Correlation of fixed-spacing thermionic converter performance with variable-spacing test vehicle data

Seven fixed-spacing thermionic converters with planar electrodes and one with cylindrical electrodes were designed and built on the basis of data from a planar variable-spacing thermionic test vehicle operated at emitter temperatures of 1600-2000 K in 100 degree increments. Electrodes in the variable-spacing device and in five of the planar converters were of polycrystalline rhenium; those in the other two planar converters, and in the cylindrical converter were of vapor-deposited rhenium. The design performance of all fixed-spacing planar converters, compared with the test vehicle data, fell within 6.5 percent of predicted values; performance of the cylindrical converter was within 1.1 percent of the predicted value. The performance of the cylindrical converter was also compared with that of a corresponding planar converter and was found to be in excellent agreement. The converter designed for a 2000 K emitter temperature had completed 11 600 hours of life testing as of October 15, 1958, at a constant power output of approximately 27 W/cm²at 0.77 volt. It was also observed that the design points for the five planar converters with polycrystalline rhenium electrodes fell on a straight line when optimum power output was plotted as a function of emitter temperature.