High temperature energy conversion for the Integrated Solar Upper Stage (ISUS)

Preliminary studies were conducted to assess the benefits of the Integrated Solar Upper Stage (ISUS) concept and key components including high temperature thermionic converters, have been tested and evaluated. Advanced radiatively coupled heat pipe cooled thermionic converters with rhenium and tungsten emitters were characterized individually for integration in a modular power unit. The converter with the tungsten emitter was performance mapped in the temperature range of 1750 K to 2400 K in order to conform to the ISUS requirements. Higher off-design temperatures yielded power densities as high as 12 watts/sq. cm. in the cesium pressure range of 4 to 9 torr. The converter with the rhenium emitter was tested in the temperature range of 1575 K to 1950 K and produced 10.5 watts/sq. cm. at the highest temperature. Dynamic switching characteristics were also measured to evaluate the possibility of interfacing a pulse width modulated (PWM) power regulator directly to a thermionic source.