Investigation of the helium afterglow II. Langmuir probe observations

Measurements are described of slow electron density decay rates in pure helium afterglow plasmas, using a very small single Langmuir probe, a technique not hitherto applied to the study of ambipolar diffusion. Values are obtained for the rate of conversion of He + ions to He 2 + ions and for the respective mobilities of these ions at a wall temperature of 295 °K, which are in reasonable agreement with values obtained by other workers using different techniques. This agreement is considered to indicate that depletion problems, which are expected to be most severe in slowly decaying plasmas, have been overcome by the utilization of very small probes. Evidence is also presented for the existence of elevated electron temperatures at times as late as 10 ms in the afterglow, and these results, together with those above, are critically discussed and compared in detail with the complementary mass spectrometric observations described in I.