ON THE THERMOIONIZATION IN HOT CAVITIES

Thermally equilibrated gases of low pressure are characterized by a high degree of ionization at temperatures easily reached with laboratory devices. This property is generally believed to be the principle behind ion sources of the hot-cavity-type. The necessary condition of thermal equilibrium for the cavity plasma is, however, often very unlikely, though in general tacitly assumed. It is shown that especially in the most widespread application - low pressure operation for isotope separation on-line - the cavity plasma is far off thermal equilibrium. The consequence is that the influence of the cavity parameters (work function, temperature, plasma density and pressure of the neutral particles) on the ionization efficiency is decisively different from what is expected for a thermalized system. © 1990.