Correlation of the Electron-Emitting Properties of Cathodes in Vacuum and in Gas Discharges

Experimentation involving the transients of the starting behavior of gas discharges reveals that this proceeds, in general, in three distinct stages: (1) a time lag for breakdown, termed plateau A, (2) a period of transition due to interaction between discharge and cathode, termed plateau B, and (3) the establishment of a final steady state for both discharge and cathode, termed plateau C. The initial state of plateau B is identified as a state of cathode and discharge that would constitute a steady state if the effects of ionic bombardment and Joule heating were absent. This state is found to obey the following relation, which is analogous to the Schottky equation for cathodes in vacuum: lnI=lnIθ+[d(lnI)/d(V1/2)](V1/2−Vθ1/2),where Iθ=AT2 exp (−∈φ/kT), the Richardson-Dushman thermionic emission; Vθ is constant with a value near the ionization potential of the gas in the diode, and the voltage at which occurs a readily determined rearrangement of the cathodic sheath; the slope d(lnI)/d(V1/2) is a constant dependent on cathode material and the atomic number of the gas in the diode. It is shown that the extrapolated linear dependence of Eq. (1) intersects the ordinate (V=0) at a singular point IR, independent of the atmosphere of the diode (vacuum, gas or type of gas) and identical with the zero-field current of the cathode in vacuum. All experimental observations appear consistent with a theoretical model postulating a Schottky effect at the cathode. A consequence of these results is a firm correlation of cathode behavior in vacuum and in gas discharges.