R. Umstattd

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.

Carbon velvet field-emission cathode

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 com...

Emission uniformity and emission area of explosive field emission cathodes

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 emissio...

Cathode and anode plasmas in short-pulse explosive field emission cathodes

Explosive field emission cathodes have been a subject of research for a number of years. These cathodes offer high current densities and total current without requiring a heater for the production of electrons. Generally these cathodes consist of some structure with a series of tips or metal–dielectric regions in which a large electric field enhancement can occur. A cathode plasma is then formed from these discharge points that then supplies the electrons necessary for space charge limited emission. This article reports on a series of optical measurements in which the cathode and anode plasmas of explosive field emission cathodes are observed. Three types of cathodes are investigated. These types are a polymer velvet cathode, a metal–dielectric cathode, and a tufted carbon fiber cathode in which the fibers have been coated with a cesium iodide salt. Cesium iodide coated carbon fiber cathodes have shown a great deal of promise for various field emitter applications. From these high speed photos, the evolut...

Improved electrostatic design for MILO cathodes

Recent experimental and computer simulation results on a magnetically insulated transmission line oscillator (MILO) have indicated that the large beam-current density emitted from each end of the cathode leads to anode plasma formation. This initiates bipolar space-charge flow in the anode-cathode gap that severely perturbs the electron flow at the launch point. The result is significant microwave power reduction on a 600-ns time scale. The field-shaper cathode, used previously to extend the MILO RF pulse duration beyond 400 ns, is shown to have several deficiencies concerning anode plasma formation. We report on implementation of miniature Pierce focusing electrodes on each end of the MILO cathode as a way to control the beam current density, and hence, to minimize anode plasma.

Effects of anode materials on the performance of explosive field emission diodes

Explosive field-emission cathodes have been the electron emitter of choice, and often necessity, for high-power microwave (HPM) tubes for many years. The materials typically used for these cathodes range from polymer and cotton velvets, to metals such as stainless steel, and to carbon materials such as bulk carbon and carbon fibers. With several notable exceptions, the issues of the anode and its composition have been largely ignored. Generally, the diode performance, such as current levels, impedance collapse, and out-gassing, have been attributed to the cathode alone rather than to the combination of the cathode and anode. In this paper, we investigate the affects of various anode materials on the performance of explosive field emission cathodes. We show that bipolar flow significantly and rapidly alter diode performance at lower voltage and energy densities than usually observed. We show also the effects of anode material choice on out-gassing, and diode conditioning. Experiments have shown that bipolar flow is a significant issue in diode performance for even short pulses. The theoretical aspects of the diodes are discussed, with a comparison of experiment to theory.

Emission uniformity and emittance of explosive field-emission cathodes

Explosive field-emission cathodes as well as plasma-flashover cathodes are important for high-power microwave tubes. These cathodes have the advantage of being lightweight as well as requiring no heater for electron emission. However, this class typically suffers from large amounts of outgassing, nonuniform emission, and very high emittance. In this paper, we review research into the uniformity and emittance of tufted carbon-fiber cathodes that have been coated with cesium iodide (CsI) salt. The CsI cathode is compared to polymer velvet, metal-dielectric, and carbon-slat cathodes. We find the uniformity and emittance are related for all of these cathodes. In general, the more uniform the electron emission, the lower the emittance of the cathode. This article shows that given proper diode design, the tufted carbon-fiber cathode with CsI has some promise as an electron emitter producing a good quality electron beam.

Design and implementation of a new UHV threshold cathode test facility

In support of cathode development at the Air Force Research Laboratory, a new ultra-high vacuum cathode test facility is being assembled to complement the existing repetition-rate test pulser. The existing test bed is a 500 kV, 100 Ohm, 1 microsecond(s) duration pulser capable of firing at up to 1 Hz. The new facility is designed to operate at lower voltages (20 - 200 kv), lower impedance (50 - 75 Ohm), and variable pulse lengths (200 - 800 ns) in a single-shot mode. This Threshold Cathode Test Facility (TCTF) will be used to generate data regarding emission turn-on field strengths, outgassing volumes and constituents, vacuum level effects, and anode effects for a variety of field-emitting and explosive- emitting cathode materials. Presented herein are the design parameters of TCTF including diagnostic capabilities and electrostatic simulations of the diode region both with and without beam current.

Cathode testing at the Air Force Research Laboratory

An integral part of any vacuum rf device is the cathode. Many rf and microwave tubes utilize thermionic cathodes. However, these cathodes are generally limited to current densities less than 100 A/cm2, a limitation too great for the majority of High Power Microwave tubes. At the Air Force Research Laboratory, Directed Energy Directorate, we have to study a variety of explosive emission cathodes. This paper presents results on studies of several types of cathodes tested in a simple circular geometry. We also present results of research on different types of anode material. The data includes measurements of current, voltage, cathode lifetime, and cathode/anode out-gassing.15

Cesium iodide cathodes for high-power microwave devices

A variety of cesium iodide-coated carbon fiber cathodes have been designed and tested for high voltage (300 - 500 kV), high current operation. Here, we describe the conditioning process and present the most recent experimental data for a thin annular beam test diode. During these reproducible microsecond-duration pulses, this diode exhibits promising beam impedance maintainability and gap closure speeds of less than 0.4 cm/microsecond(s) ec. Also presented are recent results for a carbon fiber cathode tested in a relativistic klystron oscillator and future plans for such a cathode in the magnetically insulated line oscillator.