An Engineering Guide to Photoinjectors

349 Pages·15.155 MB·English

by  Rao T., Dowell D.H. (Ed.)

A E G P
N NGINEERING UIDE TO HOTOINJECTORS
Edited by
Triveni Rao
David H. Dowell

Copyright © 2013 Triveni Rao, David H. Dowell
All rights reserved.
ISBN-13: 978-1481943222
ISBN-10: 1481943227
DEDICATION

The editors dedicate this book to:

Rama Rao, Krishna Rao and Kavitha Rao for T. Rao

and

Alice Pitt and my parents, Harry and Betty Dowell for D. Dowell

An Engineering Guide to Photoinjectors, T. Rao and D. H. Dowell, Eds. iii
LIST OF CONTRIBUTORS
Wolfgang Anders
Helmholtz-Zentrum Berlin for Materials and Energy
Albert-Einstein-Str. 15, D-12489
Berlin, Germany
Email: [email protected]

Ivan Bazarov
Physics Department
373 Wilson Laboratory
Cornell University
Ithaca, NY 14853
Email: [email protected]

Luca Cultrera
Cornell Laboratory of Accelerator-based Sciences and Education
Cornell University
Ithaca, NY 14853
Email: [email protected]

Dimitre Dimitrov
TechX Corporation
5621 Arapahoe Ave. Suite A
Boulder, CO 80303
Email: [email protected]

David H. Dowell
SLAC National Accelerator Laboratory
Menlo Park, CA 94025-7015
Email: [email protected]

Bruce Dunham
Department of Physics and CLASSE, Wilson Laboratory
Cornell University
Ithaca, NY 14853
Email: [email protected]

John W. Lewellen
Naval Post Graduate School
Monterey, CA 93943
Email: [email protected]
iv An Engineering Guide to Photoinjectors, T. Rao and D. H. Dowell, Eds.
Axel Neumann
SRF Science and Technology/BESSY II
Helmholtz-Zentrum Berlin for Materials and Energy
Albert-Einstein-Str. 15, D-12489
Berlin, Germany
Email: [email protected]

Matthew Poelker
Thomas Jefferson National Accelerator Facility
M/S Bldg. 5A, Room 500-17
12050 Jefferson Ave.
Newport News, VA 23606
Email: [email protected]

Triveni Rao
Brookhaven National Laboratory
Upton, NY 11973
Email: [email protected]

Siegfried Schreiber
Deutsches Elektronen-Synchrotron
Notkestraße 85, D-22603
Hamburg, Germany
Email: [email protected]

John Smedley
Brookhaven National Laboratory
Upton, NY 11973
Email: [email protected]

Thomas Tsang
Brookhaven National Laboratory
Upton, NY 11973
Email: [email protected]
An Engineering Guide to Photoinjectors, T. Rao and D. H. Dowell, Eds. v
PREFACE
Since the discovery of electrons more than a century ago, the generation, transport and characterization of
electron beams have been an active field of research. Breakthroughs in this field have led to applications as
far reaching as cancer treatment, investigation of nanoscale material and dark matter. In this book, we
present a snapshot of the photoinjector technology which has contributed to these advances.

The brightness of a charged particle beam, defined as the number of electrons within the 6-D phase space
volume, dictates its luminosity, a critical parameter in a linear collider. Likewise, since wavelength λ of the
free-electron laser (FEL) is governed by the inequality, ε /γ < λ/4π, the emittance ε of the electron beam
n n
(6-D phase space volume) sets the lower limit on the wavelength deliverable by an FEL while the charge in
this emittance envelope sets the gain of the FEL. As the wavelength of the FEL gets shorter, the required
emittance gets lower and thereby brightness increases. Pushing the limits even further, recently, there has
been greater emphasis on delivering not only low average current beams with high peak brightness, but also
upon increasing the average current to a significant fraction of an ampere as well.

It has been shown consistently in the past two decades that a photoinjector can meet these needs set by the
accelerator community for photon sources and high energy colliders. Over this period, the transverse
emittance of the electron beam has decreased steadily to nearly the intrinsic limit of the electron source; its
bunch length at the source has reduced to femtosecond and the charge per bunch greater than a
nanocoulomb can be delivered routinely. Better understanding of the electron beam production and
transport, resulting from rigorous bench marking of computer codes against experimental measurements,
along with improved diagnostics have led to reliable performance in a user facility environment.

