Capp Spindt

Application of field emitter arrays to microwave power amplifiers

This paper describes the operation of a field emitter array (FEA) as the electron source of a traveling-wave tube (TWT) amplifier. Issues of beam control and focus at high current density and low magnetic field are addressed as well as issues relating to the inherent high emittance of the FEA beam and cathode protection from ion bombardment. Large signal, nonlinear RF-modulated FEA-TWT interaction simulations show circuit efficiencies that approach 50%, even for minimal bunching of average-to-peak current ratios of 0.7-0.9. Direct radio frequency (RF) modulation at the cathode is predicted to significantly improve linearity in the high-efficiency regime as well as reduce harmonic power levels. An unmodulated C-Band FEA-TWT was built to test the focusing approach as well as the robustness of the emitters in an operating vacuum device. The device uses a 1-mm diameter Spindt emitter with a custom-designed electron gun and helix circuit. The FEA-TWT has operated to date with a maximum current of 91.3 mA and shows 99.5% transmission under both drive and no-drive conditions. Output power of the device is 55.0 W at 1.5 GHz with a saturated gain of 23.4 dB and efficiency of 17%, and performs as predicted by simulation. During all operation, the FEA emission appears extremely stable, with no temporal variations observed at any time.

Experimental demonstration of an emission-gated traveling-wave tube amplifier

This paper reports the results of the development of a traveling-wave tube (TWT) amplifier designed and operated using a high-frequency emission-gated field emitter array (FEA) cold cathode. The TWT was conservatively designed to operate with only 1% cathode current modulation but results show that 30% modulation of the current was achieved in the C-Band frequency range. The emission-gated TWT prototype was operated up to a current of 5 mA and RF output power of 280 mW using a 300-/spl mu/m diameter FEA cathode having 10 000 emitter tips with testing performed in single-pulse mode using 100-/spl mu/s pulses. Excellent beam control was demonstrated under all experimental conditions tested. Simulation shows that, with the same TWT circuit and demonstrated cathode modulation level, a 1-mm diameter cathode would generate /spl sim/60 W of output power in the same frequency band and /spl sim/80 W if the circuit were optimized for the measured level of modulation. Measurements also show that performance of the device does not degrade with frequency up to at least 7.0 GHz, which is the maximum operating frequency of the TWT. Cold measurements of the FEA electron gun alone indicate operation of the cathode up through 20 GHz might be possible. These results represent the first operation of an emission-gated cathode in a TWT and the highest power operation ever recorded in a microwave vacuum device using an emission-gated electron source.

11.1: A reliable improved Spindt cathode design for high currents

Recent work with Spindt cathodes has shown that past unreliable behavior has not been due to poor tip uniformity and the failure of over-achieving tips as has been widely believed. Rather the failures have been due to flashover along oxide walls in the cathode cavities. A new cavity architecture has solved the problem, and enabled greatly improved performance.

High average power field emitter cathode and testbed for X/Ku-band cold cathode TWT

A new field emitter geometry that utilizes a dielectric shield between the emitter tip and gate has been optimized to eliminate flashover in the cathode emitter cavities and improve cathode reliability. A high average power testbed has been designed and fabricated to test these cathodes at currents and current densities required for TWT operation in the X/Ku-Band frequency regime. Experimental tests in the water-cooled testbed demonstrated a record 100 hours of CW operation at 100 mA. A 200 mA X/KuBand TWT was designed to integrate these cathodes and to provide RF gain and power across the entire 6 - 18 GHz frequency band. Preliminary results taken at a cathode current of 50 mA show excellent focus of the emittance-dominated electron beam. RF results at 50 mA demonstrated positive gain over the entire frequency band with a maximum of 13.5 dB gain and 10 W output power at 10 GHz and low duty. Cathode testing as well as TWT design and preliminary operation will be described.

Development of spindt cathodes for high frequency devices and flat panel display applications

Microfabricated field emitter arrays have attracted interest for a variety of applications. The most prominent of these applications are flat panel displays, microwave amplifiers, x- ray tubes, electron beam probes, ionizers for vacuum pressure gauges, mass spectrometers, and electronic charge management on spacecraft. From a commercial point of view, the most exciting application has been flat panel displays, while high frequency applications are the most challenging with respect to cathode performance. Displays require attention to issues related to economic high-volume production, very low-voltage operation, and a very high level of uniformity over large areas with a low emission current loading. Microwave and other high frequency applications require small areas, with high tip packing density and the highest possible current loading per tip. Ionization and charge management applications require moderate emission performance, but present special problems with regard to stability and lifetime in relatively harsh environments. Designing an emitter array to meet the requirements of any of these applications involves dealing with lithography issues concerning emitter size and packing density; materials issues as they relate to fabrication processes; stability and lifetime issues with regard to hostile environments, and electronic properties such as dielectric constant, resistivity, and work function of the emitter tip; and the cost of large-scale production.

