Dissertation Application of Waveguide Mode Diagnostics for Remote Sensing in Accelerator Beam Pipes ausgef¨uhrt zum Zwecke der Erlangung des akademischen Grades eines Doktors der Technischen Wissenschaften unter der Leitung von Arpad L. Scholtz E389 Institut f¨urNachrichtentechnik und Hochfrequenztechnik eingereicht an der Technischen Universit¨atWien Fakult¨atf¨urElektrotechnik und Informationstechnik von Thomas Kroyer 9726048 Mitterweg 632, A-7201 Neud¨orfl Wien, 24. August 2005 Zusammenfassung In der vorliegenden Arbeit werden zwei Systeme zur Ferndiagnostik in Strahlrohren von CERN Teilchenbeschleunigern untersucht. Zu diesem Behufe werden Mikrowellen, die sich als Hohlleitermoden im Strahlrohr ausbreiten, verwendet. Das erste System ist eine Anwendung der Reflektometrie in Wellenleitern. Da das Auftreten von St¨orungen im Strahlrohr des Large Hadron Collider (LHC) zu erheblichen Problemen fuhren¨ kann, besteht großes Interesse daran, solche St¨orstellen finden und lokalisieren zu k¨onnen. Zeitbereichsreflektometrie auf Wellenleitern unter Verwendung synthetischer Pulse wird dafur¨ eingesetzt. Das System baut auf einem Netzwerkanalysator auf, wobei der funda- mentale TE und TM Modus im LHC Strahlrohr verwendet wird. Mittels numerischer Signalverarbeitung wird die Verschmierung der Pulse durch Hohlleiterdispersion ent- fernt. Zwei Verwendungsm¨oglichkeiten fur¨ das Reflektometer werden vorgeschlagen, die “Assembly Version” fuer Kontrollen w¨ahrend der Bauphase des LHC und die “In Situ Version” fur¨ Messungen am fertiggestellten LHC, auch nachdem das Strahlrohr eva- kuiert wurde. Fur¨ beide Versionen wurden Koppler entworfen, simuliert und an bis zu 400 m langen Rohren getestet. Die zweite Ferndiagnostikanwendung befaßt sich mit dem Elektronenwolkeneffekt im Super Proton Synchrotron (SPS). Es wurde versucht, die uber¨ die L¨ange gemittelte Elektronendichte im Strahlrohr uber¨ die Transmission von Mikrowellen zu messen. Eine von der Elektronenwolke stammende Phasen- oder Amplituden¨anderung wird mit der SPS-Umlauffrequenz moduliert, was hochempfindliche Seitenbandmessungen erm¨oglicht. Starke strahlinduzierte Signale, die zu unerwunschten¨ Effekten fuhren¨ koen- nen, wurden gefunden. M¨ogliche Erkl¨arungen der beobachteten mit dem Strahl zusam- menh¨angenden Modulation werden diskutiert. Abstract In this work, two remote sensing systems using waveguide modes in beam pipes of CERN accelerators are studied. The first is an application of time domain reflectom- etry in waveguides. Since the emergence of unexpected obstacles in the Large Hadron Collider (LHC) beam screen may lead to major disturbances, it is highly desirable to have a tool for detection and localization of such a fault. Waveguide mode time domain reflectometry using the synthetic pulse technique has been selected for this purpose. The system is based on a vector network analyzer using the fundamental TE and TM mode on the LHC beam-screen. Numerical signal processing is used to remove waveguide dispersion. Two modes of operation for the Reflectometer are proposed, the Assembly Version for inspection during the installation of LHC and the In Situ Ver- sion for measurements with the machine under vacuum. Coupling structures for both versions were designed and simulated, and tests on lines of up to 400 m length were performed. The second remote sensing application turns around the electron cloud effect in the Super Proton Synchrotron (SPS). It was tried to measure the line-averaged electron cloud density by transmitting microwaves along the beam pipe. Any electron cloud- related phase or amplitude shift is modulated by the SPS revolution frequency, making highly sensitive sideband measurements possible. Large beam-induced RF signals lead- ing to parasitic effects had to be coped with. Possible explanations of the observed beam-related modulation are discussed. Contents I Introduction 1 CERN 1 1.1 Accelerators .................................. 1 1.2 Brief introduction to synchrotrons ..................... 3 2 Large Hadron Collider 9 2.1 Machine lay-out ................................ 9 2.2 Beam screen .................................. 12 2.2.1 Properties at cryogenic temperatures ................ 17 2.2.2 Overall attenuation .......................... 20 2.3 Beam vacuum interconnects ......................... 21 2.4 Installation and commissioning ....................... 23 II LHC reflectometer 25 3 Concept 26 3.1 Methods for obstacle detection ....................... 26 3.2 Synthetic pulse reflectometry ........................ 27 3.2.1 Resolution ............................... 28 3.2.2 Range & Sensitivity ......................... 29 3.2.3 Obstacles ............................... 31 3.3 Summary ................................... 