How much power can a vacuum thermionic converter produce per square centimetre of emitter surface?

In a vacuum thermionic power converter, the electric power produced is typically 0.1 to 1 watt per square centimetre of emitter surface at a temperature of 1,100 K (approximately 800 °C or 1,500 °F).1

This output is limited by the buildup of space charge between the electrodes, which occurs as emitted electrons repel one another and disrupt the current flow. To mitigate this effect, vacuum converters utilize a very small gap between the emitter and collector, typically ranging from 0.025 to 0.038 mm (0.001 to 0.0015 inch). Due to the manufacturing difficulties associated with such small spacings, vacuum converters have seen only limited practical application.1