Donald A. Shiffler

Comparison of velvet- and cesium iodide-coated carbon fiber cathodes

Presents results of an experimental comparison of a velvet cathode- and a carbon fiber cathode-coated with cesium iodide (CsI) salt. Each cathode had a planar geometry, with similar emission areas. The cathodes were tested at electric field strengths of 50 kV/cm at anode-cathode (A-K) gaps of 4.0 cm. The applied voltage had a 1-/spl mu/s duration and the pulser was operated at up to a 1-Hz repetition rate. The system had a low base pressure (<1.0/spl times/10/sup -7/ torr). This paper reports the results and comparisons of experiments on each cathode. We address the current and voltage characteristics, the shot-to-shot reproducibility, the pressure evolution of the diode under 1-Hz operation, and the lifetime of the cathodes.

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...

Low level plasma formation in a carbon velvet cesium iodide coated cathode

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 sp...

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 shot-to-shot variation in cold field emission cathodes

High-power microwave tubes require currents and voltages generally in excess of 1 kA and 100 kV. In the past, these system requirements led to the use of single shot machines, with repetition rates well under 1 Hz. With advances in pulsed power, the Air Force Research Laboratory recently began to investigate the performance of field emission diodes at repetition rate operation. Greater numbers of shots allowed better accuracy in measurements and the application of better statistics to experimental data. In this paper, we report on new measurements that, with better experimental accuracy, show the statistical correlation between emission uniformity and the shot-to-shot variation in diode current. We report on a comparison to particle-in-cell simulations. These comparisons show the importance of randomly occurring nonemission regions on the cathode surface in dictating the spread in current data. These results imply that uniformity, in addition to playing an important role in any electron interaction with radiation, also affects the current stability for any device using these cathodes. Finally, these experiments show that for repetition rate machines, shot-to-shot variation quantified in terms of Gaussian distributions characterized by a standard deviation and skewness, provide a diagnostic capable of inferring beam uniformity in situations where direct uniformity diagnostics prove extremely difficult or impractical.

Klein tunnelling model of low energy electron field emission from single-layer graphene sheet

By considering the effect of Klein tunneling for low energy electrons with linear energy dispersion, a model has been constructed to calculate the amount of emitted line current density from a single-layer graphene sheet, which is vertically aligned inside a dc gap. It is found that the current-voltage scaling obtained from the constructed Klein tunneling model is very different from the traditional field emission model based on the Fowler-Nordheim (FN) law. Under the same geometrical field enhancement factor, our model predicts a much higher emitted current as compared to the FN law at low voltages.

A high-current, large-area, carbon nanotube cathode

Carbon nanotubes (CNTs) have attracted great attention as electron emitters. These field emission cathodes operate at room temperature without a heater, in contrast to thermionic emitters that often require considerable heater power for proper operation. Furthermore, CNTs have the advantage of large aspect ratios, allowing large local electric field enhancements at the nanotube tips, further increasing their attractiveness as field emitters. However, due primarily to materials issues such as limited emission site density, screening effects, and weak interfacial bonding, no large area cathodes have been operated at high current to date. In this paper, we report on the design, fabrication, and testing of a large-area CNT cathode operating at high-voltage (>200 kV) and high-current density (30 A/cm/sup 2/). The cathode lifetime exceeded four thousand pulses. The current density and voltage achieved represent a significant achievement and indicate that such a cathode design can prove valuable as a high-current, large-area emitter.