Yoshinori Yanagisawa

Recent Developments in High-Temperature Superconducting Magnet Technology (Review)

The use of magnets made of high temperature superconductors (HTS) such as BSCCO and REBCO easily provide higher magnetic fields and higher operating temperatures, enabling dramatic improvements in superconducting magnet technology. The LTS magnet technology is very well summarized in text books written by M. N. Wilson (Superconducting magnets, Clarendon Press Oxford, 1983) and Y. Iwasa (Case studies in superconducting magnets, 2nd edition, Springer, 2009), covering such topics as stability, protection, ac loss and so forth. To the contrary, HTS conductors were commercialized only recently and therefore the magnet technology for HTS conductors remains undeveloped, especially so in the case of REBCO conductors. The technological problems for HTS coils thus far encountered are 1) an enormous effect of a screening current-induced magnetic field, 2) degradation in the coil performance due to excessive mechanical stresses applied along the longitudinal and transverse direction, and 3) the difficulty in protecting the magnet in the case of an abrupt thermal runaway. This paper reviews recent progress in overcoming these technological problems for HTS magnets. Both BSCCO and REBCO conductors have been used for HTS magnets in areas such as high field facilities, NMR, MRI, magnetic levitation trains and so forth. The effect of the screening current is the major problem for NMR, MRI, and accelerators, as it substantially distorts spatial field homogeneity and temporal field stability; on the other hand, degradation due to excessive stresses is substantial for high field magnets. Additionally, coil protection is a common and substantive problem among high current density HTS magnets in general. World-wide activities in developing BSCCO and REBCO magnets are overviewed in this paper.

Achievement of 1020 MHz NMR

We have successfully developed a 1020MHz (24.0T) NMR magnet, establishing the worlds highest magnetic field in high resolution NMR superconducting magnets. The magnet is a series connection of LTS (low-Tc superconductors NbTi and Nb3Sn) outer coils and an HTS (high-Tc superconductor, Bi-2223) innermost coil, being operated at superfluid liquid helium temperature such as around 1.8K and in a driven-mode by an external DC power supply. The drift of the magnetic field was initially ±0.8ppm/10h without the (2)H lock operation; it was then stabilized to be less than 1ppb/10h by using an NMR internal lock operation. The full-width at half maximum of a (1)H spectrum taken for 1% CHCl3 in acetone-d6 was as low as 0.7Hz (0.7ppb), which was sufficient for solution NMR. On the contrary, the temporal field stability under the external lock operation for solid-state NMR was 170ppb/10h, sufficient for NMR measurements for quadrupolar nuclei such as (17)O; a (17)O NMR measurement for labeled tri-peptide clearly demonstrated the effect of high magnetic field on solid-state NMR spectra.

Effect of YBCO-Coil Shape on the Screening Current-Induced Magnetic Field Intensity

A numerical simulation method which deals with the screening current-induced magnetic field for YBCO coils, including the self field effect induced by the transport current, has been developed. The simulation agrees well with the experimental results for an YBCO solenoid. Based on the numerical simulation, the effect of coil shape on the screening current-induced magnetic field intensity for the YBCO coils has been investigated. The field was demonstrated to reach a maximum if the solenoid corresponds to the minimum-volume design; it amounts to as large as -18% of the central magnetic field. Two major problems must be considered for YBCO coils regarding the screening current: (a) a reduction in the central magnetic field by the screening current and (b) a temporal drift of the apparent magnetic field due to relaxation of the screening current by flux creep. It is suggested that the latter can be suppressed by a current sweep reversal technique.

