The Ya-21u unit of the Soviet-made TOPAZ-II power system has recently
been tested at the Thermionic Evaluation Facility in Albuquerque, New
Mexico. A change in the unit performance was measured during these res
ts. In an attempt to identify the causes of this change performance, d
ata were examined and used to estimate surface properties of electrode
s of thermionic fuel elements (TFEs) of the power system. The effectiv
e emissivity was estimated at similar to 0.03 to 0.035 higher than for
as-fabricated TFE and cesiated work functions of the electrodes, whic
h were higher than for as-fabricated TFEs. These changes in the effect
ive emissivity and cesiated work functions, caused by gaseous impuriti
es and air incursion in the TFEs interelectrode gap, lowered both the
emitter temperature and the output load voltage thus contributing to t
he measured decrease in output power.