Effect of negative ions on current growth and ionizing wave propagation in air

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Abstract
The spatiotemporal development of electron and ion densities, electric fields, and luminosity are calculated for electron pulse experiments in overvolted parallel‐plane gaps by numerically solving continuity equations together with Poisson’s equation. Experimental coefficients for primary ionization, cathode photoemission, photoionization, and luminosity are used. Unambiguous determination of the coefficients for attachment, detachment, and charge transfer is not possible from available experimental results. Therefore, the calculations are repeated for three sets of coefficients for these processes, corresponding to the following assumptions: unstable negative ions, stable negative ions, and no negative ions. The results of the calculations show, in each case, that the electron pulse initiates an avalanche which grows exponentially until the onset of space‐charge effects. The calculated growth rate is strongly affected by the assumed attachment, detachment, and charge‐transfer coefficients. When the total number of electrons in the avalanche reaches ∼108, anode‐ and cathode‐directed ionizing waves, or streamers, develop from the avalanche and produce a weakly ionized filamentary plasma. The calculated ionizing wave velocities are strongly increasing functions of the space‐charge–enhanced electric field within the waves and are insensitive to the assumed attachment, detachment, and charge‐transfer coefficients. The numerically calculated ionizing wave velocities are in approximate agreement with a simple analytical theory.