Sean Sengele

Georgia Tech Research Institute, Atlanta, Georgia, United States

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Publications (30)22.85 Total impact

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    ABSTRACT: 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.
    No preview · Article · Dec 2014 · IEEE Transactions on Plasma Science
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    ABSTRACT: The 1-D small-signal theory for the fundamental spatial harmonic mode developed by Pengvanich et al. is adapted to include the effect of space charge forces in the electron beam. This model allows us to look at how traveling-wave tube (TWT) performance is affected by random fabrication errors, which are modeled as random perturbations of the phase velocity, interaction impedance, and loss along the TWT's length. In particular, we examine the effect on TWT gain and instantaneous 1-dB bandwidth. Random variation of the phase velocity is found to have the largest effect on both the gain and bandwidth, but the impact is reduced as the amount of space charge in the beam is increased.
    No preview · Article · Mar 2013 · IEEE Transactions on Electron Devices
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    ABSTRACT: A one-dimensional small-signal theory for the backward-wave mode in a traveling-wave tube (TWT) is developed, which includes the effects of random fabrication errors. This is of interest since the backward-wave mode is the spatial harmonic typically responsible for instability in a TWT. The described model examines how gain and instantaneous 1-dB bandwidth of the backward-wave mode is affected by random fabrication errors, which are modeled as random perturbations of the phase velocity, interaction impedance, and loss along the TWT's length. Random variation of the phase velocity is found to have the largest effect on both the backward-wave gain and the bandwidth while having only a minor effect on fundamental, forward-wave mode behavior.
    No preview · Article · Feb 2013 · Journal of Applied Physics
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    ABSTRACT: The development and microfabrication of a millimeter-wave ring-bar slow-wave structure is presented. Dimensional measurements and subsequent HFSS analyses were completed to predict the impact of fabrication errors on phase velocity and interaction impedance. The effect of random fabrication errors on the fundamental and backward-wave gain and bandwidth were explored using a modified Pierce analysis which allowed for random variation of the Pierce parameters along the axis of the TWT.
    No preview · Conference Paper · Apr 2012
  • Xin He · John Scharer · John Booske · Nishant Sule · Sean Sengele
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    ABSTRACT: A recently published field emission cathode parameter extraction method [X. He etal, J. Appl. Phys. 102, 056107 (2007)] provides unambiguous and reliable cathode parameters. The method utilizes a transfer matrix method (TMM) to solve a one-dimensional model that includes both thermionic and field electron emission. However, there are modest differences between our TMM simulation results and experimental data near the transition point between thermal-dominated and field-dominated electron emission. We hypothesize that this discrepancy is due a dependence of effective emitting area with applied electric field. Incorporating surface field enhancement factors and emission area variations that are physically intuitive as the applied field is varied within our TMM simulation, we obtain much better agreement with the experimental data. This result supports the hypothesis that the effective emission area varies as a function of the applied electric field between the A-K (anode-cathode) gap.
    No preview · Article · Jun 2009 · Journal of Applied Physics
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    ABSTRACT: A method has been developed to fabricate waveguide-to-waveguide couplers and tapered dielectric rod antennas for the millimeter-wave regime from microetched silicon. A proof-of-concept study shows that the structures can be realized using relatively simple wet etching and robotic process control. Experimental measurements of the waveguide-waveguide couplers agree in key features with simulations. The results indicate that two-stepped tapers perform nearly as well as smooth linear tapers, but are much easier to fabricate. Coupling transmissivity of better than -1 dB, and peak antenna gain of 8-10 dB are indicated at W-band frequencies. Lateral dimension etch control of 5-mum precision was realized. To solve a challenge of controlling the length of the first step, either an improved masking method or a switch to dry etching processes is required.
    Full-text · Article · Jun 2009 · IEEE Transactions on Electron Devices
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    ABSTRACT: Vacuum electronic devices offer significant potential for increased power and performance at millimeter-wave frequen-cies. However, new approaches are required to reliably manu-facture 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. Index Terms—Meander-line, microfabrication, millimeter-wave (mm-wave), slow-wave structure (SWS), traveling-wave tube (TWT), W-band.
