Ken Golby

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.

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...

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...

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.

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.

Experimental study of electric field screening by the proximity of two carbon fiber cathodes

This paper describes the first experiments that use only two carbon fiber field emitters with different separations to quantify and isolate the effect of electric field screening. Experiments show that when the separation between the two carbon fiber cathodes decreases, both the effective field enhancement factor, βeff, and the current emission decreases. For a two-emitter geometry, our experiment suggests a height of approximately 1.5 times the separation between the two cathodes as the optimum ratio to optimize the emitted current. The paper shows the analysis of the turn on voltage of the field emitters for different separations. The authors compare experimental data with Fowler–Nordheim field emission theory and particle-in-cell simulation, showing good agreement between experiment, theory, and modeling.

Materials characteristics and surface morphology of a cesium iodide coated carbon velvet cathode

Cesium iodide (CsI) coated carbon fiber cathodes have shown promise as a cold cathode for microwave and x-ray devices. In particular, the cathodes have demonstrated over 1 000 000 shots lifetime at operating voltages at or in excess of 165 kV and current densities greater than 25A/cm2. While the vacuum emission characteristics have been well studied, the materials characteristics of the cathodes themselves, particularly after operation, have received little attention. Furthermore, while researchers at University of Wisconsin have demonstrated a reduction in work function of carbon due to the CsI coating, the details of the emission mechanism remain poorly understood. This article gives results of a series of materials diagnostics investigating the cathode surface morphology as well as the changes in the carbon fiber structure with cathode shot history. We demonstrate that the cathode surface undergoes several changes in relation to the bond line along the fiber-substrate interface as well as at the fiber ...

Demonstration of an Acid-Spun Single-Walled Nanotube Fiber Cathode

Field emission dc cold cathodes continue as an important area of research for uses such as electron microscopy, novel X-ray sources, vacuum electronic devices, terahertz sources, and high-power microwave tubes. Each of these applications typically requires high current densities with high-brightness electron beams driven by cathodes exhibiting long lifetime in the presence of deleterious conditions such as ion back bombardment and excessive heating. The Air Force Research Laboratory (AFRL) now investigates cathodes operating in dc mode for use in a terahertz traveling wave tube (TWT). The TWT requires an electron beam of 50 μm in diameter or less, at 10s of kiloelectronvolt energy with energy spreads of less than 10 eV. While AFRL has tested numerous cathodes in this regime, this paper reports on the first demonstration of a dc cathode utilizing a highly aligned carbon nanotube (CNT) rope for the electron emitter. The rope consists of individual single-walled CNTs that have been subjected to a nitrogen-enhanced acid etch and then spun into a rope configuration. Thus, the single rope emitter has an overall diameter of 100 m and a length of 1.5 mm. We report on preliminary results from this cathode, in particular the fabrication of the cathode, the dc cathode test system, and the cathode operation up to a voltage of 5 kV. The cathode operates stably to within 0.6% with a 5-mm anode-cathode gap at 5 keV and 1.0-mA current for hundreds of hours. Finally, we provide estimates of the cathode parameters such as the effective field enhancement factor (βeff) and emitting area (A) through a Fowler-Nordheim plot and comparison of the experimental data with simulations utilizing the particle-in-cell code Improved Concurrent Electromagnetic Particle-in-Cell.

Surface chemical analysis and ab initio investigations of CsI coated C fiber cathodes for high power microwave sources

CsI coated C fiber cathodes are promising electron emitters utilized in field emission applications. Ab initio calculations, in conjunction with experimental investigations on CsI-spray coated C fiber cathodes, were performed in order to better understand the origin of the low turn-on E-field obtained, as compared to uncoated C fibers. One possible mechanism for lowering the turn-on E-field is surface dipole layers reducing the work function. Ab initio modeling revealed that surface monolayers of Cs, CsI, Cs2O, and CsO are all capable of producing low work function C fiber cathodes (1 eV<Φ<1.5 eV), yielding a reduction in the turn-on E-field by as much as ten times, when compared to the bare fiber. Although a CsI-containing aqueous solution is spray deposited on the C fiber surface, energy dispersive x-ray spectroscopy and scanning auger microscopy measurements show coabsorption of Cs and I into the fiber interior and Cs and O on the fiber surface, with no surface I. It is therefore proposed that a cesium...