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Close-Spaced High Temperature Knudsen Flow.

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Final technical rept. 1 Feb 83-15 May 86,

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This work studied discharge processes in Knudsen mode collisionless, thermionic energy converters. Mechanisms for reducing the effects of electron space charge in such devices are essential for thermionic converters to produce useful current and power densities. The mechanisms studied are reduction of space-charge through a very close interelectrode gap less than 10 microns transport and retention of positive cesium ions generated by surface ionization transport of positive cesium ions generated in an arc external to the electrodes and the mechanism for enhanced current output due to a structured emitter in a mixed barium-cesium vapor. The experimental work used SAVTEC Self-Adjusting, Versatile Thermionic Energy Converter diode structures which were tested in a chamber containing 0.1 - 1.0 torr of cesium vapor. Comparison of measured volt-ampere curves with theory gave excellent agreement and indicated an interelectrode gap of 6.5 microns at an emitter temperature of 1250 K. A theoretical model of the collisionless thermionic diode was developed which included surface ionization, auxiliary ions from an external source, and trapping of charged particles in potential wells due to infrequent collisions. Studies showed that trapping of positive ions leads to a large, beneficial increase in current density. Using a diffusion analysis to couple this model to models of collision-dominated discharges gave predictions of the performance of SAVTEC devices in the presence of an auxiliary discharge, and led to design criteria for electrode size in order for auxiliary ionization to be effective.

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  • Electric Power Production and Distribution

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