a high - output energy converter of an output - improving thermionic generator , thermally connected to other generators without moving parts that utilise the residual energy from the thermionic generator . the thermionic generator comprises a rαultilayered vacuum diode , the layers of which are very thin and the gaps between the layers are also thin and kept at a distance from one another by selectively flexible spacer elements . piezo elements or heating elements can precisely adjust the height of the gaps .