Field-emitter array development for field-emission displays

In this paper we report recent results from an ongoing program designed to develop a fundamental understanding of the effects of materials, vacuum deposition parameters, and post fabrication processing on the performance of field-emitter arrays for displays. Molybdenum and silicon have been the materials of choice for first generation displays, and have produced acceptable results for the first trials. However, investigations of other emitter materials such as diamond- like-carbon (DLC) and zirconium carbide (ZrC) have produced intriguing improvements in emission performance. In addition in situ processes such as coating of molybdenum and silicon emitters with alternate materials and aggressive emitter- surface cleaning processes such as hydrogen-plasma cleaning and emission-stimulated desorption by high-current pulses, have also been shown to be beneficial. It has also been shown that when using the Spindt emitter fabrication process the emitter cone can be tailored to a preferred shape by appropriate materials selection and manipulation of the emitter deposition parameters. Finally, it is shown that the details of the emitter tip shape can have an impact on the performance of the emitter due to the dynamics of temperature and field-induced surface diffusion during cathode operation. Emitter tips of the same material, operated in the same environment and at the same emission levels can behave very differently depending on the details of the emitter-tip geometry.

Thermal field forming of Spindt cathode arrays

This paper describes changes observed in the emission from a 100-tip Spindt cathode array operated at emission levels that produced tip self-heating and resulting temperatures sufficient to cause surface self-diffusion. This is the well-known thermal-field-forming effect that can produce smoothing and blunting—or buildup and sharpening—of the emitter tips, depending on whether the surface energy or the electrostatic field energy dominates. Although a greater than 50% decrease in emission current for a set voltage was observed, Fowler/Nordheim analysis of the emission data produced the unexpected result that the field-voltage proportionality factor had increased and the emitting area had decreased.

High-speed photo-modulated spindt cathode for FELs

SRI International is integrating a Spindt-type field-emission source with a high-mobility GaN photoconducting semiconductor switch (PCSS) to develop cathodes capable of producing Ampere-level currents in the picosecond pulse regime. Semi-insulating Fe-doped GaN was selected for this application because of its high breakdown voltage, low resistance, and high carrier mobility. Both thin-film and bulk GaN are being investigated. Measurements of GaN carrier lifetime have shown that switch response times of 3-50 ps should be possible. Initial PCSS measurements with a moderate power femtosecond laser operating between 400-530 nm are limited by instrumentation but have shown sub-100 ps switch rise times. Subnanosecond field-emission current modulation has also been measured. A radiofrequency (RF) cavity based streak camera is being developed to measure the electron beams pulse width.

Operation of a low-voltage high-transconductance field emitter array TWT

Summary form only given. L-3 Communications continues development and testing of a moderate power helix TWT incorporating a new low-voltage high-transconductance field emitter array cold cathode developed by SRI International. The TWT employs a custom electron gun required to control the high beam space charge density created in the acceleration region between the HV cathode surface and the grounded input TWT helix. A custom FEA mounting header was also developed to correctly position the emitter within the electron gun and to allow for electrical connection to the base and gate. The TWT itself uses periodic permanent magnet focusing employing a single sever and a high power collector. The collector was redesigned to maximize pumping speed and improve overall TWT vacuum. The program goals are 100 mA operation at 100 W and 5 GHz. Additional goals include evaluating emitter operation in a TWT vacuum environment, determining lifetime issues and limitations, and evaluating the impact on FEA-generated beams on beam focus, circuit interaction, and RF noise characteristics. The Spindt-type field emitter arrays employed in this program demonstrate significantly improved emission characteristics over those used in previous FEA-TWT experiments using SRI cathodes. These cathodes are comprised of 50,000 Mo tips in a 1 mm diameter area with Fowler Nordheim coefficients of aFN = 0.01 A/V2 and bFN = 470 V. Emission characteristics for currents up to 50 mA and cathode current densities up to 6.3 A/cm2 have been measured and show a 63% reduction in operating voltage and a 9X increase in transconductance, dl/dV, from the previous generation of SRI emitters. Program development details and FEA TWT RF data for beam currents up to 50 mA including RF power, efficiency, small signal gain, noise power density, and noise figure will be presented.

Thermal field forming of spindt cathode arrays

This paper describes observed changes in the emission from a 100-tip Spindt cathode array operated at peak emission levels that produced Joule heating and temperatures sufficient to cause surface self-diffusion at the tips. This well-known thermal field forming effect can produce blunting or sharpening of the emitter tips, depending on whether the thermal self-diffusion, or the field-assisted thermal diffusion dominates. A Fowler/Nordheim analysis of the emission produced the unexpected result that tip sharpening had occurred.