Investigation of novel cathode surface coatings for microsecond pulse conditions

The University of Missouri - Columbia (UMC) is investigating cathode surface coatings applied to microfibrous carbon emitters in an effort to increase low field electron emission. Low field, high electron density cathodes are highly desirable components of nonrelativistic high power microwave sources, for they require no external energy source to drive thermionic emission. The objective of the research is to improve upon the already impressive performance of cesium iodide doped carbon fibers by decreasing the work function of the emitters utilizing specialized subcoating technologies. Cathode tests were conducted on a modulator, which generated an electron beam. The modulator, designed and built at UMC, uses a modified type E pulse-forming network (PFN) switched by a hydrogen thyratron to provide the electrical energy. The electron gun was designed to deliver up to 10 A. The PFN can be configured to deliver pulse lengths from 2.5 to 5 μs at voltages from 50 to 120 kV. An electron gun was designed around a carbon fiber emitter element that was 3 cm in diameter. The emission fibers were grown onto graphite bases by ESLI, and the fibers were several millimeters long and 8 μm in diameter. Several emitters were prepared, one having no treatment, one having a conventional cesium-iodide coating, and one having novel coatings deposited on the carbon prior to cesium-iodide treatment. The results of the emission research are reported.