The rapidly advancing technology of high density, relativistic electron beams has made possible a wide
range of new and exciting tools for research and industry. Some of the new applications include linear
particle colliders, Compton scattering sources, electron cooling of protons and heavy ions stored in a ring,
energy recovery linac (ERL) light sources, FELs, inverse FELs and ultrafast electron diffraction. In many
cases these new devices would not be possible without the invention of the photocathode gun. This is
because it alone has the capability to deliver the high quality beam necessary to drive an X-ray FEL,
produce the low divergence beam required to observe an electron diffraction pattern or generate the high
density polarized beams desired for the International Linear Collider (ILC).

This progress motivates the need for a comprehensive presentation of this critical photoelectron gun and
injector technology. Therefore we have taken the audacious step toward writing the first such book on the
technology of photoinjectors in collaboration with our distinguished colleagues. This is an ambitious
undertaking since this broad topic covers subject matter ranging from the chemistry of photocathodes, to the
lasers which drive them, to the HV and RF power systems which extract the electrons and accelerate them
to high energy and myriad of other topics. Because of the wide breadth of this topic, there is no one person
who can be an expert on all these subjects; and so we have collected contributions from renowned
researchers who are builders of the instruments described. This collection of writings not only provides
practical information, but also reflects each author’s unique view of the technology to which they
themselves have made significant contributions. We thank them for their efforts and hope this book
becomes the resource accelerator engineers and scientists open first for information on cathodes, lasers, RF
systems and other parts of the photoinjector.

vi An Engineering Guide to Photoinjectors, T. Rao and D. H. Dowell, Eds.
This book primarily addresses the engineering aspects of a photoinjector. For the purpose of the book, the
injector system is defined to consist of a cathode, a gun for rapidly accelerating the electrons from rest,
followed by the booster accelerator that increases the electron energy to ~10 MeV. In some systems, it may
also include low energy ballistic compression of the bunch.

The book is mainly directed to the practicing scientist or engineer interested in designing and building an
injector. The comprehensive coverage makes it a useful tool supplementing practical considerations for
theoretical understanding. Abstract mathematical equations are kept to a minimum in keeping with the
objective of an engineering guide. For readers interested in delving deeper into the subject, sufficient
references are provided for a more detailed account. Although the latest techniques and results are presented
in the book, the reader should keep in mind that this field is still evolving.

The book is organized as follows: Chapter 1 gives a theoretical underpinning of an injector. Chapters 2, 3
and 4 describe the most common photoinjectors: i) Normal Conducting RF (NCRF) injector, ii)
Superconducting RF (SCRF) injector, and iii) DC injector. Since the photocathode is a common element in
all these injectors, Chapter 5 provides the fundamental physics of photoemission – namely the three-step
model and its relevance to the typical cathodes used in operating systems. Chapters 6 and 7 describe the
most common metal and semiconductor cathodes used to generate unpolarized electrons, their properties
and preparation techniques, as well as how they are incorporated into the gun. Chapter 8 discusses the
system to deliver polarized electron beams. Chapter 9 deals with the choices one has with the laser systems,
the synchronization of the laser to the RF and longitudinal- and transverse-shaping of the laser profile for
low emittance beams. Typically, the RF systems in the photoinjectors are commercial devices; hence, care
must be taken in determining the system and its specifications that are appropriate for the application. This
is discussed in Chapter 10, along with transmitting and coupling the high power to the cavity, controlling
the amplitude and phase of the RF power delivered to the cavity, and design of higher-order mode dampers.
All relevant diagnostics are dealt with in Chapter 11.

The editors wish to specifically acknowledge our co-authors and thank them for finding time in their busy
day jobs to write, review and correct chapters for this book. It simply would not have been possible without
the generous sharing of their knowledge, expertise and time. We are confident that students of the field will
benefit from the unique insights provided by their intimate knowledge of the subject.

T. Rao would like to acknowledge the immense support provided by V. Radeka and Brookhaven National
Laboratory. D. Dowell wishes to thank his many colleagues who have supported and inspired him over the
years. And finally, all the authors are indebted to M. Rumore and A. Woodhead who have read the
manuscript carefully, suggested changes, worked diligently to format the text, figures, references and
equations and design of the final product. They vastly improved its readability and made it look great.

