Emitter-sheath effects on thermionic-converter performance

Emitter-sheath phenomena are important in thermionic energy converters because the emitter sheath forms the boundary conditions for the plasma in the gap and controls both the ion loss rate and the loss rate of hot (3000 K) plasma electrons to the emitter. This thesis examines three expected emitter-sheath phenomena and their effects on converter performance: (1) reflection of ions coming from the plasma; (2) ions trapped in the double emitter sheath; and (3) surface-emission ions. Inclusion of these three phenomena combined with the elimination of previous sheath approximations requires careful analysis and calculation of the sheath structure. It is shown that the Bohm matching condition must be generalized to insure that self-consistency prevails throughout the entire sheath and not just at the plasma/sheath interface. Further, it is shown that plasma ion distribution coming into that sheath must have its low-energy ions cut off to produce a self-consistent collisionless sheath, and that each of these emitter-sheath phenomena reduce the normalized (by plasma density) net ion loss rate to the emitter. Each of these phenomena also raise the normalized plasma density adjacent to the emitter. The higher plasma density at the emitter causes a greater increase in the loss of hot plasma electron energy to the emitter than the corresponding decrease in the loss of ionization energy (carried by the ions) to the emitter