Modeling and simulation of magnetron discharges inside a vacuum interrupter as a method to analyze the vacuum status

Abstract

Vacuum Interrupters (VIs) rely strongly on the vacuum level to be below 10-2Pa to be able to operate safely. The vacuum status is normally tested during production to be well below this level (typically below 10-5Pa) and then assumed to be sealed for life (fixed at 30 years). With a larger number of vacuum interrupters now reaching their end of life, there is a renewed interest in assessing their vacuum status in the field. The magnetron gauge principle can be applied for this. The basis for the principle is the formation of an electron trap by electric and magnetic fields across the VI. This article investigates the measurement principle using different modeling and simulation approaches. This allows getting insight into the working principle. Some basic design properties can be understood from the electron orbits. More important is the formation of a closed magnetic trap, which is investigated using an effective potential. More detailed calculations are done using a particle-in-cell approach. This allows the calculation of the build-up of the space charge of the electrons inside the trap, as well as the position and total numbers of electrons trapped. With this, one can determine the proportionality constant between the electrical current measured and the particle density in the VI. A comparison of the outcome of these simulations to own experimental findings and to results found in the literature is finally done.