Thermionic Emission Characterization of M-Type Cathodes Using Kelvin Probe in an Ultra-High Vacuum Environment

Thermionic emission characterization is vital for understanding the performance and lifetime of the many vacuum electron devices (VEDs) that make use of cathodes. To characterize the performance of thermionically emitting cathode surfaces with efficiency in one test setup, the Cathode Characterization Chamber (C3) has been assembled at the University of Kentucky. The C3 principally monitors the work function change and the current density emitted by thermionic surfaces using a Kelvin probe in an ultra-high vacuum (UHV) chamber (approaching
$10^{-{10}}$
torr). The chamber also employs ion polishing, optical pyrometry, and residual gas analysis to produce a single comprehensive cathode characterization apparatus. One widely used cathode in VEDs is the M-type cathode, developed in the mid-1960s. M-type cathodes are useful because of their relatively high current densities of emission (< 10 A/cm2) at only moderately high activation temperatures (
$< 1100 ^{\circ }\text{C}$
) for tens of thousands of hours. Using the M-type cathode as a “standard candle,” the capabilities and limits of the C3 are highlighted here and the results are compared to those from pure tungsten samples—all of which are presented as a powerful and comprehensive tool for thermionic emission characterization.