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Systematic Studies of a Cavity Quench Localization System

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Systematic Studies of a Cavity Quench Localization System SYSTEMATIC STUDIES OF A CAVITY QUENCHLOCALIZATIONSYSTEM Systematische Studien eines Systems zur Lokalisierung von Cavity Quenches Master Thesis BOSSEBEIN Prof. Dr. Wolfgang Hillert Dr. Lea Steder University of Hamburg Deutsches Elektron Synchrotron, DESY May 23, 2019 ZUSAMMENFASSUNG Es gibt eine Vielzahl von Techniken zur Lokalisierung von Quenchen bei Supraleitenden Radio Frequenz Cavities. Eine davon nutzt dafür Temperaturwellen im flüssigen Helium unterhalb des Lambda-Punktes. Sie werden von einem Quench der Cavity Oberfläche ausgelöst und werden zweiter Schall geannt. Sensoren wie die Oscillating Superleak Transducers sind in der Lage diese Wellen zu detektieren und somit auch die Signallaufzeit des zweiten Schalls zu bestimmen. Diese Technik erlaubt es während eines Cavity Leistungstests parallel den Quench-Ort zu bestimmen. Andere Methoden benötigen einen eigenen kalten Cavity Test oder sie bergen das Risiko die Cavity Oberfläche zu verschmutzen, die dann vor weiteren Tests gereinigt werden muss. Um den wahrscheinlichsten Quench-Ort mit Hilfe der Laufzeiten des zweiten Schalls zu berechnen, existieren unterschiedliche Methoden. Zwei davon werden am DESY genutzt: Multilateration und Raytracing. Auf Basis dieser beiden Methoden wurden im Rahmen dieser Arbeit neue Algorithmen implementiert. Diese schließen einen Algorithmus zur Einführung eines Laufzeit Offsets und einen von der Ausbreitungs- geschwindigkeit der Wellen abhängigen Rekonstruktionsmeachnismus für den Quench-Ort ein. Die Genauigkeiten und die zugrundeliegen- den Randbedingungen dieser Algorithmen werden in der vorliegenden Arbeit untersucht. Ein Datensatz von fast 200 Quench-Ereignissen der 18 getesteten Einzellercavities wird genutzt um eine stochastische Beur- teilung zu erhalten. Um die große Anzahl von Ereignissen auszuwerten, wurden zwei Automatisierungsprozesse eingeführt. In dem Datensatz wurde eine Winkel- und eine Höhenabhängigkeit des rekonstruierten Quench-Ortes beobachtet und den spezifischen Eigenschaften des Experimentaufbaus zugeordnet. Ein allgemeines Phänomen, nämlich dass der rekonstruierte Quench-Ort mehrere Milli- meter oberhalb der Cavity Oberfläche erscheint, ist mit allen genutzten Algorithmen beobachtbar. Um eine Fehlerfortpflanzung zu erhalten, wurde eine Monte Carlo Simulation geschrieben. Mit den entwickelten und verbesserten Algorithmen konnte eine räumliche Auflösung des rekonstruierten Quench-Ortes von 6 oder 8 mm, je nach verwendeter Methode, erreicht werden. Eine Aussage über die Tauglichkeit der ver- schieden Algorithmen zur Quench-Ort-Lokalisierung wird getroffen. Um eine experimentelle Fehlerabschätzung zu ermöglichen, wurde ein Werkzeug zur Kalibrierung, welches eine vom Quench ausgelöste Welle des zweiten Schalles simuliert, entwickelt. Die Simulation basiert auf der Erzeugung von Hitzepulsen an genau bekannten Ortskoordina- ten und Zeitpunkten. Erste Messungen mit diesem Kalibrationswerk- zeug werden am ende dieser Arbeit gezeigt. ii ABSTRACT Several tools for quench localization at Superconducting Radio Fre- quency cavities exist. One of these techniques uses the excitation of temperature waves in liquid Helium below the lambda point. These waves are created by a quench of the cavity surface and are called second sound waves. Special sensors, such as Oscillating Superleak Transduc- ers (OST), are able to detect these waves. With the help of the OSTs the propagation times of the signals are determined. This method allows a parallel measurement of the quench-spot during a standard cavity performance test, whereas other methods require their own dedicated cold test or bear the risk of a contamination of the cavity surface, which requires a re-treatment of the cavity surface afterwards. Using a set of algorithms a most probable spot for the quench location can be calculated via the reconstruction of the second sound wave propagation path. Two such methods are used at DESY: Multilateration and Raytracing. New algorithms are implemented for these two basic methods in the scope of this thesis. They are including a method to introduce a propagation time offset and a second sound velocity dependent recon- struction. The precision, constraints and limits of these algorithms are studied in this thesis. A data set of nearly 200 quench events measured at 18 different tested single-cell cavities is used for a stochastic evalua- tion. To analyze the large number of events two automation processes are introduced. In the data set an angular and height dependency of the reconstructed quench-spot is observed and attributed to specific properties of the experimental setup. A common anomaly that the reconstructed quench- spot appears several