Testing of a close spaced thermionic converter

A close-spaced thermionic converter was recently tested at the New Mexico Engineering Research Institute, USA. The converter operates in the Knudsen mode with an interelectrode gap of 5 μm at operating conditions (1400 K emitter temperature and 800 K collector temperature). Direct measurements of such a small gap in an operating thermionic converter have not been reported previously. The gap was measured by directing a laser beam through the interelectrode gap, which produced a diffraction pattern whose width is a function of the gap width. The interelectrode gap was measured for emitter temperatures ranging from 500 K to 1400 K. The measured value at 500 K was 250 μm, and the interelectrode gap decreased with increasing emitter temperature up to 5 μm at 1400 K. In this test of the prototype close-spaced converter, the emitter and collector became electrically shorted whenever the gap was less than 10 μm. This short made it impossible to obtain I-V performance data from the converter. Post test disassembly revealed that the short was due to a foreign particle embedded in the collector surface. A new converter is under construction, and will be tested in the near future