It has been a long process and we extend our heartfelt gratitude to everyone for their patience and
persistence to make this book a reality.

Triveni Rao and David H. Dowell
Upton, NY and Seattle, WA
September 10, 2012
An Engineering Guide to Photoinjectors, T. Rao and D. H. Dowell, Eds. vii
Table of Contents
List of Contributors .......................................................................................................................................... iv
Preface.............................................................................................................................................................. vi
Table of Contents ........................................................................................................................................... viii
CHAPTER 1: PHOTOINJECTOR THEORY .........................................................................................................1
1.1 INTRODUCTION ...................................................................................................................................1
1.1.1 The First Photocathode RF Gun ........................................................................................................4
1.1.2 Summary of Advances ......................................................................................................................4
1.1.3 Organization of this Chapter .............................................................................................................6
1.2 THE RF PHOTOINJECTOR ..................................................................................................................7
1.2.1 The Photocathode RF Gun, Drive Laser and First Accelerator Section ...........................................7
1.2.2 The ERL Injector System ..................................................................................................................8
1.3 BEAM DYNAMICS WITHOUT SPACE CHARGE .............................................................................8
1.3.1. RF Fields and Gun Geometries ........................................................................................................8
1.3.2. Transverse Beam Dynamics in the RF Field ..................................................................................11
1.3.3. Longitudinal Beam Dynamics in the RF Field ...............................................................................14
1.4 BEAM DYNAMICS WITH SPACE CHARGE ...................................................................................17
1.4.1 Space Charge Limited Emission .....................................................................................................17
1.4.2 Space Charge Emittance due to the Bunch Shape ...........................................................................20
1.4.3 Space Charge Emittance due to Non-uniform Transverse Emission ..............................................22
1.4.4 Emittance Compensation Theory ....................................................................................................24
1.5 GUN-SOLENOID OPTICS AND OPTICAL ABERRATIONS ..........................................................30
1.5.1 First-order Optics Model of the Gun ...............................................................................................30
1.5.2 Chromatic Aberration of the Solenoid ............................................................................................32
1.5.3 Geometric Aberrations ....................................................................................................................34
1.5.4 Aberrations due to Anomalous Quadrupole Fields and Emittance Recovery .................................34
1.6 SPACE CHARGE SHAPING ...............................................................................................................39
1.7 SIMULATION CODES ........................................................................................................................42
1.7.1 General Comments on Simulation Fidelity .....................................................................................42
1.7.2 Particle Pushers ...............................................................................................................................44
1.7.3 Particle-in-Cell Codes .....................................................................................................................45
1.7.4 Other Types of Beam Dynamics Codes ..........................................................................................46
1.7.5 Electromagnetic Design Codes and Accelerator Component Modeling .........................................47
1.7.6 General Approach to Injector Modeling .........................................................................................47
viii An Engineering Guide to Photoinjectors, T. Rao and D. H. Dowell, Eds.
Appendix 1.1: Useful Formulae ..................................................................................................................52
Appendix 1.2: Mathematical Symbols ........................................................................................................53
CHAPTER 2: NORMAL CONDUCTING RF INJECTORS .................................................................................55
2.1 INTRODUCTION .................................................................................................................................55
2.2 GUN CAVITY SHAPE AND RF COUPLING SCHEMES ................................................................57
2.2.1 Pillbox and Reentrant Cavity Shapes ..............................................................................................57
2.2.2 Gun RF Theory ................................................................................................................................59
2.2.3 RF Coupling Schemes .....................................................................................................................62
2.3 GUN THERMODYNAMICS AND RF POWER SYSTEM ................................................................63
2.3.1 Thermal Management of High Power RF .......................................................................................64
2.3.2 The Photoinjector Control System ..................................................................................................66
2.4 RF CAVITY FIELDS AND EMITTANCE ..........................................................................................68
2.4.1 RF Transverse Fields and Emittance ...............................................................................................68
2.4.2 RF Modes and the Emittance ..........................................................................................................71
2.5 SOLENOIDS, WAKEFIELD MITIGATION AND DIAGNOSTICS .................................................73
2.5.1 The Emittance Compensation Solenoid ..........................................................................................73
2.5.2 Wakefield Mitigation and Diagnostics in the Gun-to-Linac Beamline ...........................................77
