Why are vacuum thermionic converters difficult to manufacture?

Vacuum thermionic converters are difficult to manufacture primarily due to the extremely small spacing required between the emitter and collector electrodes. To minimize the disruptive effects of the space-charge effect—where emitted electrons repel one another and impede current flow—these devices must maintain a gap typically between 0.025 and 0.038 mm (0.001 to 0.0015 inch). Achieving and maintaining such precise, microscopic tolerances in a functional device is a significant engineering challenge, which has limited the practical application of vacuum-type converters.1