Frontiers in Thermionic Cathode Research

The high electron emission resulting from low-work-function scandate-added dispenser cathodes has generated a great deal of interest and research since the late 1970s. Despite the reported high current density at low temperatures, scandate cathodes have not yet seen wide-scale industrial adoption due to poor emission uniformity, inadequate reproducibility, and short lifetimes. This lack of industrial adoption stemming from the above issues is likely due to insufficient fundamental understanding of the role that scandium plays in the emission process. Recent work by five research teams under the U.S. Defense Advanced Research Projects Agency Innovative Vacuum Electronics Science and Technology Program aims at advancing this fundamental understanding and the cathode manufacturing processing. Emission microscopy of model surfaces of adsorbed Ba-Sc-O on single crystal tungsten and detailed characterization techniques, such as Wulff shape analysis, are being used to understand the morphology, composition and phase of each of the species that comprise the cathode emitting surfaces. The structure of tungsten grains at the cathode surface of activated and unactivated scandate cathodes and the resulting emission performance are observed. 3-D printing is being investigated as an advanced manufacturing method to meet the demanding requirements of modern vacuum electron devices. Finally, density functional theory is being employed to study the work function of perovskite oxides as a novel class of alternative materials to tungsten dispenser cathodes.