Michael D. Haworth

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.

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.

Evidence of a new pulse-shortening mechanism in a load-limited MILO

Experimental evidence shows that the electron flow in a load-limited magnetically insulated transmission line oscillator (MILO) is only partially magnetically insulated at the launch point, yet fully insulated along the downstream slow-wave structure (SWS). This results in a microwave pulse duration substantially less than that of the beam. Computer simulations suggest that anode plasma formation is responsible for this pulse shortening and are able to replicate key features of the experimental data.

Cathode effects on a relativistic magnetron driven by a microsecond e-beam accelerator

Experiments have been performed on a relativistic magnetron driven at e-beam accelerator peak parameters: voltage = -0.4 MV, current = 16 kA, and pulselength = 0.5 /spl mu/s. The magnetron is a six-vane device operating at about 1 GHz with extraction from two cavities. For equal power in both extraction waveguides, the peak microwave power of this device is between 200 and 300 MW. Microwave pulse-shortening limits pulselengths to the range of 10-100 ns. Time-frequency analysis of microwave emission indicates operation at about 1.03 GHz, close to the pi mode frequency identified from cold tests and the three-dimensional MAGIC code. Two cold cathodes were tested: 1) an emitting aluminum knob in the vane region with no endcap and 2) an extended cathode with a graphite fiber emission region in the vanes and endcap outside the vanes. Electron endloss current has been measured for the two cathodes. With no endcap, the cathode exhibited endloss current fraction up to 50% of the total; with one endcap, the cathode reduced the endloss current fraction to as little as 12%. Both cathodes produced peak total-electronic efficiency in the range of 14%-21%.

Improved cathode design for long-pulse MILO operation

An improved cathode design for a magnetically insulated transmission line oscillator (MILO) has resulted in extending the radiated microwave pulse duration from 200 ns to over 400 ns. This was accomplished by maximizing the emission uniformity in the launch-point region of the cathode which, in turn, minimized anode plasma formation. The extended RF pulse duration has allowed us to find evidence of a new pulse-shortening mechanism late in the beam pulse: anode plasma formation in the load region.

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.

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.

Increasing the RF energy per pulse of an RKO

The Air Force Research Laboratory RKO source has recently demonstrated the ability to convert electron beam power to RF power until the termination of the electron beam pulse, achieving a power of 1.5 GW at an energy of 170 J. These results represent an increase in power of 25-30% in power and energy extracted from this source. This paper discusses the principal research areas encountered in lengthening the RF pulse (FWHM) from 50 ns to the present 120 ns and the associated increase in the RF energy.

All Cavity-Magnetron Axial Extraction Technique

A compact axial π-mode extraction scheme, which is based on a patent by Greenwood, is demonstrated in conjunction with the UM/L-3 relativistic magnetron using the particle-in-cell code ICEPIC. Cases utilizing Greenwoods extraction technique were compared with power extraction using traditional radial waveguides. Average extracted power values in all simulated axial cases were found to be within +/-6.5% of the radial cases. Cases utilizing 85 ° and 90° sector waveguides were found to have efficiencies up to ten percentage points higher than the radial case. The best performing case was found to use a set of three axially oriented 90 ° sector waveguides, shorted on the upstream side, with the short located 15 cm from the center of the magnetron apertures.

High-power microwave-induced TM/sub 01/ plasma ring

Open-shutter photography was used to capture the air breakdown pattern induced by a TM/sub 01/ mode radiated by a high-power backward wave oscillator. The resultant plasma ring was formed in air adjacent to a conical horn antenna fitted with a membrane to keep the experiment under vacuum. This image was digitized and further processed using Khoros 2.0 software to obtain the dimensions of the plasma ring. This information was used in an air breakdown analysis to estimate the radiated power, and agrees within 10% with the power measured using field mapping with an open-ended WR-90 waveguide.