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Persistent current joints between NbTi superconducting wires

Abstract: Persistent current joints are critical components of superconducting magnets,and improvements in their properties are key to the production ofnext generation devices. Despite having been made routinely for morethan half a century, the science underpinning the performance of jointsbetween NbTi wires remains poorly understood. At this critical juncture,where new jointing methods are being sought for NbTi, it is essential thatwe develop a sound understanding of the factors inuencing joint quality.The aim of this thesis was to identify the characteristic microstructuraland superconducting properties of joints produced by the three standardjointing methods for NbTi: soldering, spot welding and cold pressing.There were three main components to this work. The first was to obtaina complete characterisation of Jc(B, T) and Bc2(T) for the raw materialsfrom which the joints were made, which was done by standard magneticmethods. Where relevant, the results have been interpreted in terms ofthe basic pinning mechanisms and microstructures in the materials tested.The implications for joint performance are discussed.Secondly, a novel magnetic method was developed to measure the superconductingperformance of joints over an unprecedented range of operationalconditions. The method involves winding a very small NbTi coil,closing the coil with the joint of interest, and testing the sample in a conventionalSQUID magnetometer. The current carrying ability of jointscan be measured simply from magnetic hysteresis loops, which enabledthe performance of a range of joints to be analysed in fine detail and withconsiderable convenience. Furthermore, owing to the extremely low coilinductance and highly sensitive SQUID magnetometer employed, currentdecay curves corresponding to voltages as low as 10-14 V could be obtainedin very short measurement times of just 20 minutes. Unfortunately, unexpectedlylarge drift in the background magnetic field was found to dominatein most measurements, but nevertheless the measurements providedan extremely rapid method of proving that the joints are truly superconducting.Suggestions are made as to how the technique could be improvedto permit precise measurements of joint Ic(B, T) and V-I curves in thepersistent mode regime, which would enable much needed fundamentalstudies into superconductivity in joints.Finally, the joint testing technique was employed to measure the superconductingperformance of soldered, spot welded and cold pressed NbTijoints. The results were analysed along with the raw material superconductingproperties and microstructures in the joints, which were obtainedby scanning electron microscopy (SEM). This allowed the performancelimitingfactors in each joint to be established. As a result of this study, anew soldering/cold pressing method is proposed which may permit directmetal-to-metal bonding between NbTi materials without the need for Pbor harmful chemicals.

Type of Award:DPhil Level of Award:Doctoral Awarding Institution:University of Oxford

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Engineering and Physical Sciences Research Council more by this funder

Grant numberEP/K503113/1 Received ByProject

  Item Description

Type: Thesis Language: English Keywords: Persistent current joints Current decay method Superconducting joints Inductive resistance testing Niobium titanium wires Subjects: Superconductors Superconducting joints Superconducting magnets

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Author: Greg Brittles - AffiliationUniversity of Oxford Role Author - - - Chris Grovenor More by this supervisor - Role Supervisor - - -

Source: https://ora.ox.ac.uk/objects/uuid:0468d27b-4d79-4ff0-a130-b9ce38b1adcb



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