Development of demountable superconducting contacts for MRI magnet switches

The static background field in commercial magnetic resonance imaging (MRI) body scanners is generated by superconducting magnets operating in persistent mode without a power supply. The magnets typically contain more than 10 joints between separate lengths of Nb-Ti multifilamentary superconducting wire, each of which must have a resistance <10-13 omega to enable persistent mode operation. In order to charge the magnet in the first instance, a special joint is needed to form a superconducting switch. On installation of the magnet, the switch is opened to allow current to be driven into the magnet from a power supply and then closed to form a continuous superconducting circuit so the power supply can be removed. Current technology to fabricate a switch exploits the superconducting/normal transition to drive a length of special superconducting switch wire into the normal state during the charging process by heating. Siemens wish to design and test an alternative switch concept which will offer greater flexibility in the design of their commercial magnets.

Aims and objectives: This project will develop and test a new switch system capable of controlling a high current superconducting circuit through physically demountable superconducting contacts rather than a separate switching circuit. A successful engineering solution will require a much better understanding of how to control the structure and electrical (superconducting) properties of the contact surfaces that form the switching element. The project is intended to take the concept to Technology Readiness Level 4 - offline concept validation. It will involve investigating and selecting suitable contact materials and explore the processing techniques for creating suitable interfaces or intermediary layers between the main current carrying superconducting materials.

Novelty of the research methodology: It is known that superconductor-to-superconductor interfaces rarely allow simple contact joints to operate in the superconducting state because of the short coherence lengths in most available materials and contamination and/or oxidation processes on exposed surfaces. The Superconductivity Group has over the past decade built up a methodology for the design of joints between different classes of superconducting materials, in testing their performance and in correlating this performance with the joint microstructure and nanoscale chemistry analysed using a wide variety of advanced microscopy techniques. The novelty of the project lies in applying these materials selection and analysis techniques to the surfaces and interfaces of a wide range of superconducting materials in order to design and fabricate prototype switches that can be tested using high current facilities in the industrial sponsor, Siemens Healthcare Limited.

This project is supported by EPSRC Industrial Collaborative Award in Science and Engineering voucher number 20000165 awarded to Siemens Healthcare Limited, and falls within the EPSRC research themes Engineering, Healthcare Technologies and Manufacturing the Future. Dr Adrian Thomas and M'hamed Lakrimi will be the industrial supervisors of this project.