Reducing the Space Charge
The emitted or boiled-off electrons between the converter plates ([Figure 8]) form a cloud of negative charges that will repel subsequently emitted electrons back to the emitter plate unless counteraction is taken. To circumvent these space charge effects, we fill the space between the plates with a gas containing positively charged particles. These mix with the electrons and neutralize their charge. The mixture of positively and negatively charged particles is called a plasma.
The presence of the plasma makes the gas a good conductor. The emitted electrons can now move easily across it to the collector where, to continue the gas analogy, they condense on the cooler surface.
Figure 8 THERMIONIC CONVERSION
Thermionic converters may be flat-plate types or cylindrical types. The cylindrical converter (a) is an experimental type for ultimate use in nuclear reactors. Courtesy Los Alamos Scientific Laboratory.
a INSULATOR COOLED COLLECTOR INCANDESCENT URANIUM FUEL ELEMENT CESIUM PLASMA CIRCULATING COOLANT VACUUM INSULATOR CESIUM POOL b WASTE HEAT OUT LOAD ELECTRONS LOW WORK FUNCTION COLLECTOR Tc CESIUM ION PLASMA FILLED GAP BOILED OFF ELECTRONS HIGH WORK FUNCTION EMITTER Tc HEAT IN