ranged from 4 x 10 to the 10th power to 5 x 10 to the 12th power watts/sq cm. The ion expansion velocity, electron temperature and x-ray emission from the carbon surface were measured. X-ray continuum emission was only detected at the highest flux densities, approximately 10 to the 12th power watts/sq cm, when an effective electron temperature of approximately 1 keV was measured. The laser induced breakdown thresholds for Ar, N2 and CO2 were measured using CO2 laser pulses of length 3 to 40 nanoseconds. In argon the threshold becomes energy dependent for pulse length less than 5 nanoseconds and is power dependent for pulse lengths greater than 14 nanoseconds. A similar transition between energy and power dependence is also exhibited in nitrogen. In a mixture of CO2/N2/He the threshold for laser induced breakdown exhibited a pressure dependence of p to the -0.3 power for 3 nanosecond pulses.