ectrical and Computer Engineering


Gyrotrons are well recognized sources of high-power coherent
electromagnetic radiation. The power that gyrotrons can radiate in the millimeter- and
submillimeter-wavelength regions exceeds the power of classical microwave tubes by
many orders of magnitude. In this work, the author considers some problems related
to the operation of gyro-devices and methods of their solution. In particular, the self-
excitation conditions for parasitic backward waves and effect of distributed losses on
the small-signal gain of gyro-TWTs are analyzed. The corresponding small-signal
theory describing two-stage gyro-traveling-wave tubes (gyro-TWTs) with the first
stage having distributed losses is presented. The theory is illustrated by using it for
the description of operation of a Ka-band gyro-TWT designed at the Naval Research
Laboratory. Also, the results of nonlinear studies of this tube are presented and
compared with the ones obtained by the use of MAGY, a multi-frequency, self-
consistent code developed at the University of Maryland. An attempt to build a large
signal theory of gyro-TWTs with tapered geometry and magnetic field profile is made
and first results are obtained for a 250 GHz gyro-TWT. A comparative small-signal analysis of conventional four-cavity and three-
stage clustered-cavity gyroklystrons is performed. The corresponding point-gap
models for these devices are presented. The efficiency, gain, bandwidth and gain-
bandwidth product are analyzed for each scheme. Advantages of the clustered-cavity
over the conventional design are discussed.
The startup scenarios in high-power gyrotrons and the most important
physical effects associated with them are considered. The work presents the results of
startup simulations for a 140 GHz, MW-class gyrotron developed by
Communications and Power Industries (CPI) for electron-cyclotron resonance heating
(ECRH) and current drive experiments on the Wendelstein 7-X stellarator plasma.
Also presented are the results for a 110 GHz, 1.5 MW gyrotron currently being
developed at CPI. The simulations are carried out for six competing modes and with
the effects of electron velocity spread and voltage depression taken into account.
Also, the slow stage of the startup in long-pulse gyrotrons is analyzed and
attention is paid to the effects of ion compensation of the beam space charge,
frequency deviation due to the cavity wall heating and beam current decrease due to
cathode cooling. These effects are modeled with a simple nonlinear theory and the
code MAGY.












ANALYSES OF ADVANCED CONCEPTS IN MULTI-STAGE GYRO-
AMPLIFIERS AND STARTUP IN HIGH POWER GYRO-OSCILLATORS.



By


Oleksandr V. Sinitsyn.





Dissertation submitted to the Faculty of the Graduate School of the
University of Maryland, College Park, in partial fulfillment
of the requirements for the degree of
Doctor of Philosophy
2005










Advisory Committee:
Professor V. L. Granatstein, Chair
Doctor G. S. Nusinovich
Professor T. M. Antonsen, Jr.
Professor I. D. Mayergoyz
Professor D. Boyd



















© Copyright by
Oleksandr V. Sinitsyn
2005
Dedication

















To my dear parents.

ii Acknowledgments


I owe my gratitude to all the people who have helped me in preparation of this
dissertation and who have made my graduate school experience one of the most
special periods of my life.
First, I would like to thank my academic advisor, Professor Victor
Granatstein, for giving me an invaluable opportunity to work on challenging and very
interesting projects over the past five years.
Second, I owe a tremendous gratitude to my co-advisor, Dr. Gregory
Nusinovich. He always made himself available for help and advice and there has
never been an occasion when I have knocked on his door and he has not found time
for me. It has been a pleasure to work with and learn from him.
Thanks are due to Professor Thomas Antonsen, Professor Isaak Mayergoyz
and Professor Derek Boyd for agreeing to serve on my dissertation committee and for
spending their time for reviewing the manuscript.
I would like to especially thank Dr. Alexander Vlasov for his help with
simulation codes and his useful suggestions and discussions.
I would also like to acknowledge help and support from some of the staff
members. Dorothea Brosius help with the software is highly appreciated, as is the
computer hardware support from Edward Condon and traveling help from Janice
Schoonover.

iii I owe my deepest thanks to my family my parents and my brother who have
always stood by me and guided me through my career, and have pulled me through
against all the odds. Words cannot express the gratitude I owe them.




iv Table of Contents



Dedication..................................................................................................................... ii
Acknowledgements...................................................................................................... iii
Table of Contents.......................................................................................................... v
List of Figures .............................................................................................................. vi
Introduction................................................................................................................... 1

I.1: Principles of operation of gyro-devices. Choice of parameters ......................... 9

Chapter 1: Two-Stage Gyro-TWTs ........................................................................... 16

1.1: Basic device configurations ............................................................................. 16

1.2: General formalism ........................................................................................... 18
1.2.1: Self-consistent set of equations for the gyro-TWT................................... 18
1.2.2: Linear theory of the gyro-TWT ................................................................ 24

1.3: Two-stage gyro-TWTs with distributed losses................................................ 27

1.3.1: Results of the linear theory. Gain and bandwidth studies......................... 27
1.3.2: Analysis of backward wave excitation ..................................................... 31

1.3.3: Results of the nonlinear analysis .............................................................. 36

1.4: Theory of the gyro-TWT with tapered parameters.......................................... 47

1.4.1: General formalism .................................................................................... 47
1.4.2: Results of the numerical analysis.............................................................. 50

Chapter 2: Theory of Multi-Stage Gyroklystrons...................................................... 59

2.1: Basic device configurations ............................................................................. 59

2.2: General formalism ........................................................................................... 61

2.2.1: Gyro-averaged equations of electron motion and the balance equation... 61
2.2.2: Point-gap model........................................................................................ 64

2.3: Comparison of two concepts: Conventional multi-cavity versus clustered-
cavity gyroklystrons................................................................................................ 66

2.3.1: Point-gap model for the three-stage clustered-cavity gyroklystron.......... 67
2.3.2: Efficiency studies...................................................................................... 69

2.3.3: Gain studies............................................................................................... 75

2.3.4: Bandwidth studies..................................................................................... 77

Chapter 3: Startup Scenarios in High-Power Gyrotrons............................................ 80

3.1: Preliminary remarks......................................................................................... 80

3.2: Excitation of the gyromonotron....................................................................... 85

3.3: Analysis and simulations ................................................................................. 90

3.3.1: Starting current and the growth rate of oscillations.................................. 90

3.3.2: Simulation results for the 140 GHz, 1 MW CPI gyrotron ........................ 93

3.3.3: Simulation results for the 110 GHz, 1.5 MW CPI gyrotron ................... 102

3.4: Slow stage of startup scenarios...................................................................... 115

3.4.1: Slow processes in CW and long-pulse gyrotrons ................................... 115

3.4.2: Analysis of the effects............................................................................. 117

Summary ................................................................................................................... 123

Bibliography ............................................................................................................. 126


v List of Figures




I.1. Typical gyrotron configuration