Impulse reaction resulting from the in-air irradiation of aluminum by a pulsed CO2 laser

The impulse reaction resulting from the in-air irradiation of aluminum by a high-intensity pulsed CO2 laser is experimentally examined in this study. Power densities of the order of 107−108 W/cm2 are obtained for focused beam energies between 100 and 700 J and pulse durations of between 15 and 50 μ sec. The targets are mounted in a sensitive pendulum in order to measure the total recoil impulse. The time history of the impulse during irradiation is determined by an interferometric technique. In addition, thermocouples are used to monitor the rear surface temperature of the thin (0.05 cm) targets. The thermal and mechanical response of the target is dominated by the effects of a high-intensity leading edge of the laser pulse. The precursor spike generates an opaque plasma at the target surface which effectively shields the target from the incident energy. Impulse and pressure transmitted to the target are related, therefore, to reaction of the expanding plasma against the target surface. Impulse coupling coefficients (I/E) between 5 and 7.8 dyn sec/J are measured for this mechanism of impulse production. The major contribution to the total impulse is associated with the plasma pressure during the initial stages of the plasma-surface interaction. The peak pressures generated during this time are found to depend on the second power of the initial beam intensity. It follows that the corresponding impulse coupling coefficients should increase linearly with beam intensity. However, a net increase in I/E is not realized at higher beam energies because of absorption losses (air breakdown) along the beam path.