  Most attention to improving vacuum thermionic energy conversion device (TEC)
technology has been on improving electron emission with little attention to
collector optimization. A model was developed to characterize the output
characteristics of a TEC where the collector features negative electron
affinity (NEA). According to the model, there are certain conditions for which
the space charge limitation can be reduced or eliminated. The model is applied
to devices comprised of materials reported in the literature, and predictions
of output power and efficiency are made, targeting the sub-1000K hot-side
regime. By slightly lowering the collector barrier height, an output power of
around $1kW$, at $\geq 20%$ efficiency for a reasonably sized device ($\sim
0.1m^{2}$ emission area) can be achieved.
