Occurrence of population inversion in unsteady rarefaction waves behind the exit section of a supersonic nozzle

Experimental results are presented on population inversion at the unsteady stage of outflow from a 10 percent N2O - 20 percent N2 - 70 percent He mixture from a supersonic nozzle, such as that in a gasdynamic laser. It is shown that, at the unsteady stage of outflow, there exist zones where sounding radiation is amplified, these zones being connected not with the shock heating of outflowing gas behind the secondary shock front, but with unsteady expansion upstream from the secondary wave. Here the gain can exceed its quasi-steady value.