Measurement of subsonic laser absorption wave propagation characteristics at 10.6 μm

An experimental study of subsonic laser absorption waves arising from the interaction of 10.6-μm radiation with metal and insulator target materials is described. A shock-tube-driven gas-dynamic laser operating up to 400 kW for 4 msec was used as the radiation source. Instrumentation included streak photography to determine wave speed, holographic interferometry to determine the gas density field, time-integrated visible spectroscopy to determine the principal radiating species, and CO2 laser attenuation measurements to determine the plasma absorption coefficient. The results reported here emphasize the characteristics of waves in free and clean air, far from the influence of the target. The measured wave speeds in air at 1 atm pressure ranged from Mach 0.05 to 0.2 for CO2 laser intensities ranging from 1×105 to 5×105 W/cm2. The wave speeds were also found to depend on the laser beam diameter. The spectral emission and CO2 laser absorption measurements showed that the air plasma reaches a temperature of 15 000–20 000°K and is essentially fully ionized. From an analysis of the results, it is concluded that radiation transport in the plasma plays a dominant role as a wave propagation mechanism, and that radial flow ahead of and within the wave is a dominant feature of the wave structure.