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Beam Monitoring System for the ATLAS Experiment at CERN

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Beam Monitoring System for the ATLAS Experiment at CERN Department of Physics, Chemistry and Biology Master’s Thesis Phase and Intensity Monitoring of the Particle Beams at the ATLAS Experiment Christian Ohm LITH-IFM-EX--07/1808--SE CERN-THESIS-2007-055 24/05/2007 Department of Physics, Chemistry and Biology Linköpings universitet SE-581 83 Linköping, Sweden Master’s Thesis LITH-IFM-EX--07/1808--SE Phase and Intensity Monitoring of the Particle Beams at the ATLAS Experiment Christian Ohm Supervisor: Thilo Pauly CERN Examiner: Patrick Norman ifm, Linköpings universitet Linköping, 24 May, 2007 Avdelning, Institution Datum Division, Department Date Division of Computational Physics Department of Physics, Chemistry and Biology 2007-05-24 Linköpings universitet SE-581 83 Linköping, Sweden Språk Rapporttyp ISBN Language Report category — Svenska/Swedish Licentiatavhandling ISRN Engelska/English Examensarbete LITH-IFM-EX--07/1808--SE C-uppsats Serietitel och serienummer ISSN D-uppsats Title of series, numbering — Övrig rapport URL för elektronisk version http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-9614 Titel Intensitets- och fasövervakningssystem för partikelstrålarna vid ATLAS- Title experimentet Phase and Intensity Monitoring of the Particle Beams at the ATLAS Experiment Författare Christian Ohm Author Sammanfattning Abstract At the ATLAS experiment at CERN’s Large Hadron Collider, bunches of protons will cross paths at a rate of 40 MHz, resulting in 14 TeV head-on collisions. During these interactions, calorimeters, spectrometers and tracking detectors will look for evidence that can confirm or disprove theories about the smallest constituents of matter and the forces that hold them together. In order for these sub-detectors to sample the signals from exotic particles correctly, they rely on a constant phase between a clock signal and the bunch crossings in the experiment. On each side of the detector, 175 m away from the interaction point, electro- static button pick-up detectors are installed along the accelerator ring to monitor the beam. A model describing how these detectors function as beam information transducers is constructed and analyzed in order to understand the signal. The focus of this thesis is the design, implementation and testing of a system that uses this signal to monitor the phase between the clock signal and the arrival time of the bunches in the center of the detector. In addition, the system extracts information about the proton beam structure as well as the individual bunches. Given the interaction rate and the complexity of the processes the experiment wants to study, vast amounts of data will be generated by ATLAS. To filter out well-understood phenomena, a trigger system selects only the most interesting events to be saved for further offline analysis. A proposal for how the signals from the button pick-ups can be used as input to the trigger system is therefore also presented. Nyckelord Keywords CERN, ATLAS, particle beam monitoring, experimental physics Abstract At the ATLAS experiment at CERN’s Large Hadron Collider, bunches of protons will cross paths at a rate of 40 MHz, resulting in 14 TeV head-on collisions. During these interactions, calorimeters, spectrometers and tracking detectors will look for evidence that can confirm or disprove theories about the smallest constituents of matter and the forces that hold them together. In order for these sub-detectors to sample the signals from exotic particles correctly, they rely on a constant phase between a clock signal and the bunch crossings in the experiment. On each side of the detector, 175 m away from the interaction point, electro- static button pick-up detectors are installed along the accelerator ring to monitor the beam. A model describing how these detectors function as beam information transducers is constructed and analyzed in order to understand the signal. The focus of this thesis is the design, implementation and testing of a system that uses this signal to monitor the phase between the clock signal and the arrival time of the bunches in the center of the detector. In addition, the system extracts information about the proton beam structure as well as the individual bunches. Given the interaction rate and the complexity of the processes the experiment wants to study, vast amounts of data will be generated by ATLAS. To filter out well-understood phenomena, a trigger system selects only the most interesting events to be saved for further offline analysis. A proposal for how the signals from the button pick-ups can be used as input to the trigger system is therefore also presented. v Acknowledgments First of all I would like to express warm appreciation to my supervisor at CERN, Thilo Pauly, for