Recent studies of electrical breakdown in clean vacuum systems indicate that breallown is initiated by field emission current and occurs at a value of the electric field which is approximately a constant independent of the electrode geometry and the spacing between the electrodes. It has been postulated that sharp projections on the cathode cause a local enhancement of the electric field and hence almost all of the field emission current is emitted from these projections. Breakdown is believed. to occur when a projection is heated to a critical temperature by the field emission currents. Thus, the value of the breakdown field is the field at the tip of such a' projection and not the average field at the cathode surface.nThe present investigation was undertaken to investigate the characteristics of projections that exist on electrode surfaces and to determine the correlation between the geometry of the projections and the prebreak down current-voltage characteristics of the electrodes.nFowler-Nordhelm plots of the prebreakdown currents predicted thatnthe projections should be of the order of 10 to 10 cm in diameter andnhave a length about ten times the diameter. Direct observations of electrode profiles with an electron microscope confirmed that projections of the predicted size and shape did exist on the electrode surface. Electrical breakdown resulted in the disappearance of one or more projections and sometimes caused the formation of new projections.
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