Electron Interactions in Cryogenic Helium Plasmas

Microwave-propagation and microwave-interaction techniques have been used to determine the electron collision frequency for momentum transfer in helium for electron energies in the vicinity of 0.001 eV. Measurements of the complex microwave conductivity and electron-energy relaxation rates have been performed in the afterglow of a pulsed discharge in helium in the pressure range 0.1 to 5 Torr submerged in a bath of liquid helium at 4.2°K. Electron-radiation temperature measurements during plasma decay have demonstrated monotonically decreasing electron temperatures as a function of time. For times when an extrapolation of the electron temperature decay indicated near thermal equilibrium with the parent gas, a momentum-transfer cross section in the range 10×${10}^{-16\mathrm{}}$ to 19×${10}^{-16\mathrm{}}$ ${\mathrm{cm}}^2$ was determined. Measurements of electron-energy relaxation rates for atomic densities exceeding 2.3×${10}^{18}$ ${\mathrm{cm}}^{-3\mathrm{}}$, where the electron de Broglie wavelength is becoming long in comparison to the average inter-scatterer spacing, indicate the limit of validity of binary-collision concepts.