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Values of the ratio of the longitudinal diffusion coefficient to mobility DL/μ for electrons in He, Ar, Kr, and Xe are derived from current waveforms obtained during earlier measurements of electron mobility. The electric field to gas density ratios E/N cover the wide range of 10−3 to 20 Td, thereby bridging previous experiments at low E/N to recent experiments at high E/N. Here 1 Td=1×10−21 V m2. The corresponding DL/μ values range from 0.0066 eV for thermal electrons at 77 K to 10 eV. In addition to the well-known peak in DL/μ for Ar at E/N between 0.01 and 0.1 Td caused by the Ramsauer minimum in the momentum transfer cross section, we find previously unreported low-energy peaks in DL/μ vs E/N in Kr and Xe and previously unreported pronounced leveling-off in DL/μ at E/N≳8 Td in Ar, Kr, and Xe. Calculations of transport coefficients using numerical solutions of the Boltzmann equation and cross section sets in the literature give good agreement with experiment from E/N producing thermal electrons up to average energies ≊10 eV and E/N up to 100 Td, the upper limit of our calculations. The leveling off of DL/μ at high E/N is caused by inelastic collisions.

The temperature decay of a lightning channel during the interstroke period is determined theoretically. It is shown that, in the absence of input energy to the channel, the channel temperature will decay sufficiently slowly so that conditions conducive to the initiation and propagation of a dart leader will exist in the channel after a typical interstroke period of 40 msec. Thus, it would appear unnecessary to invoke the special mechanisms suggested by Brook et al. and by Loeb to explain the 'long' interstroke period. The calculations indicate that the lightning channel radius during the latter stages of a lightning stroke is of the order of centimeters. A mechanism is suggested to explain the phenomenon of bead lightning and to account for the observed long-lasting luminosity occurring at certain points on the normal discharge channel.

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Calculations assuming thermal equilibrium have been made of properties of cylindrical arc plasmas which decay after the removal of the electric field. Account is taken of energy losses due to thermal conduction and radiation, self-absorption effects being included where possible in the treatment of radiation. The maximum possible influence of radial convection is calculated by assuming that convective flow maintains a uniform pressure in space and time. Comparisons are made of the calculated decay of electrical conductance with experimental measurements, where available, for A, N 2 , SF 6 , H 2 , and air; curves are given showing the contributions of thermal conduction, convection, and radiatllOn to the decay. The theoretical results underestimate the rapid decays of electrical conductance that have been observed for plasma temperatures of less than 10000 OK in air, and less than 7000 OK in SF 6 , but good agreement between theory and experiment is obtained for argon. Information from rate coefficients supports the view that the rate of atomic recombination to form molecules is insufficient to maintain thermal equilibrium and as a result there is an enhanced rate of cooling. Theoretical transient temperature profiles in air are also compared with published experimental profiles.(1)