The Effect of Temperature, Degree of Thoriation and Breakdown on Field Currents from Tungsten and Thoriated Tungsten

Abstract

Electron field currents from thoriated tungsten, with different degrees of thoriation, were found to be independent of temperature. The characteristic field current curve was found to be independent of the degree of thoriation of a thoriated tungsten filament. An electrical breakdown, a sudden discontinuity in the characteristic field current curve at which the current increases from small values usually considerably less than a microampere to a few milliamperes, occurs with thoriated tungsten and pure tungsten cathodes. This breakdown raises the field current curve to higher currents by an enormous factor. Current increases by factors from 10,000 to 10,000,000 were usually obtained. With pure tungsten no change in the thermionic activity of the cathode results from the breakdown; with thoriated tungsten the thermionic emission is increased; i.e., a partial thoriation of the filament occurs. Heat treatment at about 1300\ifmmode^\circ\else\textdegree\fi{}K following breakdown generally further increases the field currents from thoriated tungsten but produces no such effect with pure tungsten. The high field currents following breakdown can be erased by heating the cathode to about 2600\ifmmode^\circ\else\textdegree\fi{}K for a few minutes. The value of the breakdown field is independent of the temperature and the degree of thoriation of a thoriated tungsten cathode. The application of the usual breakdown voltage with the filament as anode produces none of the usual effects of breakdown. Breakdown does not occur if a sufficiently high resistance is inserted in the circuit. In general, successive breakdowns occur at increasingly higher values of the applied field. None of this progression can be attributed to a conditioning of the anode. A definite part of this progressive increase in the breakdown field can be attributed to glass surfaces when they are exposed to the discharge. The highest electric field that could be applied to a cathode without breakdown occurring was about 4.7\ifmmode\times\else\texttimes\fi{}${10}^{6}$ volts/cm. The evidence favors the following conclusions: The electric field applied to the cathode rather than the applied voltage is the more important factor in producing breakdown. The anode has no effect on the breakdown. When the shielding of glass surfaces is adequate, the breakdown is determined primarily by conditions at the cathode.