Introduction Thermionic Energy Conversion (TEC) was part of the space nuclear research activities being investigated at NASA Lewis Research Center (LeRC) in the early 1970s [1,2]. Before the termination of these activities in 1973, Lanthanum Hexaboride (LaB6) was one of the better materials being studied as a part of the diode-screening project [3]. The TEC activities were then continued at LeRC by NASA's applied research and technology (ART) program [4]. The TEC-ART activities were terminated due to programmatic retrenchment and transferred to the Jet Propulsion Lab (JPL) where research on metal hexaborides has since received less attention [5,6]. A twin diminiode station, a TECO diode station and diode processing station were acquired by the Aero Propulsion Laboratory of AFWAL/WPAFB in 1987. These stations are now being rejuvenated and upgraded with improved instrumentation and testing facilities. The planar diminiode tested in the twin station had an emitter made from sintered LaB6 and a collector made from arc melted LaB6. The construction and configuration are explained in detail in earlier documents [7-11]. The details of instrumentation and the data acquisition and processing system used were different and this paper provides a description of these aspects. Before actually conducting the optimization tests, an initial sequence of tests was conducted to verify the performance of the diminiode. In this sequence of tests the cesium reservoir temperature was maintained at 460 K and the collector temperature was maintained at 900 K while the emitter temperature was changed from 1500 to 1700 K at random intervals. After confirming the normal operation of the diminiode as well as the testing facility, a test plan was generated and executed for the first and second stage optimization tests. Nomenclature AC Alternating Current J-V Output Current A/D Analog to Digital Density—Voltage AFWAL Air Force Wright LaB6 Lanthanum Hexaboride Aeronautical Laboratories LeRC Lewis Research Center APL Aero Propulsion Laboratory NASA National Aeronautics and ART Applied Research and Space Administration Technology P Peak Power Density ASU Arizona State University PC Personal Computer ATI Advanced Thermionic TE Emitter Temperature Initiative TP Pyrometer Temperature DC Direct Current Tt Target Temperature EB Electron Bombardment TEC Thermionic Energy HIPP Hot Isostatic Pressing Conversion Process WPAFB Wright-Patterson Air Force HP Hewlett Packard Base 1ST Innovative Science and WRDC Wright Research and Technology Development Center JPL Jet Propulsion Laboratory Z-248 Zenith 248
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