Very-Low-Frequency Plasma Oscillations in a Spherical, Magnetized, Cold Cathode Discharge

Abstract

Coherent very-low-frequency (VLF) oscillations in the range 0–30 kHz, due to a plasma transport across magnetic field lines, have been found in a cold cathode dc discharge, where the cathode is a uniformly magnetized sphere. Similar oscillations are found to occur in coaxial cylindrical discharges with a uniform external magnetic field. The frequency of the oscillations decreases with increasing discharge current, decreasing discharge voltage, background gas pressure, magnetic field strength, ion mass, and increasing diameter of the cathode. The oscillations are rich in harmonics. At the lower frequencies the discharge revolves in the equatorial plane of the cathode. A mixing phenomenon is reported in which sum and difference frequencies are obtained between the signals of the device and an external generator. Using probes as diagnostic tools the maximum of the oscillations has been located in the negative glow of the discharge. In addition it is found that two different modes of oscillation exist, the m=0 mode having no phase difference in the angular direction, and the m=1 mode having a phase difference of 2π in the angular direction. A simple theoretical model, based on the assumption that the frequency of oscillation is proportional to the rotation frequency of the plasma, is developed for the m=1 mode of oscillation. Finally the use of the device as a sensitive pressure gauge is suggested.