31 4 Reflectometer Assembly Version 32 4.1 TE mode coupler ............................... 32 4.2 TM mode coupler ............................... 35 4.3 Data processing ................................ 38 4.3.1 Basic functionality .......................... 39 4.3.2 Advanced features .......................... 46 4.3.3 Backend functions .......................... 46 4.4 Performance .................................. 49 4.4.1 Resolution ............................... 49 4.4.2 Range ................................. 50 4.4.3 Sensitivity ............................... 50 4.4.4 Test obstacles ............................. 51 4.4.5 Multiple reflections .......................... 55 i CONTENTS ii 4.5 Reflectometry on the cryogenic ring line .................. 56 4.5.1 RF properties of the QRL ...................... 58 4.5.2 Measurements ............................ 61 4.5.3 Preliminary conclusion and possible application .......... 64 4.6 Summary ................................... 64 5 Reflectometer In Situ Version 66 5.1 Concept .................................... 66 5.1.1 Desirable features .......................... 67 5.1.2 Location ................................ 67 5.1.3 Modes of operation .......................... 68 5.2 Modified VM module ............................. 69 5.2.1 Coupler design ............................ 70 5.2.2 Realisation of adopted design .................... 81 5.3 Performance .................................. 82 5.3.1 Range ................................. 83 5.3.2 Sensitivity ............................... 83 5.4 Potential fringe applications ......................... 84 5.5 Summary ................................... 85 III Microwave transmission diagnostics 86 6 Electron cloud effect 87 6.1 Introduction .................................. 87 6.2 Electron cloud in the LHC .......................... 90 6.3 Summary ................................... 92 7 SPS microwave transmission experiment 93 7.1 Concept .................................... 93 7.2 Measurement set-up ............................. 94 7.2.1 Machine lay-out ........................... 94 7.2.2 RF measurement equipment ..................... 96 7.2.3 Testing of the measurement set-up ................. 100 7.3 Data acquisition ............................... 102 7.4 Results ..................................... 104 7.4.1 Beams and Machine Parameters .................. 104 7.4.2 Attenuation with beam ....................... 105 7.5 Bench cross-check ............................... 113 7.5.1 Electrons injected into a pipe carrying waveguide modes ..... 114 7.5.2 Resonator measurements ...................... 117 7.5.3 Discussion ............................... 118 7.6 Possible explanations ............................. 119 7.6.1 Electron cyclotron absorption .................... 120 7.6.2 Tails .................................. 120 7.7 Summary ................................... 121 CONTENTS iii Conclusion 121 Bibliography 123 List of Abbreviations 126 Acknowledgements 127 Part I Introduction iv Chapter 1 CERN After the destructions of World War II, European nuclear physics was in a very bad state. Seizing the suggestion of the French physicist Louis de Broglie, the Conseil Eu- rop´eenpour la Recherche Nucl´eaire (CERN), or European Organisation for Nuclear Research was formally established with 11 member states in 1954. For the site of the CERN laboratory complex the quiet village Meyrin a few kilometers outside Geneva close to the French border was chosen. Today CERN has 20 member states and works in close collaboration with many non- member states like the U.S., Russia, India or Japan. The current activities are con- centrated on the construction of the Large Hadron Collider (LHC), which will become operational in 2007. In addition to particle physics there are many spin-offs in other technologies, the most famous being the World Wide Web, that was developed at CERN in 1990-91. 1.1 Accelerators The wide-spread particle physics program being carried out at CERN relies on a num- ber of accelerators. As an accelerator only accepts particles in a given energy range, particles have to go through different acceleration stages for reaching very high ener- gies. CERN’s flagship project, the LHC (Large Hadron Collider) will sit at the end of CERN’s accelerator chain, as shown in Fig. 1.1. Protons being collided in the LHC experiments will already have gone a long way. Emitted by a proton source, they are first run through LINAC2 (Linear Accelerator 2), which they leave with a momentum of 50 MeV/c. Then they are further accelerated by circular machines, namely the Pro- ton Synchrotron Booster (PSB, ejection energy 1.4 GeV), the PS (Proton Synchrotron, ejection energy 26 GeV) and the SPS (Super Proton Synchrotron, ejection energy 450 GeV) before finally being injected into