always taking time for thorough discussions, pedagogical explana- tions and good feedback. I would also like to to thank all my other co-workers at CERN from whom I have learned very much. In particular, Richard Jacobsson and Thomas Aumeyr both contributed with alternative perspectives and viewpoints because of their work with similar systems for two of the other LHC experiments, LHCb and CMS. Patrick Norman, my examiner at Linköping University, also deserves mention for his invaluable help on structuring and shaping the contents of the report (and for waiting for it patiently). I would also like to thank David Sernelius for scruti- nizing the contents and contributing with additional feedback and suggestions at the last stages of finalizing the thesis. Finally, I would like to thank my friends here in Geneva for both help and motivation in my work as well as sharing the distracting activities that has made my time here unforgettable. Geneva, May 2007 Christian Ohm vii Contents 1 Introduction 7 1.1 CERN . .7 1.1.1 The ATLAS experiment at CERN . .8 1.2 Problem description . .9 1.3 Limitations . 10 1.4 Outline for this thesis . 10 2 Background 13 2.1 Accelerator principles . 13 2.1.1 Radio frequency cavities and accelerators . 13 2.1.2 Storage rings . 15 2.2 Large Hadron Collider . 15 2.2.1 Beam production chain . 16 2.3 ATLAS . 18 2.3.1 Trigger system . 19 2.3.2 Timing at ATLAS . 22 2.3.3 Clock signal distribution chain . 22 2.4 Beam Pick-up Timing Experiment . 24 2.4.1 Beam monitoring . 24 2.4.2 BPTX signal as trigger input . 24 2.4.3 BPTX usage at other CERN experiments . 24 3 Beam monitoring system design 27 3.1 The need for a beam monitoring system . 27 3.2 Requirements on a beam monitoring system . 27 3.3 Idea and choice of technology . 27 4 Signal from the beam pick-up detectors 31 4.1 Button pick-up design . 31 4.2 Mathematical modeling . 32 4.2.1 Modeling of a bunch . 32 4.2.2 Modeling of beam pick-up signals . 34 4.2.3 Transmission line effects . 39 4.2.4 Oscilloscope bandwidth effects . 41 4.3 Bunch parameter dependency . 42 ix x Contents 4.4 Beam scenarios . 44 5 BPTX signals as trigger input 47 5.1 Purpose . 47 5.2 Implementation . 48 5.2.1 Hardware . 48 5.2.2 Signal shaping . 49 6 Design of the analysis software 51 6.1 Design overview . 51 6.2 Required features . 53 6.3 Data structures . 53 6.3.1 Waveform . 53 6.3.2 Waveform descriptor . 54 6.4 Data acquisition modules . 55 6.5 Waveform processors . 55 6.5.1 Bunch signal processors . 55 6.6 Phase and BCID associator . 58 6.7 Storage . 59 6.8 Display and presentation . 59 6.8.1 Outlier finder . 59 6.8.2 Plots . 59 6.9 Configuration . 60 7 Results and discussion 61 7.1 Large Hadron Collider . 61 7.2 SPS measurements . 61 7.2.1 Experimental set-up . 62 7.2.2 Signal shape . 63 7.2.3 Satellite bunches . 65 7.2.4 Performance of the oscilloscopes . 65 7.2.5 Acceleration effects . 66 7.2.6 Bunch-by-bunch variations . 68 7.3 Software analysis . 70 7.3.1 Waveform processors . 70 7.3.2 Square pulse signal processor . 71 7.3.3 BCID and phase associator . 72 7.3.4 Data acquisition . 72 7.3.5 Display and presentation . 72 7.4 Simulated LHC data as input . 73 7.4.1 Purpose . 74 7.4.2 Stability for extreme input signals . 74 7.4.3 Stability for different sampling rates . 74 7.4.4 Correlation between input and output parameters . 75 7.5 Hardware tests . 78 7.5.1 Experimental set-up . 78 Contents xi 7.5.2 Performance evaluation . 79 8 Conclusions 81 9 Future work 83 9.1 Remote monitoring . 83 9.2 Benchmarking of the bunch intensity and length measurements . 83 9.3 Refined satellite bunch finder . 83 9.4 Data exchange with other systems . 84 9.4.1 Information Service/Data Interchange Protocol . 84 9.4.2 Conditions database . 84 Bibliography 85 A Signal calculations 87 A.1 Matlab program . 87 B Cable attenuation 94 B.1 Attenuation . 94 B.2 Cable length measurements . 94 C LabVIEW files 96 Nomenclature B The magnetic flux density F The force vector v The velocity of a particle on vector form ρ(t; z) The linear charge distribution that describes a bunch σz The bunch length c0 The speed of light in vacuum cang(z) The angular coverage of the pick-up Cpb The measured pipe-button electrode capacitance e The fundamental charge of a proton Hc(!) The transfer function of the transmission line Hosc(!) The transfer function that describes the limited bandwidth of an oscilloscope Ib(!) The Fourier transform of the button current ib(t) ib(t) The current flowing to a button electrode N The bunch intensity, the number of protons/bunch Qimg(!) The Fourier transform of the image charge Qimg(t) Qimg(t) The image charge collected on a button electrode as a function of time rb The radius of the button electrode Rc The impedance of the transmission line rpipe The radius of the beam pipe t0 The time of arrival for a bunch 1 2 Contents U(!) The Fourier transform of the voltage u(t)
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