Persistent-mode, Liquid-helium-free, Robust Bi2212 Magnets for MRI and >1GHz NMR

Solid Material Solutions, Llc, North Chelmsford MA

Investigators

Abstract

Superconducting magnets are required for MRI systems that can operate at elevated temperatures and with higher magnetic field without the expensive and in many places, unavailable liquid helium that is required with Low Temperature Superconductors (LTS), and for enabling NMR instruments to operate above the 23 T LTS field limit. High Temperature Superconductors (HTS) provide the best options for advancing the performance of these instruments at the lower-cost elevated temperatures of mechanical refrigeration provided they can be made with the required form, strength, uniformity and current density into persistent current coils. The Magnet Technology Division (MTD) of the MIT Francis Bitter Magnet Laboratory (FBML), our partner in this Phase I STTR Program, was first to recognize and propose to the NIH in 1999, that HTS-based conductors must be incorporated into MRI and NMR magnets in order to surpass these limits. Among HTS conductors, only 2212 wire can provide superior Je in much higher operating temperatures and magnetic fields while in principle enabling wire forms, for example, round and rectangular, that are proven to work with LTS. However, in order to meet this demand, 2212 coils must be developed with: 1) improved wire stress and bend tolerance in long lengths to allow for simpler coil fabrication and operation with large Lorentz forces, 2) capability for superconducting joints to allow much lower costs and more uniform fields via persistent current operation modes, and 3) high, uniform current densities in layer-wound configurations. Solid Material Solutions (SMS) is now developing coiled forms of strong, rectangular 2212 wire with focus in this program on a first-of-its-kind, persistent mode HTS coil that is made with strong 2212 wire and superconducting joints. Firstly, SMS, with its partner, the MTD at MIT-FBML, will develop a superconducting joint technique for its strong, rectangular, high current density 2212 wire. Joint configuration, melt texture parameters and wire / joint Ic?s will be investigated based on the results of a preliminary study that has already demonstrated that a superconducting joint can be produced between 2212 wire ends. Secondly, the technique will be applied to develop superconducting joints and a prototype persistent current switch. Joints will be prepared between reinforced wire ends and heat treatments completed, followed by Ic tests of joints as well as coiled wire sections. Using the best samples, a method for switching to persistent mode will be established with heaters, and with field-decay rates characterized to demonstrate that persistent current carrying joints can be achieved with coiled strong 2212 wire. Thirdly, a first persistent mode demonstration HTS coil will be designed, produced and tested, based on our strong 2212 wire and coil making know-how. This coil will be designed and built so that it can generate a central field of up to about 5 T at or below Ic, with loop-closing superconducting joints, and heater to allow switching to persistent mode. It will be tested at 4.2 K in driven current mode up to Ic and then with a background field for a total field of > 8 T. It will then be charged to different current levels, followed by switching to persistent mode. Tests will be completed to measure its field decay rates in background fields up to about 4 T, in order to characterize persistent mode properties and demonstrate our capability to produce and operate strong-wire based 2212 coils in persistent mode. When fully developed, this advance will enable the practical production of persistent mode HTS magnets based on our 2212 superconductor, for use in liquid He-free, and higher field MRI as well as >1GHz NMR systems.