Study of the Transition from Ambipolar to Free Diffusion by means of Afterglow Technique

The electron density-dependence of effective diffusion coefficient Ds for electrons in argon Maxwellianized plasmas has been verified experimentally by means of afterglow technique. A calibrated microwave resonant cavity was used for measuring the electron density. By appending a synchronous detector to a conventional circuit, the sensitivity for the electron density averaged over the volume of the cavity increased by about three orders of magnitude. Temporal behavior of the electron density between 1016 and 2.5×1011 m-3 at pressures of 46.6 and 66.5Pa was measured. In order to compare the experimental with theoretical results, was transformed into the axial dcnsity, by the decay rate of which Ds was determined. The values of Ds were compared with those obtained theoretically by several authors in the range of ratios between 4 and 500 of the characteristic diffusion length Λ of the discharge tube to the Debye length λD0 for electrons. It was found experimentally that diffusion mode changed from ambipolar diffusion to transition modes when Λ/λD0 became smaller than about 100.