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.
[Illustration: 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
T_c
CESIUM ION
PLASMA FILLED GAP
BOILED OFF ELECTRONS
HIGH WORK FUNCTION EMITTER
T_c
HEAT IN