Population inversion in helium in supersonic plasma expansion

The conditions associated with a population inversion of the He I levels with principal quantum numbers 4 and 3 are studied in the supersonic expansion of helium plasma from circular and plane nozzles into a low-pressure region. In an axisymmetric jet the population inversion arises at some finite distance from the nozzle; in plane jets the inversion occurs immediately beyond the slit nozzle. The experimental results are compared with calculations. The experimental data confirm the operation of a recombination mechanism for the population of the He I levels under the experimental conditions and show that a population inversion can be achieved by virtue of the difference in the rates of impact decay of the levels. The population inversion found experimentally corresponds to gain values of the order of 0.1 m/sup -1/. Diffusion of nitrogen from the surrounding medium into the jet reduces the population inversion.