1.5: Development of Field Emission Cathodes, Electron Gun and a Slow Wave Structure for a Terahertz Traveling Wave Tube

Nathaniel P. Lockwood, Keith L. Cartwright, P. D. Gensheimer,

D. A. Shiffler High Power Microwave Division

Directed Energy Directorate Air Force Research Laboratory Kirtland AFB, NM, USA, 87117

icepic@kirtland.af.mil

Christian Y. d’Aubigny, Christopher K. Walker,

Abraham Young Steward Observatory University of Arizona

Tucson, Arizona, USA, 85721 cdaubigny@as.arizona.edu

Steven B. Fairchild, Benji Maruyama

Materials and Manufacturing Directorate

Air Force Research Laboratory WPAFB, OH, USA, 45433

Abstract: High power terahertz (THz) sources and amplifiers hold the potential to greatly improve remote sensing and high bandwidth communication. To enable these applications, a Traveling Wave Tube (TWT) operating at 0.22 THz and a multi-cathode Field Emission (FE) electron gun are developed and characterized using a Particle-in-Cell simulation. Three candidate high current density cathode materials, Halfnium Carbide (HfC), carbon fibers, and Carbon Nanotubes (CNTs) were tested, characterized and their emission properties compared and used to verify simulations. A current of 3.0 mAmps for a single 100 micron diameter single walled nanotube rope was experimentally achieved and used as the basis of the FE gun design. Simulations of the FE gun and THz TWT were coupled and the effects of multiple and single tip FE gun beam characteristics on the TWT gain, bandwidth, and efficiencies are examined for several beam optic configurations.

Keywords: cathodes; field emission; electron beam; electron gun; carbon fiber; carbon nanotubes; terahertz amplifier; terahertz; traveling wave tube; particle-in-cell; ICEPIC.

Introduction High power THz sources and amplifiers hold the potential to revolutionize the remote sensing and communication industries. Applications of high power terahertz technology fall in two broad categories: 1) Radar and high bandwidth communications. 2) Remote detection and identification of material properties. TWTs are the most promising technology for compact high power amplifiers at THz frequencies. The gain and high frequency limit of TWTs are determined by the loss in the slow wave structure (SWS) and the ability to produce small diameter, high current density electron beams. A compact, high-power TWT operating at 0.22 THz is currently being developed utilizing a combination of computer modeling software, field emission cathodes, and modern micro-machining techniques.

Traditionally, vacuum electronic sources and amplifiers have used thermionic cathodes to generate high current

electron beams for TWTs and other electronic devices [1]. High current densities and a small beam radius are achieved for a small THz TWT beam tunnel by emitting electrons from a cathode surface much larger than the beam tunnel and focusing the electron beam using either electrostatic or magnetic field beam optics. This approach, however, introduces large transverse electron energies due to the electrons being accelerated radially by the optics. The radially accelerated electrons increase the emittance and magnetic confinement field requirements. Focusing the electron beam down to a smaller diameter also presents several problems with beam envelope oscillations if the electrons are not introduced into the confining magnetic field correctly. To avoid these issues, a micron sized high aspect ratio FE cathode can be immersed in the confining magnetic field and used to generate an electron beam with a smaller diameter than the THz TWT beam tunnel. The key technical challenge with using a micron sized FE cathode is achieving the high current densities (107 Amps/m2) and relatively high currents, long life time, and low beam emittance. A multi-pronged approach of improving the FE materials and optimizing the electric and magnetic fields using highly correlated experiment, theory, and high fidelity simulations are used to address these technical challenges. This presentation reviews the experimental and simulation results used to develop the FE electron gun and the effects of the resulting beam characteristics on the TWT performance.

Field Emission Electron Gun Field Emission Cathode Development: Carbon fibers, CNTs, SWNT ropes, and HfC hold the most potential for achieving the high current densities for 1000s of hours of operation required for a THz TWT. Advantages of carbon fibers, SWNT ropes and CNTs are that they can achieve aspect ratios of greater than 2 million respectively. Carbon fibers and CNTs also exhibit high conductivity along the cathode axis, often as large as 1,000 times that of copper [2, 3]. The advantage of HfC is that has demonstrated stable operation for periods of over 2200 hrs at 0.25 mA [4]. These four cathode materials were tested, characterized, and modified in order to achieve the appropriate beam

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characteristics for THz amplification. Beam stick tests show that 0.5 mAmps per vapor grown carbon fiber tip are attainable by annealing the carbon fiber. CNT arrays achieved up to 3.6 mAmps at 5180 Volts, but used a 1 mm diameter emission area that was ten times larger than the THz TWT beam tunnel. SWNT ropes of 100 micron diameter achieved 3 mAmps and high emission stability for over a 1000 hours of operation. Methods used to enhance the resilience of the carbon fiber and CNT cathode materials to high current densities are discussed.

Figure 1. Emission Current of a) 20 Micron Diameter Vapor Grown Carbon Fiber (VGCF) b) 1 mm Diameter CNT array and c) 100 Micron Diameter SWNT Rope Electron Gun Design: FE cathode emission, focusing optics and beam characteristics are simulated using the Improved Concurrent Electromagnetic Particle-In-Cell (ICEPIC) simulation. Electron beam optic designs are developed and characterized for both single and multiple immersed FE cathodes. Initial focusing of the beam is accomplished using a combination of a Pierce-like cathode geometry and aperture lens [5]. Additional focusing is achieved using multiple pierce electro static lens. The beam focal length, emittance, and electron convergence angle distribution for various beam optic configurations is characterized. The difference in the beam emittance and uniformity using single versus multiple tip configurations

are also investigated. To ensure the accuracy of the FE cathode emission models, geometrically accurate ICEPIC models, using Fowler-Nordheim [6] and Young [7] emission models, are compared against experimental FE cathode emission tests.

Traveling Wave Tube Simulations The THz TWT developed in this study is a folded waveguide structure based on a 46 GHz tube designed by Waterman [8]. The 46 GHz tube was scaled to operate at THz frequencies using ICEPIC. Previous ICEPIC simulations at 0.35 THz assumed a perfectly collimated 10 mA, 25 kV beam, 100 μm diameter tunnel (J=106 Amps/m2) that uses a 5.4 kG magnetic field for beam confinement. Simulations showed a gain of 21 dB, 2.7% instantaneous fractional bandwidth, and power added efficiency of ~0.16% could be obtained from a SWS of 64 periods. A sever was used at period 28 in the SWS to reduce backwards wave oscillations. The effects of non-ideal electron beam characteristics on the TWT performance is determined by coupling the FE gun and 0.22 THz TWT simulations. The impact on the gain, bandwidth, and power added efficiency of the TWT due to a non-ideal electron beam is examined for several FE cathode beam optic configurations. The results of the simulations are used to develop fabrication and alignment tolerances for the FE electron gun optics.

Figure 2. TWT SWS gain curve for 64 periods with a sever at period 28

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8. “Folded-Waveguide Millimeter-Wave Circuit Model’’, Waterman, J., Engineer thesis, Stanford University, 1979

CNT Array

VGCF

SWNT Rope

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