Explosive field emission cathodes comprise an important class of cathodes for high power microwave tubes, having the advantages of light weight as well as requiring no heater for electron emission. Generally, however, this class of cathodes suffers from large amounts of outgassing, nonuniform emission, and very high emittance. This article describes a new class of carbon velvet cathodes that have been coated with a cesium iodide (CsI) salt. We discuss two manifestations of the cathode. We review the lifetime and operation of the cathodes with two different pulse durations, as well as the outgassing from the cathodes during operation. Lifetimes in excess of 980 000 pulses have been obtained, with an outgassing rate of 3.5 atoms per electron. Finally, we discuss the uniformity and emittance of tufted carbon cathodes that have been coated with CsI salt. For comparison, we relate these results to those previously obtained from other cathodes in this class. The cathodes have an emittance of 2.5π mm rad, as compared to the theoretical value, based on computation, of 2.3π mm rad. These new cathodes differ greatly from cathodes such as polymer velvet and tufted carbon fiber cathodes in that no volatiles reside on the cathode and in that a unique coating technique allows the cathodes to function. This new class of cathodes offers a potential replacement for existing thermal cathodes, in that no heater is required for superior operation with low outgassing and long lifetime.

Field emission cathodes have been a subject of research for many years. These cathodes hold the promise of effective electron emission in the absence of a heater. Such devices find application in the high power microwave (HPM) arena, as well as the conventional microwave industry and other areas such as flat panel displays. Over the past several years the Air Force Research Laboratory began to focus on cesium iodide cathodes as a field emission cathode of some interest. Previously reported results demonstrated a cesium iodide coated carbon velvet cathode capable of over one million pulses of operation with no degradation of emission. However, the exact emission mechanism remains somewhat unclear. This paper presents results showing that plasma formation on the cathode surface remains minimal at 1 μs pulse lengths. While ionized cesium and iodine lines exist in the light spectrum from the diode, these lines remain quite small, with the fluorescent emission from solid cesium iodide dominating the optical spectra in the diode. Hence, we propose that the cesium iodide coated carbon velvet operates in a space charge limited regime with pure field emission alone.

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Explosive field emission cathodes have been used extensively in high power microwave tubes. These cathodes emit electrons without the use of cathode heaters. Recently, some theoretical and simulation work has been performed to gain further understanding of the physics of these cathodes. The purpose of this letter is to provide the experimental background and justification for the theoretical work. The general idea of how explosive field emission cathodes operate is that plasma is rapidly formed, which provides the sea of electrons for space charge limited flow. However, recent theoretical and experimental work suggests edge effects, rather than plasma formation across the entire emission area, can also provide the same effect. In this letter we review three types of cathodes which have been tested. We provide optical data on the cathode emission uniformity as well as the electrical data for the same devices. In particular, we find that a large percentage of the cathode can fail to take part in the emission process and yet the voltage and current can appear identical from the case in which the entire cathode contributes electrons to the emission process.