Work function reduction in lanthanum hexaboride hollow cathodes operated in gas discharges

Thermal characterization of lanthanum hexaboride (LaB6) hollow cathodes
has revealed lower than expected electron emitter temperatures when the cathode reaches
steady state. This phenomenon is observed at discharge currents ranging from 5 to 35 A
and xenon mass
ow rates of 5 to 25 sccm in cathodes with three di erent ori ce diameters.
Thus, the accepted value of the work function for polycrystalline LaB6, 2.66 eV, does not
describe well the emission characteristics of LaB6 hollow cathodes operating with internal
gas discharges at steady state. The measured temperatures and a model of the hollow
cathode emitter and xenon discharge were used to estimate the value of the work function
in these experiments, yielding a value ranging from 2.1 - 2.44 eV. Measurements of the
work function as a function of depth on a hollow cathode emitter using X-ray photoelectron
spectroscopy and ion beam milling indicate that the work function in a thin layer near the
emitting surface is lower than in the bulk material. We postulate that a lanthanum-rich
emitting surface with a lower work function develops as a result of lanthanum recycling in
the internal plasma.