Don Shiffler

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

Presents results of an experimental comparison of a bare carbon fiber cathode and the same cathode when coated with cesium iodide salt (CsI). An annular cathode was constructed by arranging carbon fibers in an annular tuft pattern. The cathode was then operated as a bare carbon fiber cathode and in a configuration with a CsI coating. The cathode was tested at electric field strengths ranging from 50 kV/cm to 265 kV/cm at anode-cathode (A-K) gaps of 3.175 cm. The applied voltage had a 1-/spl mu/s duration and the modulator was operated at up to 1 Hz repetition rate. The system had a low base pressure (<1.0/spl times/10/sup -7/ torr). The article reports on results concerning the conditioning of the cathodes, the shot-to-shot reproducibility of the cathodes and the pressure evolution of the diode under 1 Hz operation. We also report on the impedance evolution of each of the diodes.

Significant pulse-lengthening in a multigigawatt magnetically insulated transmission line oscillator

The Air Force Research Laboratory/Phillips Laboratory magnetically insulated transmission line oscillator (MILO) is a gigawatt-class, L-band, high-power microwave tube driven by a 500-kV, 60-kA electron beam. A previous version of this tube generated 1.5 GW pulses, but with significant RF pulse shortening, The paper reports on improvements to the tube that have allowed us to increase the output power by 25% and to increase the RF pulse duration by a factor of two and a half.

A Cerenkov-like Maser Based on a Metamaterial Structure

Microwave and radio frequency (RF) sources have been a subject of intense research for nearly 100 years, with the advent of High-Power Microwave (HPM) sources based upon intense relativistic electron beams emerging in the 1970s. These sources act to transform kinetic energy in an electron beam into radiation through the interaction of the electrons with some form of external circuit. One form of HPM device, the Cerenkov maser, consists of a hollow cylinder loaded with a dielectric material. The dielectric forms a slow-wave structure with which the relativistic beam can interact. This paper considers a new design approach and interaction mechanism for a Cerenkov maser. For the first time, a dielectric Cerenkov maser formed using microwave metamaterials in the form of a concentric ring structure was considered in the study. The authors proposed that metamaterials provide a practical new opportunity for vacuum electron devices, opening the means for new sources operating in the RF and microwave to THz regimes.

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 ...

Investigation of RF breakdowns on the MILO

Describes a series of experiments performed to isolate the RF breakdown mechanisms in the hard tube magnetically insulated transmission line oscillator (MILO) Experiment at the Air Force Phillips Laboratory, Albuquerque, NM. Specifically, several causes of RF breakdown in the region of the vacuum-air interface and the antenna region have been investigated. These causes are X-ray induced electron emission, VUV and visible photoemission of electrons, and breakdown due to large field stresses in the antenna. Each of these mechanisms has the effect of liberating electrons from a surface in a high field region which then are a seed for a breakdown. This paper discusses measurements in the X-ray, VUV, and visible regimes with support from computer simulation. Also, imagery results are shown, which in conjunction with the computer work, point to the presence of high electric field stresses in the antenna, which cause a subsequent breakdown. In particular, X-rays, VUV, visible light, and plasmas do not seem to be the major source of RF breakdown in this tube.

Analysis of electric field screening by the proximity of two knife-edge field emitters

The electric field of two semi-infinitely wide knife-edge cathodes with arbitrary separation is calculated by using a Schwarz–Christoffel transformation. This geometry could also represent a trench (or scratch) on a flat surface. It is found that the magnitude of the electric field on the knife-edge cathodes depends strongly on the ratio h/a, where h is the height of the knife-edge cathodes and 2a is the distance between the cathodes. When h/a increases, the magnitude of the electric field on the cathode’s surface decreases. This shows the screening of one cathode by another cathode; for example, keeping the height fixed and decreasing the distance between the cathodes, the field enhancement on the corner decreases. Analytic approximations for the divergent electric field in the immediate vicinity of the sharp edge are derived for the cases where h/a>>1, and h/a≪1. These results lead to insight on the relationship of the density of field emitter in field emitting arrays and field emission from rough surfaces.

Study of Split-Ring Resonators as a Metamaterial for High-Power Microwave Power Transmission and the Role of Defects

Microwave metamaterials show promise in numerous low-power applications, ranging from strip lines to antennas. In general, metamaterials allow microwave designers to obtain electromagnetic characteristics not typically available in nature, leading to new behavior as well as reductions in the size of typical devices. High-power microwave (HPM) sources were efficient in the conventional microwave source community. We consider a specific use of metamaterials as a method to reduce the size of waveguide used for power transmission, particularly, a configuration in which an array of split-ring resonators (SRRs), forming a “mu-negative” structure, allows transmission of power in a waveguide well below the cutoff frequency. This configuration would not be used in an actual HPM device, but explores the methods and considerations that might be required for developing a metamaterial structure for either making HPM sources more compact or developing new types of interaction at these high powers. For any HPM application, a microwave structure must be able to sustain high electric and magnetic fields, as well as high peak and possibly average power. The challenge for metamaterials consists of devising the subwavelength structures (a defining characteristic of metamaterials) that can sustain such fields. In particular, one must understand the sensitivity of any metamaterial system to changes in the individual elements, which in high power pertains mainly to the loss of an individual resonator element. As such a sample system, we explore the physical operating characteristics of the waveguide system loaded with an array of SRRs, particularly the role of defects on its properties. Such defects would form an important feature in any high-power application in which subwavelength structures can be damaged by high field stresses.

Electron beam coupling to a metamaterial structure

Microwave metamaterials have shown promise in numerous applications, ranging from strip lines and antennas to metamaterial-based electron beam driven devices. In general, metamaterials allow microwave designers to obtain electromagnetic characteristics not typically available in nature. High Power Microwave (HPM) sources have in the past drawn inspiration from work done in the conventional microwave source community. In this article, the use of metamaterials in an HPM application is considered by using an effective medium model to determine the coupling of an electron beam to a metamaterial structure in a geometry similar to that of a dielectric Cerenkov maser. Use of the effective medium model allows for the analysis of a wide range of parameter space, including the “mu-negative,” “epsilon-negative,” and “double negative” regimes of the metamaterial. The physics of such a system are modeled analytically and by utilizing the particle-in-cell code ICEPIC. For this geometry and effective medium representati...

Dielectric nonlinear transmission line

A parallel plate nonlinear transmission line (NLTL) was constructed. Periodic loading of nonlinear dielectric slabs provides the nonlinear capacitance and the gaps between provide linear inductive interconnects, this is essentially the same design used by Ikezi [1],[2]. The NLTL was modeled in a circuit simulation code using the experimentally measured form of the nonlinear capacitance. Dielectric loss included in the model as an equivalent series resistance derived from the measured loss tangent data affects the formation of RF oscillations. The diagnostics used on the experimental system are Bdots along the line and a current viewing resistor at the load. The diagnostics provide a measurement of the pulse evolution as it travels down the line. The waveforms from the experimental line qualitatively agree with the circuit model, showing no strong RF formation as a result of the loss.