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Electron beam welding of tungsten to tungsten/rhenium and tungsten/rhenium to niobium, discussing techniques for assembly of thermionic converter fuel elements
Out-of-core thermionic converter system using heat pipes as electrical resistive elements, discussing design and performance
Chemically vapor deposited deep etched tungsten emitter for high performance cylindrical thermionic converter
Heat rejection radiator influence on space nuclear power system as function of mass/area for Brayton, Rankine, thermoelectric and thermionic conversion schemes
Nuclear space power systems computer simulation for preliminary system and mission analysis, considering Brayton, Rankine and thermionic systems
Computer acquired I-V characteristics of thermionic fixed spaced planar diodes
Performance data from an etched-rhenium, molybdenum thermionic converter are presented. The planar converter has a guard-ringed collector and a fixed spacing of 0.254 mm (10 mils). The data were acquired by using a computer and are available on microfiche as individual or composite parametric current, voltage curves. The parameters are the temperatures of the emitter T sub E, collector T sub C and cesium reservoir T sub R. The composite plots have constant T sub E, and varying T sub C or T sub R, or both. The envelope and composite plots having constant I sub E are presented. The diode was tested at increments between 1500 and 2000 K for the emitter, 750 and 1100 K for the collector, and 540 and 640 K for the reservoir. In all, 774 individual current, voltage curves were obtained.
A Russian paper by A. A. Babad-Zakhryapina contributes much to the understanding of fuel, clad interactions, and thus to nuclear thermionic technology. In that publication the basic diffusion expression is a simple one. A more general but complicated equation for this mass transport results from the present work. With appropriate assumptions, however, the new relation reduces to Babad-Zakhryapina's version.
Description of new (III-V) photocathodes which show improvements in sensitivity of as much as ten to a hundred times over conventional cathodes in the near infrared and useful improvements at shorter wavelengths. The development stems from a combination of basic knowledge of the photoemission process, gained in the 1950s, and the advancing understanding of the technology of III-V materials, in the 1960s. The superior performance of these cathodes is due to the fact that the vacuum level at the surface lies below the bottom of the conduction band in the bulk of the material. Consequently, the threshold of response is set by the III-V bandgap. The bandgap (and the threshold of response) can be varied by alloying different III-V materials together. A reduction in thermionic emission is realized with these cathodes. At present no semitransparent III-V cathodes with comparably interesting performance are available. The problems hindering further improvements, as well as the problems of placing these cathodes in practical multipliers and image tubes, are discussed briefly.
Mission operations are presented for comet rendezvous and outer planet exploration missions conducted by unmanned Nuclear Electric Propulsion (NEP) system employing in-core thermionic reactors for electric power generation. The selected reference mission are Comet Halley rendezvous and a Jupiter orbiter at 5.9 planet radii, the orbit of the moon Io. Mission operations and options are defined from spacecraft assembly through mission completion. Pre-launch operations and related GSE requirements are identified. Shuttle launch and subsequent injection to earth escape by the Centaur d-1T are discussed, as well as power plant startup and heliocentric mission phases.
Development of a model of mercury vapor electron bombardment thrustor hollow cathode operation. The proposed model predicts that the primary electron emission mechanism is thermionic when sufficient low work function material is present, that the cathode is heated by power extracted from the plasma to a temperature sufficient to emit the current demanded by the external circuit; that for a given total arc current, attempts to reduce cathode temperature by changes in the external thermal coupling will affect only the amount of power extracted from the plasma; and that depletion of low work function material in the cathode results in higher cathode temperature.
Performance data are compared for thermionic diodes with various tungsten or rhenium emitters and niobium or molybdenum collectors. The planar converters have guard-ringed collectors and a fixed space of 10 mils (0.254 mm). The data were acquired using a computer. The parameters are the temperatures of the emitter Te, collector Tc, and cesium reservoir Tr. The composite plots have constant Te and varying Tc or Tr or both. The envelope and composite plots having constant Te are presented. The diodes were tested at increments between 1500 and 2000 K for the emitters, 750 and 1100 K for the collectors, and 520 and 650 K for the reservoirs.