millimeters above the cavity surface could be observed with all used algorithms in different manifestations. For the generation of an uncertainty propagation a Monte Carlo simulation is written. Using the improved algorithms developed in the scope of the thesis a spatial resolution of 6 and 8 mm respectively can be achieved with the two best reconstruction methods. A rating of the usability for quench-detection of the different reconstruction algorithms is given as well. In order to enable an experimental based uncertainty estimation of the quench localization a tool for calibration via simulation of a quench generated second sound wave is developed. The simulation is realized by injection of short heat pulses at exact known time and space coordinates. First measurements with the calibration tool are presented in the end. iii CONTENTS Introduction 1 1 Basics of the SRF cavity technology 5 1.1 superconducting cavities .................. 5 1.1.1 Accelerating gradient . 8 1.1.2 Quality factor Q0 .................. 9 1.1.3 The TESLA cavity . 9 1.2 superconductivity ...................... 10 1.2.1 Quench . 12 1.2.2 Quenches in superconducting cavities . 12 1.2.3 Niobium . 13 1.3 liquid helium ........................ 13 1.3.1 Second sound . 15 1.3.2 Sensors for second sound detection . 16 1.4 surface treatment of superconducting cavities ..... 19 1.4.1 Surface polishing . 19 1.4.2 Mechanical cleaning . 20 1.4.3 Thermal treatment . 20 1.4.4 European-XFEL surface treatment procedure . 20 1.5 performance test of cavities ................ 21 1.5.1 Vertical test . 21 1.5.2 Horizontal test . 22 1.6 quench detection ...................... 22 1.6.1 Mode scanning . 22 1.6.2 Temperature mapping . 23 1.6.3 Second sound wave measurements . 24 1.6.4 Optical inspection with OBACHT . 24 1.6.5 Silicone replica . 25 2 Experimental Environment 27 2.1 general setup ........................ 27 2.2 data acquisition ....................... 29 2.3 determining the helium temperature ........... 29 2.4 uncertainty contributions to the obtained data .... 31 2.4.1 Spatial uncertainty . 31 2.4.2 Second sound wave velocity uncertainty . 32 2.4.3 Propagation time uncertainty . 32 2.4.4 Uncertainty propagation . 33 v vi Contents 3 Software and algorithms 35 3.1 the software project .................... 35 3.2 quench detection via second sound measurements .. 36 3.3 time determination ..................... 36 3.3.1 Noise filtering of the OST signals . 37 3.3.2 Automation of the quench-time determination . 38 3.3.3 Running-time determination . 40 3.4 automation of the running-time selection ....... 42 3.4.1 Threshold-cutting . 42 3.4.2 Selection of the running-time . 43 3.4.3 Line-of-sight running-times . 43 3.5 trilateration and multilateration ............ 43 3.5.1 Basic concept . 44 3.5.2 Surface constraint . 45 3.5.3 Spherical Multilateration . 45 3.5.4 Velocity fitting Multilateration . 46 3.5.5 Trilateration combinatorics . 46 3.6 line-of-sight ......................... 47 3.7 raytracing .......................... 47 3.7.1 Conceptual steps of Raytracing . 48 3.7.2 Expansion of the Raytracing code . 50 4 Systematic studies on second sound quench detection 53 4.1 the data set ......................... 53 4.2 anomaly of the reconstructed quench-spot location . 55 4.2.1 Thermal conduction of Niobium . 55 4.2.2 Response time of the reflected RF power . 55 4.2.3 Amplitude dependency of 푢2 ........... 56 4.3 automation of the running-time determination ..... 56 4.4 running-time selection for the reconstruction ..... 57 4.5 results from the individual reconstruction algorithms 59 4.5.1 Combinatorics of Trilateration . 59 4.5.2 Surface constraint of the Multilateration . 60 4.5.3 Spherical Multilateration . 61 4.5.4 Raytracing . 63 4.5.5 Velocity fitting Multilateration and Raytracing . 65 4.6 uncertainty propagation .................. 66 4.6.1 Monte Carlo simulation . 66 4.6.2 Multidimensional contributions (RT, OST, cavity) 69 4.6.3 One dimensional contributions (푢2, QT) . 74 4.6.4 Combination of all uncertainties . 78 4.6.5 Conclusion . 83 4.7 general criteria for a quench detection ........ 84 4.7.1 Angular dependence . 84 4.7.2 Height dependence . 88 4.7.3 Global surface quench . 89 4.7.4 Reflection of second sound waves . 89 Contents vii 5 Experiments towards a calibration tool 93 5.1 motivation .......................... 93 5.1.1 OST speaker . 93 5.1.2 Thermal heater . 94 5.1.3 Artificial quench . 94 5.2 concepts for a resistor based calibration tool ..... 95 5.3 results obtained with a prototype calibration tool .. 96 5.3.1 Experimental setup . 96 5.3.2 Testing sequence . 97 5.3.3 Summary . 100 6 Conclusion 101 6.1 summary ........................... 101 6.2 outlook ........................... 102 Bibliography 105 INTRODUCTION A rather wide field of applications for accelerators exists worldwide. Accelerators are used from low to high
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