POPULATION INVERSION IN HELIUM IN SUPERSONIC PLASMA EXPANSION
Zhinzhikov,G.M., Luk'yanov,G.A., Nazarov,V.V., Pavlova,N.O.: 1978,
Sov.Phys.Tech.Phys., 23, 554.
The conditions associated with a population inversion of the He I levels with
principle quantum numbers 4 and 3 are studied in the supersonic expansion of
helium 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 recombining 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 rate of impact decay of the levels. The population inversion found
experimentally corresponds to gain values of the order of 0.1 per meter.
Diffusion of nitrogen from the surrounding medium into the jet reduces
the population inversion.
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