Examination of the feasibility of using the solution as a source of cesium and oxygen, with description of an experimental system designed for a systematic investigation. Preliminary data are presented. Thermionic performance of the converter was recorded before the injection of oxygen at emitter temperatures of 1600, 1700, 1800, and 1900 K. The power of this converter, which had a polycrystalline tungsten emitter and a polycrystalline molybdenum collector, is compared with other converters. Experimental results show that cesium oxide mole factors higher than 0.18 are needed to achieve oxygen effects. It appears not to be necessary to be concerned about avoiding oxygen impurities in the cesium reservoir, since mole fractions as high as 0.18 have failed to influence the performance.
A new reference design was prepared for the 5 kW(e) thermionic power supply. The safety equipment in this design is a passive containment system which does not rely on the operation of any mechanisms such as a launch escape rocket or deployment of parachutes. It includes: (1) a blast shield to protect against the explosion of the launch vehicle; (2) a combination of refractory thermal insulation and heat storage material to protect against a sustained launch pad fire; (3) a reentry body with a spherical nose and a large conical flare at the aft end to stabilize the reentry attitude and lower the terminal velocity in air; (4) composite graphite thermal protection to sustain the reentry heat pulse; (5) crushable honeycomb behind the nose to limit the deceleration of the radioisotope source due to impact on land at terminal velocity; (6) a double-walled secondary containment vessel surrounding the isotopic capsules; (7) neutron shielding to reduce external dose rates; (8) an auxiliary cooling system employing redundant heat pipes to remove the radioactive decay heat from the heat source and reject it to the surroundings or to a forced convection loop.
The problem of extending the useful life of thermocouples intended for in-core and out-of-core thermionic applications in a vacuum environment at temperatures up to 2273 K for periods of time up to 10,000 hours was investigated. Many factors that may influence this useful life were examined, and a basic probe design was developed. With a few modifications, twenty-three thermocouple assemblies were fabricated. Generally the finished thermocouple consisted of solid doped W-3% Re and W-25% Re wires and high purity and high density BeO insulators, and was sheathed in a high purity tantalum tube. In a few probes, stranded thermocouple wires were substituted; commercial grade BeO was used; and in two cases, CVD W-22% Re tubing was used. Each of the components was made of the highest purity materials available; was subjected to special cleaning steps, and was assembled in a class 10,000 clean room. Pertinent physical and chemical properties were determined on each of the components. Special processing techniques were used in the fabrication of the high purity (99.95%), high density (over 95% of theoretical) BeO.
Rhenium thermionic emitters were prepared by the pyrolysis of rhenium chlorides formed by the chlorination of rhenium pellets. The impurity contents, microstructures, degrees of (0001) preferred crystal orientation, and vacuum electron work functions of these emitters were determined as a function of deposition parameters, such as substrate temperature, rhenium pellet temperature and chlorine flow rate. A correlation between vacuum electron work function and degree of (0001) preferred crystal orientation was established. Conditions for depositing porosity-free rhenium emitters of high vacuum electron work functions were defined. Finally, three cylindrical rhenium emitters were prepared under the optimum deposition conditions.
Results of a mission engineering analysis of nuclear-thermionic electric propulsion spacecraft for unmanned interplanetary and geocentric missions are summarized. Critical technologies associated with the development of nuclear electric propulsion (NEP) are assessed. Outer planet and comet rendezvous mission analysis, NEP stage design for geocentric and interplanetary missions, NEP system development cost and unit costs, and technology requirements for NEP stage development are studied. The NEP stage design provides both inherent reliability and high payload mass capability. The NEP stage and payload integration was found to be compatible with the space shuttle.
Results of a mission engineering analysis of nuclear-thermionic electric propulsion spacecraft for unmanned interplanetary and geocentric missions are summarized. Critical technologies associated with the development of nuclear electric propulsion (NEP) are assessed, along with the impact of its availability on future space programs. Outer planet and comet rendezvous mission analysis, NEP stage design for geocentric and interplanetary missions, NEP system development cost and unit costs, and technology requirements for NEP stage development are studied.