The negative glow and Faraday dark space in near-normal neon discharges†

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Abstract
Radial and axial electrostatic probe measurements have been made of the negative glow and Faraday dark space of some weakly ‘ anomalous ’ cold cathode discharges in neon at pressures near 1 torr. These have been analysed and discussed with the aim of extending previous diffusion theories. Either an anode glow or positive column was present. The axial plasma electron concentration had a maximum in the negative glow, with much smaller values in the Faraday dark space. Some features of the axial pattern were consistent with an increase in diffusion coefficient on moving towards the anode. This could be due to increase in mean electron energy or to the onset of ‘ subnormal ’ conditions with expansion of the wall-sheath, the latter showing by development of bell-shaped radial concentration curves ; but there is doubt as to the applicability of ordinary probe theory under ‘ subnormal ’ conditions. The length of the Faraday dark space is consistent with the head of the positive column appearing when diffusion, mostly lateral, has either reduced the electron concentration to a characteristic positive column value, or has reduced the random current density to the order of the drift current density, or has caused expansion of the wall-sheath to a sine which would effectively close the tube without onset of some new régime. A number of ambiguities in the measurements and analysis are noted, and suggestions made for future work.

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