Beam Diagnostics in Superconducting Accelerating Cavities: The Extraction of Transverse Beam Position from Beam-Excited Higher Order Modes

128 Pages·2013·7.74 MB·English

by  Pei Zhang (auth.)

Springer Theses
Recognizing Outstanding Ph.D. Research
Pei Zhang
Beam Diagnostics
in Superconducting
Accelerating Cavities
The Extraction of
Transverse Beam Position
from Beam-Excited
Higher Order Modes
Springer Theses
Recognizing Outstanding Ph.D. Research
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Pei Zhang
Beam Diagnostics in
Superconducting
Accelerating Cavities
The Extraction of Transverse Beam Position
from Beam-Excited Higher Order Modes
Doctoral Thesis accepted by
the University of Manchester, UK
123
Author Supervisors
Dr. PeiZhang Prof.Dr. RogerM. Jones
BE-RF The Cockcroft Institute
CERN School ofPhysicsand Astronomy
Geneva The Universityof Manchester
Switzerland Manchester
UK
Dr. Nicoleta Baboi
Machine Diagnosticsand
Instrumentation Group
Deutsches Elektronen-Synchrotron
Hamburg
Germany
ISSN 2190-5053 ISSN 2190-5061 (electronic)
ISBN 978-3-319-00758-8 ISBN 978-3-319-00759-5 (eBook)
DOI 10.1007/978-3-319-00759-5
SpringerChamHeidelbergNewYorkDordrechtLondon
LibraryofCongressControlNumber:2013940765
(cid:2)SpringerInternationalPublishingSwitzerland2013
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Supervisors’ Foreword
Theintroductionofachargedparticlebeamintoanacceleratingcavityself-excites
a wakefield which acts back on the bunch itself (the short-range portion) and on
succeeding bunches (the long-range portion) in a train of accelerated particles.
This wakefield has both transverse and longitudinal components. This work is
focused on the transverse component in the main and on its implications on the
long-rangecomponentinparticular.Thistransversewakefieldcanbedecomposed
intoaseriesofmultipolesand,providingthebeamisguidedclosetotheelectrical
centre of the cavity, the dipole mode is the dominant one. If the progress of this
wakefield is left unchecked, it can appreciably dilute the beam’s quality. Higher
order mode (HOM) couplers are incorporated into the cavity to suppress the
wakefields.Inthiswork,thewakefieldisalsosampledinordertoprovidearemote
diagnostic as to the beam position within the cavity—the amplitude of the dipole
modes excited being directly proportional to the offset of the beam.
This technique has previously been exploited for superconducting accelerating
cavitiesoperatingat1.3 GHzbycolleaguesfromCEA,DESY,SLAC,andFNAL.
However, this work goes further in addressing the application of the method to
thirdharmoniccavitiesforbunchshapinglightsourceapplications.Here,thereare
foursuperconductingcavitieswithinamodule—andalmostallmodesarecoupled
throughout the module. This is a challenging problem to address both computa-
tionally, and to practically implement as a diagnostic. Nonetheless, substantial
progressisindicatedinthistexttoaddressbothissues.Bandsofmodesratherthan
individual modes are found to be particularly effective. Experimental results
indicate a position resolution, based on this technique, of 20 microns over a
complete module offour cavities. Means are also indicated to improve the reso-
lution and to apply this method to other cavity configurations.
v
vi Supervisors’Foreword
This research is distinguished by its clarity and potential for application to
several other international facilities, using superconducting accelerating cavities.
The material is pedagogically appropriate, and indeed highly recommended, for
both students new to the field, and to scientists familiar with the fields of accel-
erating cavities and beam diagnostics.
Manchester, April 2013 Prof. Dr. Roger M. Jones
Hamburg Dr. Nicoleta Baboi
Acknowledgments
First of all, I would like to thank my supervisors, Prof. Roger M. Jones and
Dr. Nicoleta Baboi, for their encouragement and constant support over the last 3
years. They introduced me to the field of accelerator physics and guided me
throughout my Ph.D. I am particularly grateful to Nicoleta for her patience and
immeasurablesupportfromallaspectsofmylifeatDESY.TheHOMBPMproject
