Design and Test of a 94 GHz Overmoded Traveling Wave Tube Amplifier

181 Pages·2015·32.82 MB·English

by  Corrinne Fogg

PSFC/RR-15-1

Design and Test of a 94 GHz Overmoded Traveling

Wave Tube Amplifier

Elizabeth J. Kowalski

January 2015

Plasma Science and Fusion Center
Massachusetts Institute of Technology
Cambridge MA 02139 USA

This work was supported by the Air Force Office of Scientific Research Program on
Plasma and Electroenergetics under Grant FA9550-09-1-0363. Reproduction,
translation, publication, use and disposal, in whole or in part, by or for the United States
government is permitted.
Design and Test of a 94 GHz Overmoded
Traveling Wave Tube Amplifier
by
Elizabeth J. Kowalski
B.S. Electrical Engineering, the Pennsylvania State University (2008)
S.M. Electrical Engineering and Computer Science, Massachusetts
Institute of Technology (2010)
Submitted to the
Department of Electrical Engineering and Computer Science
in partial fulfillment of the requirements for the degree of
Doctor of Philosophy
at the
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
February 2015
(cid:13)c 2015 Massachusetts Institute of Technology. All rights reserved.
Author ..............................................................
Department of Electrical Engineering and Computer Science
December 31, 2014
Certified by..........................................................
Richard J. Temkin
Senior Research Scientist, Department of Physics
Thesis Supervisor
Accepted by.........................................................
Professor Leslie A. Kolodziejski
Chairman, Committee on Graduate Students
Department of Electrical Engineering and Computer Science
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Design and Test of a 94 GHz Overmoded Traveling Wave
Tube Amplifier
by
Elizabeth J. Kowalski
Submitted to the Department of Electrical Engineering and Computer Science
on December 31, 2014, in partial fulfillment of the
requirements for the degree of
Doctor of Philosophy
Abstract
This thesis discusses the design and test of an overmoded W-band Traveling Wave
Tube (TWT). The TWT was designed to operate in the rectangular TM cavity
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mode at 94 GHz. The unwanted lower order, TM and TM , modes were suppressed
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using selectively placed aluminum nitride dielectric loading. Simulations in 3-D CST
Particle Studio confirmed suppression of unwanted modes due to dielectric loading
and operation in the TM mode. The TWT was designed to operate at 31 kV with
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310 mA and a 2.5 kG solenoid magnet. Simulations in both 1-D Latte and 3-D CST
predicted 32 dB of gain, 200 MHz bandwidth, and 300 W peak output power for the
TWT at 94 GHz. Test structures of 9- and 19- cavities were made via CNC direct
machining. Cold test measurements showed suppression of the unwanted modes and
transmission of the TM mode, which correlated well with HFSS simulations. Two
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final 87-cavity structures were built and cold tested.
The experiment was designed and built in-house at MIT (with exception of the
electron gun cathode, manufactured by industry). It was operated with a 3 microsec-
ondpulsedpowersupply. Abeamtestwasimplementedwhichconfirmedoperationof
the TWT set up and electron gun. The electron gun operated at 31 kV with 306±6
mA of current detected at the collector and 88 % transmission of current. Initial
operation of the TWT showed zero-drive stable operation and demonstrated 8 dB of
device gain and 10 W peak output power at 95.5 GHz. Following these first tests,
the magnetic field alignment was improved and the second structure, which showed
better circuit transmission in cold test, was installed. The overmoded TWT produced
21±2 dB device gain (defined as P /P ) at 94.3 GHz and 27 W of saturated output
out in
power in zero-drive stable operation. The TWT was estimated to have about 6 dB
of additional loss due to coupling into and out of the circuit. Taking that loss into
account, the gain on the TWT circuit itself was estimated to be 27±2 dB circuit gain.
CST simulations for the experimental current and voltage predict 28 dB circuit gain,
in good agreement with measurements.
This experiment demonstrated the first successful operation of an overmoded
TWT. The overmoded TWT is a promising approach to high power TWT opera-
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tion at W-Band and to the extension of the TWT to terahertz frequencies.
Thesis Supervisor: Richard J. Temkin
Title: Senior Research Scientist, Department of Physics
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Acknowledgments
This thesis would not have been possible without my advisor, Dr. Richard J. Temkin.
Always willing to help, offer advice, and teach the finer points of vacuum electronics,
he guided me through this project and helped to make my PhD successful. Every
member of the Waves and Beams Group in the PSFC also helped me in my research.
Particularly, my officemates Dr. Emilio Nanni and XueYing Lu dealt with my dis-
tracting conversation, pungent teas, and research woes. Ivan Mastovsky’s expertise
helped my experiment to be operational, while Dr. Michael Shapiro ensured that my
theory and simulations were correct. In addition, Dr. Sudheer Jawla, Dr. David Tax,
Dr. Brian Munroe, Jason Hummelt, JeiXi Zhang, Sam Schuab, Alexander Soane,
and Haoron Xu all offered their advice and help, from Friday evening brainstorming
to unexpected company in the lab on Saturday.
Graduate Women at MIT, GWAMIT, helped me to realize I was not alone in my
endeavors and introduced me to some amazing women at MIT who encouraged me
and became great friends. It was amazing to be a part of shaping GWAMIT, and I
hope that the organization continues to grow in the future.
Of course, my family and friends helped me through the long nights, weeks,
months, and years of research. My parents taught me how to learn, and my sis-
ters taught me how to have a life. My husband, Edward Loveall, provided endless
support. He listened to me practice countless presentations, so he may even under-
stand 31 % of this thesis.
Elizabeth Kowalski
Cambridge, MA
December 22, 2014
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6
to Edward
onion ("@ny@n) noun. An edible bulb with
a pungent taste and smell, composed of
several concentric layers, used in cooking.
— Oxford English Dictionary
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Contents
1 Introduction 21
1.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
1.2 A Brief History of TWTs . . . . . . . . . . . . . . . . . . . . . . . . . 29
1.2.1 Vacuum Tubes and Radar . . . . . . . . . . . . . . . . . . . . 29
1.2.2 Invention of the TWT . . . . . . . . . . . . . . . . . . . . . . 32
1.2.3 More Vacuum Devices . . . . . . . . . . . . . . . . . . . . . . 35
1.3 Modern Day W-Band Vacuum Tubes . . . . . . . . . . . . . . . . . . 36
1.3.1 W-Band TWTs . . . . . . . . . . . . . . . . . . . . . . . . . . 36
1.3.2 Other W-Band Devices . . . . . . . . . . . . . . . . . . . . . . 38
1.3.3 Overmoded TWTs . . . . . . . . . . . . . . . . . . . . . . . . 39
1.4 Overview of Thesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
2 Theory of Traveling Wave Tubes 41
2.1 TWT Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
2.2 Electron Beams in Vacuum Tubes . . . . . . . . . . . . . . . . . . . . 44
2.2.1 Pierce Electron Gun . . . . . . . . . . . . . . . . . . . . . . . 44
2.2.2 Magnetic Field Confinement . . . . . . . . . . . . . . . . . . . 51
2.2.3 Collector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
2.3 Analytical Theory for Slow-Wave Structures . . . . . . . . . . . . . . 53
2.3.1 Gain in the Circuit . . . . . . . . . . . . . . . . . . . . . . . . 59
2.4 Types of Slow-Wave Structures . . . . . . . . . . . . . . . . . . . . . 62
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