Persistent mode operation is a specific feature of superconducting magnets exploiting their lack of electrical resistance to trap a current in a closed loop indefinitely. MgB₂ superconductors have a chance to succeed in reaching this favorable operating mode, and to fully qualify for a variety of uses including MRI and NMR.

Persistent mode operation of superconducting magnets is a competitive advantage with respect to conventional electro-magnet solutions. In magnetic resonance imaging (MRI) and nuclear magnetic resonance (NMR), persistent mode (PM) operation of LTS magnets allows to design and manufacture competitive zero-boil off liquid helium systems, and to achieve a superior field stability. Both have become great selling points for superconducting devices, and so they have consequently gained a large market share.

The main components needed to enable PM operation are the superconducting joints and the superconducting switch. For low temperature superconductors (LTS), various solutions have been developed and introduced in mass production of magnets with great success. Therefore, the projected implementation of high temperature superconductors (HTS) in MRI and NMR is not only determined by the availability of long, uniform, performing and affordable wires, but also by the capability to consistently operate HTS magnets in PM.

MgB₂ is the HTS material with most promising impact on MRI systems. Length, cost and performance are steadily improving and cryogen-free operation of MgB₂ based MRI magnets in the temperature range between 6 and 20K has become a realistic target. As a matter of fact, Paramed’s 0.5T open MRI system (1) represents a further demonstration that the wire technology is on the right path, although being still operated in driven mode.

MgB₂ superconducting joints and switches need a comparable level of attention than the wire itself in the path to industrialization of this material for application in MRI and NMR. Many researchers and companies have engaged in the development of MgB₂ superconducting joints, and the typical answer has been substantially positive. In spite of the brittle nature of MgB₂, various experts in the field have reported successful loss-less jointing, mostly on in-situ wires, by means of different technologies, including purely MgB₂ to MgB₂ joints as well as through low-melting temperature superconducting alloys. MgB₂ to NbTi superconducting joints were also reported, implying that a conventional LTS switch circuit may be used for magnets operated at or close to liquid helium temperature. MIT researchers (2) have reported on an in-situ MgB₂ coil having a number of superconducting joints successfully operated at 0.5 Tesla.

At ASG we have been developing a purely MgB₂ to MgB₂ superconducting joint technology applicable to fully reacted MgB₂ wires (3). This is the most challenging goal as it requires the achievement of a negligible contact resistance between superconducting wires having freshly exposed filaments. Moisture and surface oxidation are the typical enemies of such process. In addition, wires with exposed filaments are subject to mechanical stresses that may introduce cracks and microstructural defects acting as supercurrent barriers.

In our process, we mechanically uncover filaments from rectangular ex-situ MgB₂ wires by controlled 3D milling, and then we embed the exposed surfaces in an MgB₂ bulk that is thermally reacted in order to maximize the contact area with the wires and minimize the contact resistance. By such process, the superconducting joint has a very compact size, and can be realized both in shaking and praying hands configuration. The superconducting joint performance is measured inductively, by determining the current decay in closed loops having a trapped current. The resistance criterion we select is very severe, of 10^-14 Ohm.

After the lengthy development of the basic jointing process was completed, we engaged in a reliability test phase. The very same process has been repeated multiple times in order to produce a meaningful statistics for evaluating technology readiness for industrial scale-up. After a few tens of superconducting joint trials, the achieved superconducting joint performance at 20K has been evaluated in 431 A with a standard deviation of 148 A. This value is quite promising and reflects the effective capability to produce MgB₂ superconducting joints with an industrial process. Effort is currently ongoing to further reduce the spread in superconducting joint performance. Most of the residual performance variation has been attributed to specific imperfections in the superconducting loop mounting and in the soldering method of the current leads, and not to the joint itself. Future work will aim at demonstrating PM operation of MgB₂ magnets, followed by technology transfer of the superconducting joint process to the end users of MgB₂ wires in need of such technology.

References:

Paramed's MR Open
Yukikazu Iwasa, “Towards liquid-helium-free, persistent-mode MgB₂ MRI magnets: FBML experience”, Superconductor Science and Technology, Volume 30, Number 5, pg. 53001 (2017)
ASG Superconductors SpA, patent number WO2009127956A1

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