Sean Sengele

Microfabrication and Characterization of a Selectively Metallized W-Band Meander-Line TWT Circuit

Vacuum electronic devices offer significant potential for increased power and performance at millimeter-wave frequencies. However, new approaches are required to reliably manufacture the miniature electromagnetic circuits used at these high frequencies. In this paper, we describe the design, fabrication, and testing of an innovative meander-line slow-wave structure for a W-band traveling-wave tube (TWT). The unique challenge of metallizing only the top of a high-aspect-ratio serpentine dielectric ridge using conventionally planar microfabrication techniques is overcome using a novel selective masking and metallization process. The procedure is demonstrated by fabricating a W-band meander-line circuit for a 10-W continuous-wave TWT. Cold-test S -parameter measurements are presented.

Generation of chaotic radiation in a driven traveling wave tube amplifier with time-delayed feedback

The application of chaos in communications and radar offers new and interesting possibilities. This article describes investigations on the generation of chaos in a traveling wave tube (TWT) amplifier and the experimental parameters responsible for sustaining stable chaos. Chaos is generated in a TWT amplifier when it is made to operate in a highly nonlinear regime by recirculating a fraction of the TWT output power back to the input in a delayed feedback configuration. A driver wave provides a constant external force to the system making it behave like a forced nonlinear oscillator. The effects of the feedback bandwidth, intensity, and phase are described. The study illuminates the different transitions to chaos and the effect of parameters such as the frequency and intensity of the driver wave. The detuning frequency, i.e., difference frequency between the driver wave and the natural oscillation of the system, has been identified as being an important physical parameter for controlling evolution to chao...

A selectively metallized, microfabricated W-band meander line TWT circuit

The development of a selective metallization process capable of metallizing only the top of a microfabricated, raised meander line ridge is described. This fabrication process has unique potential in the development of millimetre-wave and terahertz regime slow wave structures for travelling wave tubes. The fabrication process will be described and the latest images and measured data will be presented.

650 GHz traveling wave tube amplifier

Calabazas Creek Research, Inc. (CCR) and the University of Wisconsin, Madison (UW) are developing a 650 GHz traveling wave tube amplifier (TWTA). Simulations predict 360 mW peak output power with a 2-10% duty cycle. This paper summarizes the design and fabrication of the TWT. Testing is expected in November, 2008.

One-dimensional combined field and thermionic emission model and comparison with experimental results

An integrated theoretical model has been developed to predict the entire range of emission from thermionic to field emission, including the mixed emission regime. The model assumes a Sommerfeld free electron model supply function, for which the Fermi-Dirac distribution applies with a nonzero temperature. The electron transmission coefficient is calculated in one dimension using a transfer matrix method (TMM) to solve the steady-state Schrodinger equation. Emission current densities have been measured for a periodic copper knife-edge cathode to compare with the TMM model result. It is shown that the computational result utilizing this model provides good agreement with the experimental data. Unambiguous and reliable estimates of the effective field enhancement factor βeff (βeff=Es∕Eg, where Es is the cathode surface electric field and Eg is the gap electric field between the cathode and anode) and the effective work function ϕeff are obtained from experimental measurements using this model by simultaneousl...

Overview of W-Band Traveling Wave Tube Programs

Two research programs are in progress to develop W-band traveling wave tubes (TWTs). One uses a folded waveguide (fwg) slow-wave circuit and the other uses a novel, planar meander line circuit. Both devices are currently being assembled. The predicted performance, fabrication methods and challenges, and measured data to date were presented

Modeling of cold emission cathode by inclusion of combined field and thermionic emission processes

Emission currents have been measured at elevated temperatures for a periodic copper knife-edge cathode. To model the emission process of the cathode, we have combined thermionic and field emission processes. Electron tunneling is calculated using a transfer matrix method. Using this model, we accurately reproduced the experimental deviation and minimum in the ln(J∕Eg2) vs 1∕Eg plot, where Eg is the gap electric field and J is the emitted current density. This phenomenon has been widely observed but no comprehensive explanation has been put forth. Unambiguous and realistic estimates of the cathode effective field enhancement factor beta averaged (βeff=Esurface∕Egap) and effective work function Φ are obtained from experimental measurements using this model.

Eulerian calculations of wave breaking and multivalued solutions in a traveling wave tube

The traveling wave tube is an electron beam device that works on a similar principle to the beam-plasma instability, where the background plasma is replaced by an electromagnetic waveguiding structure. The nonlinear evolution of the instability includes wave breaking and the formation of multivalued solutions, and conventionally these solutions have been computed using Lagrangian techniques. Recently, an Eulerian method for computing multivalued solutions was developed in the context of geometrical optics and has been applied to the klystron, a relative of the traveling wave tube. In this paper we apply the Eulerian technique to solve a traveling wave tube model and compare the results to a Lagrangian technique. The results are found to be in good qualitative agreement with small quantitative differences that are attributed to the numerical methods.

Backward-Wave Suppression Analysis, and Design and Fabrication of a Prototype Millimeter-Wave Ring-Bar Slow-Wave Structure

A challenge for high-power millimeter-wave (mmw) traveling-wave tube (TWT) amplifiers is to realize high-power operation without incurring oscillation from backward-wave (BW) interaction. Conventional wisdom purports that contrawound (including ring bar) helix TWTs are superior to monofilar helix TWTs for stability against BW oscillations due to a zero or at least greatly suppressed BW interaction impedance (KBW) compared with the forward-wave (FW) interaction impedance (KFW). We use 3-D electromagnetic (EM) field calculations of a ring-bar helix to compare the BW and FW interaction impedances. Using a sine/cosine basis set and a comparison at a fixed phase velocity (rather than fixed frequency assumed in previous analyses), we show that the BW interaction impedance is not always significantly suppressed in the ring-bar/contrawound helix. In spite of the lack of ubiquitous BW stability, we use EM simulations to illustrate that there remain specific, efficient, high-gain mmw ring-bar designs with KBW <; KFW. The accuracy of the simulations is validated by experimental measurements that confirm simulation predictions of phase velocity and S-parameters.

Micromachined step-tapered high frequency waveguide inserts and antennas

Applications of high frequency radiation, in the ranges of 30-300 GHz (millimeter-wave) and 300-1000 GHz (terahertz), require the development of specialized components, specifically low-loss waveguides and antennas. Conventional waveguide antennas must be flared in both lateral dimensions requiring a large space and are challenging to fabricate. Previous work has shown the promise of tapered dielectric rod antennas for their high transmission and ease of integration into waveguide systems. However, practical repeatable machining techniques are neglected. This work develops the design, manufacturing, and analysis of step-tapered waveguide inserts and antennas, fabricated via controlled acid etching of high resistivity silicon, optimized for good transmission in the range of 75-110 GHz.