1. Overview and prospect of the NMR magnet
The superconducting materials and technological challenges¹
The first superconducting NMR magnet, 200 MHz, used NbZr wire, including shim coils, a persistent-current circuit, and a field stabilization system. The use of NbTi wire gradually replaced NbZr wire, as it had sufficient ductility for wire drawing and better superconducting performance at a high field. The magnets in those days suffered from quenches due to abrupt magnetic flux jumping during the coil charge. Multi-filamentary NbTi wire removed such flux jumping. The upper critical field, Hc2 (i.e., the magnetic field above which superconductivity disappears) of NbTi is ~11 T at 4.2 K, and hence 400 MHz (9.4 T) performance was achieved.
Using an Nb₃Sn inner coil increases the NMR magnetic field, as the H_c2 for Nb₃Sn is ~20 T at 4.2 K. The bronze process enabled forming a multi-filamentary Nb₃Sn wire. Furthermore, a joint with superconducting solder enabled the creation of a superconducting joint for Nb₃Sn wire. Thus, the NMR magnet reached 500 MHz (11.75 T) in the 1980s.
The H_c2 of the Nb₃Sn wire improves by adding titanium or tantalum elements to the bronze-matrix or Nb filaments. Supposing the NMR magnet is cooled at ~2 K, the H_c2 increases by several tesla. The performance of such an NMR magnet reached 1 GHz (23.5 T) in 2009.
Potentials for super-high field NMR magnet
The discovery of high-temperature superconductors (HTS) in 1986 drastically changed superconducting magnet technology. An HTS has a much higher H_c2 than an LTS, suitable for the inner coil of the NMR magnets. Three HTS wires, Bi₂Sr₂Ca₂Cu₃O_10-δ(Bi-2223), B_i2Sr₂Ca₁Cu₂O_8+δ(Bi-2212), and RE (rare earth) Ba₂Cu₃O_7-δ(REBCO), are available.
Japanese workers developed the world's first 1.02 GHz NMR using a Bi-2223 inner coil in 2014. Although a DC supply drove the magnet, a field-frequency lock system stabilized the field ripples. The magnet achieved excellent NMR spectra. Bruker-BioSpin manufactured persistent current 1.2 GHz NMR magnets in 2020 using REBCO inner coils; it has not been revealed whether they installed superconducting joints. ² Further work in Japan develops a 1.3 GHz NMR magnet; it comprises a REBCO inner coil, a Bi-2223 middle coil, and LTS outer coils.³ It is operated in the persistent current mode, using superconducting joints between HTS wires. MIT is developing another driven-mode 1.3 GHz NMR magnet, comprising a REBCO inner coil and LTS outer coils.⁴
The world record for the magnetic field achieved by an HTS magnet is 45.5 T, using a 32 T water-cooled outer coil and an HTS insert.⁵ For all-superconducting magnets, the best field achieved is 32.4 T ⁶, while a 40 T magnet is being designed. Hence, a 1.8 GHz (42.3 T) NMR magnet will be available in the future.
2. Prospect of the super-high field MRI magnet
The specification for an MRI magnet resembles that for an NMR magnet, i.e., excellent magnetic field strength, temporal magnetic field stability, and spatial magnetic field homogeneity are essential. An MRI with a magnetic field ≤ 3 T is used in hospitals for clinical diagnostics. As the magnetic field enhances signal-to-noise ratio and spatial resolution, higher magnetic fields are preferred for medical research, such as neuroscience. So far, one hundred 7 T (~300 MHz) MRI, six 9.4 T (400 MHz), and one 10.5 T have been installed.⁷ NeuroSpin in France recently installed an 11.75 T (500 MHz) MRI.
Like the NMR magnet, super-high field MRI magnets with a magnetic field ≤ 9.4 T (400 MHz) use NbTi coils, cooled at 4.2 K. It is an outside-notch-corrected superconducting solenoid.⁸ A typical 9.4 T (400 MHz) NMR magnet uses 354-km of NbTi wire with a field drift rate of 0.05 ppm/ h and a field homogeneity of ± 1.5 ppm over 30 cm dsv.⁹
The 11.75 T MRI magnet installed in the NeuroSpin comprises an NbTi main-coil, 2 m in outer diameter (o.d.) and 4 m in length; an NbTi screening-coil is 4m in diameter. It is operated in the driven mode using an external current supply. Coils are cooled with superfluid helium at 2 K to ensure cryostability, reducing the risk of a magnet quench.⁷
Based on an actively shielded 14.1 T (600 MHz) MRI magnet design¹⁰, the main-coil comprises an Nb₃Sn inner coil and an NbTi outer coil, 2.1 m in outer diameter, and 3.4 m in length. The total length of the conductor is as long as 1820 km.
Based on the historical evolution of the NMR magnet, a magnetic field up to 18.8 T (800 MHz) may be possible if we use NbTi/Nb₃Sn coils cooled at 2 K. A higher field MRI magnet is achievable if we use HTS inner coils, although the short lengths of the HTS wire that are available, 500 m, will be an obstacle. Main challenges for these super-high field MRI magnets are as follows: a. Suppressing the magnet quench. b. Protecting the magnet in case of a quench. c. Sufficiently reducing the hoop stress due to the electromagnetic force generated.

Acknowledgements

This work is supported by JST-MIRAI Program, JPMJMI17A2, Japan.