Ignition and maintenance of subsonic plasma waves in atmospheric pressure air by cw CO2 laser radiation and their effect on laser beam propagation

Abstract

Laser supported combustion (LSC) waves, subsonically propagating plasma, are ignited from solid targets and maintained in atmospheric pressure air using focused high−power, as high as 15 kW, cw CO₂ laser radiation. The intensity necessary to ignite LSC waves, I_i, is measured for steel and aluminum targets as a function of beam focal spot diameter and wind velocity transverse to the beam axis as is the intensity needed to maintain such waves in atmospheric pressure air, I_m. In the absence of a transverse wind, the quantities I_i and I_m are essentially identical for diameters smaller than 0.1 cm, but I_i is substantially larger at larger diameters. The reason for this is discussed in terms of the ignition mechanism itself. In the presence of a 20−m/sec transverse wind, I_m and I_i are essentially identical at 0.2 cm, the largest diameter for which I_i was determined. The measurements of I_m extend to 0.5 cm beam diameter and indicate, in the absence of the transverse wind, that while thermal conduction determines the value of I_m at small (0.007 cm) beam diameters, radiative losses are important at the larger diameters considered. An interferometric technique is utilized to obtain spatially resolved LSC wave plasma properties, and the latter are used to calculate the decrease in focal spot laser intensity due to absorption and refraction by the plasma. The calculated results, as well as those obtained from independent experiments, indicate that while inverse bremsstrahlung absorption can account typically for a 50% loss in focal spot intensity, scattering or beam blooming by the plasma has a significantly greater effect as the LSC wave propagates away from the focal spot.