Laser propagation through an absorbing transonic flow

Experiments were carried out to investigate the propagation of a cw CO₂ laser beam through an absorbing slightly supersonic flow. A 500-W CO₂ beam was propagated through a small blow-down wind tunnel operating with an air/SF₆ mixture in order to simulate a high-power beam propagating through the atmosphere. A collinear schlieren system was used to observe the density gradients caused by the absorbed power. Large density gradients were observed at a Mach number of 1.17; however, they were not shock initiated and their magnitude was not inconsistent with a heat-balance analysis. Under the conditions of the experiment no shock waves were observed, even though their occurrence was predicted by a simple one-dimensional analysis. From this study it is concluded that shock waves will not occur under most conditions encountered by high-power cw CO₂ laser radiation in the atmosphere. The presence of subshock density gradients and their effect on the propagating beam require further study.