Space-charge effects in secondary emission

Secondary emission devices fall into two classes. (1) Those in which the emitter-collector potential is such that all emitted secondary electrons are received by the collector. (2) Those in which the emitter-collector potential is such that the number of collected secondary electrons equals the number of incident primary electrons. For either class of device this potential is determined by the nature of the secondary emitter, the electrode configuration, and the secondary-electron current density. For the first class the collector must always be positive with respect to the emitter. For the second class there are cases where this potential may be negative and others where it may be positive. This paper shows how Langmuir´s treatment of thermionic emission in the plane diode can be applied to secondary emission to compute the emitter-collector potential for either class of device. This treatment accounts for space-charge and initial electron velocities. The effects predicted theoretically are supported by experimental measurements. Included are nomographs which simplify the computations involved in applying Langmuir´s plane-diode equations to either thermionic or secondary emission.