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The opportunities and advantages of an innovative cylindrical thermionic converter for solar power and propulsion systems are discussed. The converter consists of coaxial cylinders, where the emitter is located on the exterior cylinder and the collector on the interior, resulting in an inverted design compared to traditional thermionic converters. Design considerations and parameters for a solar thermal power system led to the basic concept of the cylindrical inverted thermionic converter. The materials and design of an experimental prototype, consisting of a single-cell inverted thermionic converter are described. Results of calculations on output parameters, emitter temperatures during operation, and efficiency are presented. Fabrication and testing of this device are in progress.

This paper presents trade studies that address the use of the thermionic/AMTEC cell - a cascaded, high-efficiency, static power conversion concept that appears well-suited to space power applications. Both the thermionic and AMTEC power conversion approaches have been shown to be promising candidates for space power. Thermionics offers system compactness via modest efficiency at high heat rejection temperatures, and AMTEC offers high efficiency at modes heat rejection temperature. From a thermal viewpoint, the two are ideally suited for cascaded power conversion: thermionic heat rejection and AMTEC heat source temperatures are essentially the same. In addition to realizing conversion efficiencies potentially as high as 35-40%, such a cascade offers the following perceived benefits: Survivability - capable of operation in the Van Allen belts. Simplicity - static conversion, no moving parts. Long lifetime - no inherent life-limiting mechanisms identified.

An integral space power concept provides both the electrical power and propulsion from a common heat source and offers superior performance capabilities over conventional orbital insertion using chemical propulsion systems. This paper describes a hybrid (bimodal) system concept based on a proven, inherently safe solid fuel form for the high temperature reactor core operation and rugged planar thermionic energy converter for long-life steady state electric power production combined with NERVA-based rocket technology for propulsion. The integral system is capable of long-life power operation and multiple propulsion operations. At an optimal thrust level, the integral system can maintain the minimal delta-V requirement while minimizing the orbital transfer time. A trade study comparing the overall benefits in placing large payloads to GEO with the nuclear electric propulsion option shows superiority of nuclear thermal propulsion.

The goal of the Thermionic Fuel Element Verification Program (TFEVP) is to demonstrate the technological readiness of a TFE suitable for use as the basic element in a thermionic reactor having an electric power output in the 0.5- to 5-MWe range, and a full-power life of seven years. The TFEVP has made significant progress in developing components capable of withstanding the required neutron fluenee (4x1022 n/cm2, E>0.1 MeV) and the required burnup (5.3%). Technology developed under the TFEVP also supports the 5- to 40-kWe thermionic systems currently of interest to the Strategic Defense Initiative Organization and the Air Force. The fast neutron flux in certain 5- to 40-kWe systems is up to a factor of five less than that in 0.5- to 5MWe systems. Component technology that has been developed for 0.5- to 5-MWe systems will thus be suitable for use in long-life, high-performance, 5- to 40-kWe systems.

A hybrid space nuclear reactor which can provide both electrical power and thermal propulsion has been found to have significant benefits for near term military satellites. It combines the STAR-C thermionic reactor with NERVA rocket propulsion technology. This hybrid reactor would use a W/UO2 cermet core which was developed in the nuclear rocket program of the 1960's. Both the reactor core and the thermionic converters can be tested prior to launch. The design of a 10 kWe reactor capable or producing over 1300 N of thrust is described.