Review of Cold Cathode Research at the Air Force Research Laboratory

Over the last decade, the Air Force Research Laboratory, Directed Energy Directorate (AFRL/DE) has engaged in a high current density field emission cathode research program. This program explored the aspects of cathode materials as well as the details of cathode geometries and emission physics. This paper summarizes the results of this ongoing research effort to date. We review the history and motivation for the program, which provide insight into the physics issues of concern for various vacuum electronic sources. One important aspect of the program consists of the investigation of new cathode materials. For many high power microwave (HPM) sources, neutral out-gassing, which ties critically with cathode materials, plays a key role in the effective operation of the source. These material properties influence plasma formation, which in turn dictates the operation of an HPM device. For a cathode material, AFRL chose to focus on cesium-iodide-coated carbon fiber cathodes, which we discuss in detail here. A second important aspect of the program consists of understanding emission physics and the optimum geometries for a cathode. This aspect couples closely with electron beam quality, which in turns effects the electron beam interaction with microwaves in the HPM structure. This paper concludes with a discussion of the implementation of the cathode material on both a Magnetically Insulated transmission Line Oscillator and a relativistic magnetron.