Emission Mechanism of High Current Density Thermionic Cathodes

Abstract

Summary form only given. In this paper, the research on the emission performance and the nonuniform emission of M-type cathode, impregnated scandate cathode, oxide cathode, and novel scandia-doped dispenser cathode are mentioned, the factors affecting the emission uniformity are discussed. The cathode's normal and abnormal "Schottky" phenomena in the case of high current density are also involved in by the ways of Miram curves and practical work function distribution (PWFD). According to the research results and experimental data, the authors proposed the "semiconductor-monolayer superposition" emission model, which is suitable for all kinds of thermionic cathode on the basis of barium-emission mechanism. It means Richardson-Dushman equation introduced from the phenomenon of pure metal emission is suitable for barium-tungsten cathode of monolayer emission mechanism (including M-type, mixed matrix, and reservoir cathode, etc). Though the semiconductor model based on traditional oxide cathode is not so perfect, it is still presumed suitable for the oxide cathode. As for the emission mechanism of all the kinds of scandate cathodes (no matter impregnated scandate, top-layer scandate or scandia-doped tungsten matrix, etc) are between the types of "semiconductor" and "monolayer" - a kind of superposition of semiconductor and monolayer models. No matter oxide cathode, barium tungsten cathode (M-type cathode) or scandate cathode can be explained by the model of "semiconductor and monolayer superposition" , and the "field assistance thermionic emission" mechanism decides the main factors leading to emission high or low and normal / abnormal "Schottky" effect. Essentially, barium tungsten cathode (including M-type, mixed matrix, and reservoir cathode, etc), which is typical thermionic emission, almost follows the pure metal thermionic emission formula. Therefore, the normal "Schottky effect" exists. However, the oxide cathode and all kinds of scandate cathodes (including impregnated, mixed pressed, top-layer or scandia-doped tungsten matrix) are greatly influenced by the field penetration, especially under the condition of delivering high current density. It is considered as "field-assistance thermionic emission". An obvious emission characteristic is that the work function decreasing with current density (or applied voltage) increasing. The authors predicted that a new kind of emitter (perhaps covering improved oxide and scandate cathode) exists between the ways of "field assistance thermionic emission" and "thermal assistance field emission". This kind of emitters should have better emission capability under low temperature than usual thermionic cathode and can sustain more awful environment than field emission array cold cathode. Moreover, its emission characteristics on high current density don't follow either the Richardson-Dushman equation or the existing Fowler-Nordheim equation. The equation that can explain this kind of novel cathode emission characteristics coupling both thermal effect and field effect must be modified formula which includes field assistance and/or thermal assistance effect.