A 3.3 μm helium–neon laser and 2.25 and 8.6 mm microwave interferometer systems have been used to measure electron densities over the range 1.8 × 1015 to 8 × 1011 electrons/cm3 in a decaying helium plasma without a confining magnetic field at gas pressures ranging from 82 mtorr to 1.2 torr. Collisional radiative recombination is considered to be the dominant mechanism of electron removal above a pressure of 700 mtorr.
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Newton et al. (1966) studied this question.