    Full-text · Article · May 2009 · IEEE Transactions on Electron Devices
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    ABSTRACT: We report the experimental study of field emission from cold cathodes having a raised ridge or knife-edge structure and analyses using a Transfer Matrix Method (TMM) model used to extract the cathode parameters. Local emission current measurements were done on a copper cathode, which was fabricated using electric discharge machining (EDM) and chemical etching yielding ridges of thickness 15 mum. Measurements of total cathode currents and lateral scans of the localized field emission current distribution with a 40-micron resolution are reported and analyzed.
    No preview · Conference Paper · Jan 2009
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    ABSTRACT: 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.
    No preview · Conference Paper · Oct 2008
  • Xin He · John Scharer · John Booske · Sean Sengele
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    ABSTRACT: Not Available
    No preview · Conference Paper · May 2008
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    ABSTRACT: 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.
    No preview · Conference Paper · May 2008
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    ABSTRACT: 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.
    No preview · Conference Paper · May 2008
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    Xin He · John Scharer · John Booske · Sean Sengele
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    ABSTRACT: 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 Schrödinger 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 eff eff = E s / E g , where E s is the cathode surface electric field and E g is the gap electric field between the cathode and anode and the effective work function eff are obtained from experimental measurements using this model by simultaneously fitting thermionic and field emission data for the cathode. Comparing the experimental and theoretical results reveals that finite temperature thermal contributions to the current emission can be significant in the operation of many field emission cathodes. © 2008 American Vacuum Society.
    Full-text · Article · Mar 2008 · Journal of Vacuum Science & Technology B Microelectronics and Nanometer Structures
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    Xin He · John Scharer · John Booske · Sean Sengele
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    ABSTRACT: 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.
    Full-text · Article · Sep 2007 · Journal of Applied Physics
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    ABSTRACT: We report measurements and analyses of field emission from both copper and aluminum cathodes. To analyze the data, we have developed a numerical model of electron emission. We note that localized heating of high current density field emission cathodes can provide regime where thermionic as well as field emission effects must be considered. Our analysis of experimental data fitting in both the thermionic and field emission dominant regime provides a robust method for determination of effective work function and field enhancement factor for field emission cathode.
    No preview · Conference Paper · Aug 2007
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    ABSTRACT: We report measurements and analysis of metal cold field emission cathodes utilizing an advanced cathode test facility. The facility is designed to measure the field emission current densities from cathodes on the Micro-and Macro-scale. Measurements are obtained under UHV vacuum (10~10 Torr) conditions. The vacuum chamber is a stainless steel six-way cross. We use a scroll pump, turbo pump and a Vacion pump to achieve UHV. We bake the system at 450 C for several days to eliminate residual water vapor and other possible contaminants. An emitting tungsten filament is used to clean the anode surface. The current vs. electric field (I-E) characteristics incorporating the effects of cathode uniformity, edge effects, space charge, thermal effects and the Fowler-Nordheim coefficients are examined. With a Glassman high DC voltage supply and a DEI high voltage pulse supply, 0~20 kV, 1~5000 mus duration negative pulses with rise times as fast as 60 ns are applied between the cathode and anode to obtain current-voltage characteristics. A Tektronics high voltage probe is used to monitor the cathode-anode voltage. A Lesker manual linear shifter is used to adjust the gap between the cathode and anode from 0~2.5 cm with a resolution less than 0.1 mm. Another MDC XYZ micro-positioner moves a second small anode (with a diameter between 100 mum and 1 mm) located behind a Mo anode screen along the whole cathode surface to obtain the emission current distribution. This is used to examine emission current density uniformity and edge effects. Carefully designed shielded circuits and signal averaging techniques are used to measure the emission current down to the nano-ampere range. Measurements have been made on ALF (Ablation-Line-Focus) aluminum cathodes and CKE (Copper-Knife-Edge) cathodes. The I-E curve and extracted beta are obtained for these cathodes. Simulations of temperature effects on quantum electron tunneling that affects field emission have been made. It demonstrates that for - low electric fields, the emission current density is much higher at room temperature than that of the 0 K Fowler-Nordheim equation. A break point divides the expected linear relationship between ln(J/E") and 1/E into two regions with different slopes. This prediction will be compared with experimental measurements. Both single sharp tip and multi-tip cathodes made of different materials (Cu, Al and W) are being tested, in order to determine the interaction between local neighboring tips during the electron emission process.
