New Experimental Results for Plasma Electron Oscillations†

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Abstract
This paper contains an account of some experimental investigations of plasma electron oscillations in low-pressure mercury vapour discharges with plane oxide coated cathodes. The appearances of some of the discharges are described. The primary electrons were deviated laterally in or near the meniscus-shaped region of ionized gas brighter than the surrounding plasma, which appeared a few millimetres from the cathode. Variation of anode-cathode separation was found to have a marked effect on both the appearance of the deflected beams and the meniscus, and on the high-frequency oscillatory properties of the discharges. Measurements of plasma electron concentration and temperature were made simultaneously with measurements of oscillation intensity and frequency. A strong tendency was found for the oscillations to grow in amplitude when the beam of primary electrons moved down a concentration gradient and to die out when the beam moved up a gradient. The frequency of the oscillation was found to agree closely with that calculated from the Langmuir formula, using the plasma electron concentration at the position of appearance of the oscillation. Several checks were applied to the measurements of concentration, including measurements of the positive ion current flowing to a probe and measurements with double probes.

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