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Investigation of the properties of the Penning discharge with effects due to helium gas in a vacuum interrupter / Lance D. Benson.

by Benson, Lance D.

Electric circuit-breakers, Vacuum.

Vacuum technology

Non-destructive testing

Chemical detectors

92 leaves : ill. ; 29 cm.

Advisor: Dr. Richard Mett.

Committee Members: Jeffrey Korn, Dr. Robert Strangeway.

Introduction -- Electrical conduction through vacuum and gases -- Gaseous discharges -- Pressure testing at Cooper Power Systems -- Experimental test setup -- Theoretical model -- Experimental results -- Electrical field analysis -- Particle trajectory models -- Analysis of theoretical models -- Conclusions and recommendations -- References -- Project approval form.

The purpose of this project is to determine the effect of changes in testing parameters such as the magnitude and direction of electric and magnetic fields, and gas composition (helium gas) when using the Penning discharge for the testing of vacuum interrupters. This paper seeks to determine how these parameters influence the discharge vs. pressure curves for different types of vacuum interrupter designs. Several references were analyzed and then used to examine the basic fundamentals of gaseous discharges, the basic theory behind the Penning discharge, and charged particle motion in crossed fields. Experimental data was then taken to verify the effects of changes in various testing parameters for the pressure testing of vacuum interrupters.

A theoretical model was developed based on a particle balance within the vacuum interrupter under steady state conditions. In conjunction with the theoretical model, the potential distribution, electric field distribution, and electron trajectories for each vacuum interrupter configuration were evaluated using finite element analysis software.

The results of the software analysis indicate that the electrons produced by the discharge move in complex helical paths. The electrons tend to become confined in specific regions of the vacuum interrupter due to the combination of the effects of crossed fields and the electrical repulsion that is imparted to an electron that is constrained between two negatively charged surfaces.

The results of the experimental data, when combined with calculations based on the theoretical model, indicate that the electron density tends to remain constant in the regions of electron confinement due to effects caused by the buildup of space charge. In addition, the non-linear characteristics that were measured for the discharge current versus pressure curves can be explained by effects due to secondary ionization caused by electrons that escape from the confinement region.

Test results indicate that the Penning discharge has a reduced sensitivity to helium. Due to the smaller molecular cross-section and the higher ionization potential of the helium atom, the magnitude of the discharge current that is produced at any given pressure is reduced by a factor of 0.18 that of air. Because of this, the upper test limit when testing vacuum interrupters processed in a helium atmosphere should be adjusted to account for this reduced sensitivity.

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