    No preview · Conference Paper · Jul 2007
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    ABSTRACT: The development of terahertz (THz) millimetre-wave (MMW) regime vacuum electronics necessitates the concurrent development of adequate fabrication techniques. Since the size of vacuum electronic devices is proportional to their frequency of operation, fabricating them for the THz and mmw regimes requires fabrication techniques capable of producing millimeter and micron-scale features. Monolithic microfabrication technologies such as deep reactive ion etching (DRIE) have been used widely in the MEMS community for years to achieve these sorts of feature sizes.
    No preview · Conference Paper · Jun 2007
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    ABSTRACT: Summary form only given. Vacuum electronic sources of THz regime (0.1-10 THz) radiation will require advanced methods to precisely fabricate miniature waveguides. We are exploring the design and fabrication of THz-regime waveguides using bulk silicon-based microfabrication, specifically, deep reactive ion etching. Due to the waveguide's dimensions it is advantageous to fabricate it in halves on two separate silicon wafers. Final assembly consists of metallizing the halves and thermocompressively bonding them together. Finding an appropriate diffusion barrier and bonding technique is vital to the success of this design and has recently been our primary focus. Along with the development of THz-regime waveguides, we are exploring a novel coupling technique that consists of a tapered silicon tip, made by wet chemical etching. These silicon tips, fabricated from square silicon rods approximately the size of the waveguide, can be fabricated with a tolerance of less than one micron by monitoring the electrolytic current in the etchant bath and controlling the submersion depth of the silicon rod via robotic controls. In conjunction with the development of the waveguides and couplers, we are also investigating how THz regime radiation interacts with metallic thin films. We are developing computational models to predict effective RF conductivity of metallic thin films at THz frequencies, including the effects of surface roughness scattering at the interfaces. Experimentally, our micro fabricated THz-regime waveguides provide us with an excellent platform for validating the model. By measuring the throughput power for different lengths of waveguide, we can determine the ohmic loss per unit length. This presentation will discuss the latest developments in all of these research efforts namely the fabrication of THz regime waveguides, coupling techniques and our investigation into electron transport in metallic thin films at THz frequencies.
    No preview · Article · Jan 2007 · IEEE International Conference on Plasma Science
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    ABSTRACT: The millimeter-wave (MMW) and terahertz (THz)-regime portions of the electromagnetic spectrum have enormous application potential, including high-data-rate communications, remote sensing and threat detection, high resolution radar, biomedical imaging, and spectroscopic analysis. To exploit this potential, new sources of coherent electromagnetic radiation are needed in the frequency range of 100 - 1000 GHz. The ideal sources would provide high power with high efficiency in a compact, lightweight, and low-cost package. Many of the applications require bandwidths of several percent (relative) or greater and both amplifiers and oscillators are needed. Vacuum electronic devices, such as traveling wave tubes (TWTs) meet many of these requirements but are constrained by complex fabrication methods that become impractical at frequencies of 100 GHz and above. This research is investigating new methods for TWT fabrication, derived from semiconductor microfabrication technologies. Various microfabrication techniques are under investigation, to identify those that are optimally suited. One critical piece of research is to measure the passive microwave losses of a 400 GHz waveguide made by these microfabrication methods. The final goal includes a study of the characteristics of a microfabricated TWT using a new electron-beam source designed for MMW and THz-regime vacuum device research
    Full-text · Article · Oct 2006
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    ABSTRACT: A chaotic waveform generator based on a TWT amplifier with delayed recirculated feedback driven with an external harmonic signal has been developed. In this paper, a numerical model of the oscillator is described and results of numerical and experimental investigation are presented. Such as oscillator is of great interest for novel communication and radar systems utilizing noise-like waveform signals.
    Full-text · Conference Paper · Apr 2006