2.6 EXAMPLES OF NCRF GUNS ............................................................................................................78
2.6.1 Low Duty Factor Guns ....................................................................................................................78
2.6.2 High Duty Factor Guns ...................................................................................................................81
2.7 SUMMARY ..........................................................................................................................................83
CHAPTER 3: SUPERCONDUCTING RF PHOTOINJECTORS ...........................................................................87
3.1 INTRODUCTION AND ASSUMPTIONS ..........................................................................................87
3.2 WHY SHOULD WE USE AN SCRF PHOTOINJECTOR? ................................................................88
3.2.1 High Average Current or High Duty Factor Operation ...................................................................89
3.2.2 Small Stand-Alone Installations ......................................................................................................90
3.3 PRESENT STATE-OF-THE-ART .......................................................................................................90
3.3.1 Multi-Cell Gun Designs ..................................................................................................................91
3.3.2 Single-Cell Gun Designs .................................................................................................................92
3.3.3 Hybrids ............................................................................................................................................97
3.4 INITIAL QUESTIONS TO CONSIDER ..............................................................................................97
3.4.1 What are the Performance Goals for the Injector? ..........................................................................98
3.4.2 What System Characteristics are Already Fixed? What Might be Changed
Later? ...................................................................................................................................................98
3.4.3 What Degree of Operational Flexibility is Desired? .......................................................................99
An Engineering Guide to Photoinjectors, T. Rao and D. H. Dowell, Eds. ix
3.4.4 What Cathode(s) will be Used? .....................................................................................................100
3.4.5 What is the Cathode’s Position? Should it be Adjustable? ...........................................................100
3.4.6 How will the Cathode be Mounted? ..............................................................................................101
3.4.7 Is a Single-Cell or Multi-Cell Gun Design Preferred? ..................................................................102
3.5 CAVITY GEOMETRY CONSIDERATIONS ...................................................................................103
3.5.1 Field Emission and Multipacting ..................................................................................................103
3.5.2 Power Dissipation ..........................................................................................................................104
3.5.3 Emittance Compensation ...............................................................................................................104
3.5.4 Power Coupler Options .................................................................................................................104
3.5.5 Injector Cryomodule Diagnostics ..................................................................................................105
3.6 DESIGN PROCESS ............................................................................................................................106
3.6.1 Initial Modeling .............................................................................................................................106
3.6.2 Initial Beam Dynamics ..................................................................................................................107
3.6.3 Refinement ....................................................................................................................................108
3.6.4 Validation ......................................................................................................................................108
3.7 CONCLUDING THOUGHTS ............................................................................................................109
CHAPTER 4: DC/RF INJECTORS .................................................................................................................112
4.1 INTRODUCTION ...............................................................................................................................112
4.2 SYSTEM-LEVEL REQUIREMENTS AND DEFINITIONS ............................................................113
4.3 SUB-SYSTEM DESCRIPTION .........................................................................................................116
4.3.1 Overview .......................................................................................................................................116
4.3.2 High Voltage Gun .........................................................................................................................116
4.3.3 Bunching and Focusing Section ....................................................................................................142
4.3.4 Superconducting Acceleration Section .........................................................................................145
4.4 SUMMARY ........................................................................................................................................149
CHAPTER 5: PHOTOCATHODE THEORY .....................................................................................................154
5.1 HISTORY ............................................................................................................................................154
5.2 THREE-STEP MODEL ......................................................................................................................155
5.2.1 Step 1 – Photon Absorption and Electron Excitation ....................................................................155
5.2.2 Step 2 – Photon Absorption Length and Electron Transport ........................................................157
5.2.3 Step 3 – Escape ..............................................................................................................................159
5.2.4 Yield and QE in the Three-Step Model .........................................................................................160
5.3 THE TRANSVERSE EMITTANCE IN THE THREE-STEP MODEL .............................................161
5.4 MODIFICATIONS TO THE THREE-STEP MODEL.......................................................................163
x An Engineering Guide to Photoinjectors, T. Rao and D. H. Dowell, Eds.