Some Aspects of the Sparkover Time of a Low-Pressure Magnetic Switch

In this paper an approximate equation for the calculation of sparkover time of a low-pressure magnetic switch is presented. The theory is based on the assumption that all of the positive ions generated in the gap are situated at the anode. The concept of the effective reduced electric field (EREF) is applied to take into account the influence of a crossed magnetic field on the discharge parameters. The results obtained show that on application of a magnetic field the sparkover time in oxygen decreases very rapidly and reaches a minimum value. The spark over time remains at this minimum value over a range of magnetic field strengths, the minimum value depending upon the molecular number density and the applied voltage. A further increase in the magnetic field increases the sparkvoer time very rapidly, reaching a "cut-off" value at which breakdown of the gap is not possible. Application of Davidson's theory for a low pressure discharge shows very good agreement between the calculated and measured sparkover times in electric fields alone. Davidson's theory has been extended to a coaxial cylindrical geometry and also to the case of an applied crossed magnetic field. The sparkover times are calculated using the extended Davidson's theory and compared with the present author's theory.