Close-space, high temperature Knudsen flow

The discharge processes and performance characteristics of thermionic energy conversion systems operating in the collisionless (Knudsen) mode were investigated. The interelectrode space charge can be reduced by close interelectrode spacings, contact ionization of cesium at the emitter surface or introduction of ions produced in an externally generated arc. It is suggested that structured emitters can increase the current and power densities in the unignited mode by a factor of four. Small diode structures, SAVTECs, with spacing set by thermal expansion, to measure converter performance at interelectrode spacings below 20 micron were used. Measurements covered both the vacuum and unignited regimes. Two variable-spacing planar converters containing mixed cesium and barium vapor are planned to investigate the structured emitter effect. Theoretical studies and computer models of the collisionless thermionic converter and its interaction with an external arc supplement the experimental work.