Thermionic Emitter & Collector
The emitter's temperature and the emitter-collector work-function difference set the current and voltage a thermionic stage can produce.
The two electrodes
A thermionic converter is defined by its two surfaces. The emitter is held hot and should have a work function high enough to emit copiously at operating temperature yet be matched to the source heat. The collector is kept cooler and should have a low work function so that electrons arriving there give up as much voltage as possible to the circuit. The useful output voltage is essentially the difference between the two work functions minus internal losses.
Richardson emission
The emitted current density follows the Richardson-Dushman law: it rises steeply with emitter temperature and falls exponentially with the emitter work function. This is why thermionic converters demand genuinely hot emitters — the emission current is a strong function of temperature, so a modest temperature increase can raise output substantially, up to the point where materials or space charge intervene.
Material choices
- Emitters: refractory metals and carbides that survive high temperature and particle impact.
- Collectors: low-work-function coatings (e.g. cesiated or oxide surfaces) to maximize output voltage.
- Both must tolerate the burner's radiation environment and stay dimensionally stable across the narrow gap.
Coupling to the source heat
In the burner, the emitter is heated by the very energy the DEC train did not convert directly — intercepted ions, absorbed radiation, and residual flow. The emitter temperature is therefore set by that incident heat flux and by how fast the collector side removes the rest. Getting a useful thermionic contribution means running the emitter hot enough for strong emission while keeping the collector cool, which the next page shows is limited by space charge in the gap.