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The in-core thermionic nuclear reactor is a leading candidate for low power space systems requirements. The thermionic converters are static devices which convert heat directly to electricity in the form of high current, low voltage output power. The nuclear fuel cladding is used as the emitting electrode and is surrounded with close spacing by the collector electrode. The stability and lifetime of the system depend on the maintenance of the interelectrode gap established by the resulting coaxial geometry. Emitter distortion, therefore, can be a life-limiting factor. This is exacerbated by high emitter temperature and high fuel power density present in typical applications. In the particular case of low nuclear power level systems, the added complexity of a fast driver core section is necessary to ensure sufficient excess reactivity for power control.

The present work is aimed at developing a computational model of the interelectrode phenomena in thermionic energy converters which will be accurate over a very wide range of plasma conditions and operating modes. Previous models have achieved only moderate degrees of accuracy and, in a limited range, of validity. This limited range excludes a number of advanced thermionic devices, such as barium-cesium converters. The model under development promises improved accuracy in prediction of conventional devices and extension of predictive capability to advanced devices. The approach is to adapt the “Converted Scheme”, or CS method, to the cesium vapor plasma diode. This method, developed at the University of Wisconsin- Madison, is an extremely efficient algorithm for the solution of charged-particle kinetic equations and has been successfully used to simulate helium RF glow discharges.