The starting mechanism of the first stage of the ring discharge

Abstract

When the high frequency magnetic field of a coil surrounding a cylindrical vessel filled with hydrogen is increased above a critical value, a faint discharge starts which emits the H 2 -band spectrum. If the current in the coil is raised further, an intense ring discharge suddenly develops showing predominantly the atomic hydrogen lines. The first stage is shown to be initiated by the axial component of the electric field of the coil, even when conventional electrostatic screens are used. When the axial component is suppressed, a circumferential component of even twice the axial starting field fails to initiate the discharge. This axial uniform field is calculated for low pressures, at which the mean free electron path exceeds the size of the vessel, by assuming that electrons are multiplied by collisions with the glass wall; it is thus independent of the gas and its density. The dependence of the starting field on length, pressure and frequency is measured using a vessel of variable length and 4 cm radius. Values of 9 and 5V/cm peak at 15 an d 5 Mc/s respectively are found between 10 -4 and 10 -2 mmHg a t 20 cm length, in agreement with theory. For pressures between 0.1 and 1 mmHg the starting field is calculated by balancing the rate of gas ionization by electron collisions and the loss of charge by diffusion to the wall, field-dependent energy losses being allowed for. Its value, now depending on the nature of the gas, rises with increasing pressure and becomes frequency-independent at higher pressures. Again, theory and experiment agree quantitatively.