Magnetron

3D Computer Modeling of Magnetrons

by Lili Ma

A thesis submitted to the University of London in partial fulfillment of the requirements for the degree of Doctor of Philosophy

Department of Electronic Engineering Queen Mary, University of London United Kingdom
December 2004

TO MY FAMILY

Abstract
The research work presented in this thesis is primarily based on computer modellingof magnetrons using a 3D FDTD/PIC code called MAGIC. The usual modelling process has been substantially extended to include realistic details of two e2v technologies’ S and X band magnetrons. Considerable insight into the magnetron operation has thus been obtained. An investigation into the melting of the output coupling rods in the S band MG5193 magnetron revealed that multipacting taking placebetween the coupling rods and the central support rod is the most likely cause, rather than an RF mismatch or RF current overheating. Also, a good agreement between the simulated and measured frequency spectrum of the S band MG5193 anode has been achieved by suitably adjusting the dimensions of the bare anode, the strapping system and the ‘tuner’. Possible reasons for the high overall efficiency ofthe X band MG5241 magnetron model have been investigated by modelling the end hats emission, the output coupling circuit, the copper surface losses and the rate of rise of anode voltage, but none proved to be relevant. However, the investigations revealed some important features of magnetron operation. Computer modelling revealed for the first time non-uniform electron emission and back bombardmentalong the length of the cathode. Also it was shown that in computer modelling it is essential to use a real rather than a uniform magnetic field profile. Simulation runs with different cathode emission profiles have indicated marked influence of the emission distribution on the overall efficiency of the magnetron. Computer modelling also indicated that high energy back bombardment electrons are present inthe X band magnetron and that an artificially high initial velocity of emitted electrons significantly reduces the overall efficiency. It is therefore concluded that the high energy of secondary electrons is the likely reason for a relatively low efficiency of i

the X band magnetron. Perhaps one ought to add that all the new and additional information could not have been obtained without a powerful3D computer code.

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Acknowledgments
I would like to express my gratitude to my supervisor, Dr. X. Chen, for his supervision, guidance and support throughout this project. Many thanks are also due to the following people who have kindly offered their help and support: - Mr. M. Esterson specifically, who guided me through my four year’s PhD study. His knowledge of magnetrons and intelligentway of analyzing has been great help. - Professor P. A. Lindsay for his inspiring suggestions, from the view of fundamental magnetron theory. - Dr. L. Ludeking for his support on MAGIC simulation techniques. - Mr. P. Burleigh, Mr. M. Brady, Mr. K. Saleem and Mr. D. Wilson for their suggestions regarding engineering problems and many useful discussions. - Dr. Z. Wang for his help on computermodelling. - All my colleagues at the Electronic Engineering Department, Queen Mary College, for their assistance. Finally I would like to convey my gratitude to my husband and my parents, for their understanding, encouragement and everlasting support.

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Table of Contents
Abstract Acknowledgments Table of Contents List of Figures List of Tables List of Abbreviations 1 Introduction 1.1Introduction to Microwave Tubes . . . . . . . . . . . . . . . . . . . . . . . 1.1.1 1.1.2 1.2 Fundamentals of microwave tubes . . . . . . . . . . . . . . . . . . Different microwave tubes and their applications . . . . . . . . . . i iii iv ix xviii xx 1 1 1 2 5 5 6 7 8 9 11

Motivation and Contributions . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.1 1.2.2 1.2.3 Characteristics of magnetron...