Plasmoidal High-Frequency Oscillatory Discharges in "Non-Conducting" Vacua

Abstract

Plasmoidal discharges. A new type of vacuum tube discharge is described, excited by comparatively low voltage high-frequency oscillations (1.7 meters up to 30 meters wave-length) in which remarkable luminous balls, spindles and pear-shaped bodies are formed. These have been reported in earlier papers in Nature and the Phil. Mag. A study of the conditions under which these bodies are formed and their behavior in magnetic fields has lead to the conclusion that surface charges play a part in their formation. They have been shown to be built of excited molecules, as their spectra are molecular band spectra (${\mathrm{O}}_{2}$ or CO) and since Langmuir has seen in their formation the possibility of plasma oscillations, such as he observed in the mercury arc; they have been named "plasmoids" provisionally.Red phosphorescence of the glass wall. This occurs in all of the tubes containing oxygen at very low pressure. It appears to be due to low velocity secondary electrons in combination with excited oxygen. Its behavior in magnetic fields has been studied, and it appears when a plasmoid is deflected against the wall by a magnet. The red phosphorescence does not appear until after the high frequency discharge has operated on the glass for some minutes, becoming finally a bright ruby red. The same glass surface phosphoresces green under the impact of high velocity electrons given off from the inner surface of the glass when the external cathode is excited by higher voltages. It may happen that superposed streaks of red and green phosphorescence may occur simultaneously and they can be separated by a magnetic field.Clean-up effects. Remarkable clean-up effects have been observed, it being possible to have a tube sealed off from the pump which can be made to show either a pure Balmer spectrum of atomic hydrogen (purple discharge) or the green discharge of molecular oxygen. The pressure changes occurring in the sealed off tubes as the type of spectrum changes have been followed with a vibrating quartz fiber manometer.Spectroscopic investigation. The spectra of the plasmoids, their dark sheaths, and the less luminous gas in which they are imbedded have been investigated by projecting their images on the slit of a spectroscope. The band spectrum of molecular oxygen is seen in the whole tube, but is concentrated in the plasmoids and diluted in their dark sheaths; atomic lines appear as well, the local concentration being different for singly and doubly ionized oxygen.General nature of discharge. There are two types of discharge, one at very low potentials, suggesting the Townsend discharge (unmodified by space or surface charges) and a more brilliant one which comes in suddenly as the potential is gradually raised. Plasmoids, however, appear to form with both types, though their shape and distribution in the tube changes as we pass from one type to the other. External electrodes are used in all cases.