could never stand in the current strong position without her passion, experience,
coordination, and strong involvement. Coming from a high energy physics
background,myachievementsinacceleratorphysicstodaywouldcertainlynotbe
possible without Roger’s excellent tutorship.
Based at DESY for the last 3 years, I thank all my colleagues in Hamburg for
their support and guidance, especially Dirk Lipka, Bastian Lorbeer, and Thomas
Wamsat. I also thank Kay Wittenburg and Dirk Nölle for supporting my research
atDESYfrommanyaspects.IextendmygratitudetocolleaguesinMDIgroupfor
helpingmewithHOMmeasurementsatFLASH,namelyKlausKnaack,Reinhard
Neumann and Norbert Wentowski. Inspiring remarks from colleagues in MPY
group deserve a special mention, namely Martin Dohlus and Rainer Wanzenberg.
IthankAlexandrIgnatenkoforhiscompanywhenwesharedanofficetogetherin
Building 30 at DESY. I thank all members of Manchester Electrodynamics and
Wakefields (MEW) group for discussions on the weekly meetings. I particularly
thank Ian Shinton for his numerous help and wise advice. I extend my thanks to
collaborators in Rostock University and FNAL, namely Nathan Eddy, Thomas
Flisgen, Hans-Walter Glock, and Manfred Wendt. Our discussions were always
constructive and our collaborations were both successful and pleasant.
I acknowledge the funding support of EuCARD, DESY, The University of
Manchester and Cockcroft Institute. During my Ph.D., I received bursaries to
attend three international conferences: LINAC2010, IPAC2011 and DITANET
conference on Beam Diagnostics, and financial support for two international
workshops: DIPAC2011 and HOMSC12, I would like to thank the respective
organizationcommitteesandinstitutes.Ialsoappreciatetheopportunitiestoattend
two CAS, one JUAS, one LC, and one DITANET school. My research was
benefited a lot from these conferences and schools.
vii
viii Acknowledgments
Finally, I would like to express my thanks to my parents for their love and
support.MyMumtransmittedmethestrengthtopursuemydreams,whilemyDad
instilled inme the curiosity for all things in nature. Last but not least, I thank my
wife, Xi Wang, for her love and company.
Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1 Free-Electron Laser in Hamburg . . . . . . . . . . . . . . . . . . . . . . . 2
1.2 Third Harmonic Cavities . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.3 Wakefields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
1.3.1 Electromagnetic Field in a Cavity. . . . . . . . . . . . . . . . . 9
1.3.2 Definitions of RF Parameters. . . . . . . . . . . . . . . . . . . . 10
1.3.3 Definition of Wakefields . . . . . . . . . . . . . . . . . . . . . . . 11
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2 Electromagnetic Eigenmode Simulations of the Third
Harmonic Cavity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2.1 The Third Harmonic Cavity as a Periodic Structure. . . . . . . . . . 17
2.2 The Beam Pipe as a Circular Waveguide. . . . . . . . . . . . . . . . . 22
2.3 Eigenmodes of an Ideal Third Harmonic Cavity . . . . . . . . . . . . 25
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
3 Measurements of HOM Spectra. . . . . . . . . . . . . . . . . . . . . . . . . . 31
3.1 Transmission Spectra of an Isolated Single Cavity. . . . . . . . . . . 31
3.2 Module-Based Transmission Spectra . . . . . . . . . . . . . . . . . . . . 35
3.3 Beam-Excited HOM Spectra. . . . . . . . . . . . . . . . . . . . . . . . . . 37
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
4 Analysis Methods for Beam Position Extraction from HOM . . . . . 43
4.1 Data Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
4.2 Direct Linear Regression . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
4.3 Singular Value Decomposition . . . . . . . . . . . . . . . . . . . . . . . . 48
4.4 k-means Clustering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
4.5 Comparison of DLR, SVD and k-means Clustering. . . . . . . . . . 54
4.5.1 Fixed Sample Split . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
4.5.2 Cross-Validation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
ix