8-4. Optimization of Lanthanum Hexaboride Electrodes for Maximum Thermionic Power Generation M. L. RAMALINGAM & M. MORGAN Summary Experimental research activities relating to thermionic energy conversion were initiated at the Aero Propulsion Laboratory (APL) of Wright Research and Development Center (WRDC) with the acquisition of several test stations from NASA Lewis Research Centre through Arizona State University. The diode test stations have been provided with new cooling systems, a new data processing/acquisition system and an output load circuit with a biasing power supply. The data acquisition system is capable of handling 100,000 readings/s and has a built-in multiplexer and analog to digital (A/D) converter. It interfaces with a Z-248 PC that controls the operations with the ASYST program. Tests are currently being conducted manually by controlling the electrode and cesium reservoir temperatures between data points using their respective cooling and heating systems. Efforts are currently directed towards developing an automated pulse generating circuit to sweep the diode voltage and generate an ignited mode J- V characteristic in less than 5 ms thereby taking advantage of the converter's thermal inertia. Each performance test consists of driving the diode at fixed electrode temperatures from a positive bias voltage to ignition and further on to a fixed negative bias voltage, then tracing the path back to the original starting positive bias voltage. The three primary variables of interest are the emitter, collector and cesium reservoir temperatures. The response of the diode is such that the output current density peaks at some optimum temperature and has to be optimized with reference to each one of the primary variable temperatures. Optimization tests were conducted in two stages. In the first stage the emitter and collector temperatures were maintained constant at 1700 K and 900 K respectively and the cesium reservoir temperature was varied from 350 to 550 K in suitable intervals. The performance peaked at a cesium reservoir temperature of 485(±10) K. Similar optimization tests on the collector revealed an optimum collector temperature of 885(±10) K. The optimum emitter temperature was limited to 1700 K due to bonding strength problems. A second stage of optimization tests was carried out to reduce uncertainties and predict optimum temperatures more accurately but this did not reveal any useful information as the spread in the experimental data was of the same order of magnitude as the difference in short circuit densities at various temperatures. M. L. Ramalingam, Research Scientist, Universal Energy Systems, Inc., 4401 Dayton-Xenia Road, Dayton, OH 45432, USA and M. Morgan, Engineer, Aero Propulsion Laboratory, AFWAL/POOS-3, Wright- Patterson AFB, OH 45433, USA. Paper number IAF-ICOSP89-8-4.
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