Magnitude of the Screening Field for YBCO Coils

Screening current induced in a YBCO-coated conductor coil causes two major problems; (i) reduction in the central magnetic field and (ii) temporal magnetic field drift due to flux creep. They constitute disadvantages for YBCO coil applications such as NMR, MRI, accelerator and high field magnets. The second problem is effectively suppressed by current sweep reversal, while the first remains unsolved. The present paper demonstrates that the screening current-induced magnetic field (screening field) is dominated by (a) the YBCO coil shape, (b) the YBCO-coated conductor width, (c) the coil inner diameter and (d) the ratio of operating current to the coil critical current. The dependence on these quantities is systematically investigated by numerical simulations. We conclude that coils with a smaller width of YBCO-coated conductor, a larger inner diameter and a higher ratio of operating current to the coil critical current generate a smaller central screening field ratio.

Operation of a 500 MHz high temperature superconducting NMR: towards an NMR spectrometer operating beyond 1 GHz.

We have begun a project to develop an NMR spectrometer that operates at frequencies beyond 1 GHz (magnetic field strength in excess of 23.5 T) using a high temperature superconductor (HTS) innermost coil. As the first step, we developed a 500 MHz NMR with a Bi-2223 HTS innermost coil, which was operated in external current mode. The temporal magnetic field change of the NMR magnet after the coil charge was dominated by (i) the field fluctuation due to a DC power supply and (ii) relaxation in the screening current in the HTS tape conductor; effect (i) was stabilized by the 2H field-frequency lock system, while effect (ii) decreased with time due to relaxation of the screening current induced in the HTS coil and reached 10(-8)(0.01 ppm)/h on the 20th day after the coil charge, which was as small as the persistent current mode of the NMR magnet. The 1D (1)H NMR spectra obtained by the 500 MHz LTS/HTS magnet were nearly equivalent to those obtained by the LTS NMR magnet. The 2D-NOESY, 3D-HNCO and 3D-HNCACB spectra were achieved for ubiquitin by the 500 MHz LTS/HTS magnet; their quality was closely equivalent to that achieved by a conventional LTS NMR. Based on the results of numerical simulation, the effects of screening current-induced magnetic field changes are predicted to be harmless for the 1.03 GHz NMR magnet system.

Generation of 24 T at 4.2 K using a layer-wound GdBCO insert coil with Nb3Sn and Nb–Ti external magnetic field coils

High-temperature superconducting (HTS) magnets are believed to be a practical option in the development of high field nuclear magnetic resonance (NMR) systems. The development of a 600 MHz NMR system that uses an HTS magnet and a probe with an HTS radio frequency coil is underway. The HTS NMR magnet is expected to reduce the volume occupied by the magnet and to encourage users to install higher field NMR systems. The tolerance to high tensile stress is expected for HTS conductors in order to reduce the magnet in volume. A layer-wound Gd–Ba–Cu–O (GdBCO) insert coil was fabricated in order to investigate its properties under a high electromagnetic force in a high magnetic field. The GdBCO insert coil was successfully operated at a current of up to 321 A and an electromagnetic force BJR of 408 MPa in an external magnetic field generated by Nb3Sn and Nb–Ti low-temperature superconducting coils. The GdBCO insert coil also managed to generate a magnetic field of 6.8 T at the center of the coil in an external magnetic field of 17.2 T. The superconducting magnet consisting of GdBCO, Nb3Sn and Nb–Ti coils successfully generated a magnetic field of 24.0 T at 4.2 K, which represents a new record for a superconducting magnet.

Operation of a 400 MHz NMR magnet using a (RE:Rare Earth)Ba2Cu3O7−x high-temperature superconducting coil: Towards an ultra-compact super-high field NMR spectrometer operated beyond 1 GHz

High-temperature superconductors (HTS) are the key technology to achieve super-high magnetic field nuclear magnetic resonance (NMR) spectrometers with an operating frequency far beyond 1GHz (23.5T). (RE)Ba2Cu3O7-x (REBCO, RE: rare earth) conductors have an advantage over Bi2Sr2Ca2Cu3O10-x (Bi-2223) and Bi2Sr2CaCu2O8-x (Bi-2212) conductors in that they have very high tensile strengths and tolerate strong electromagnetic hoop stress, thereby having the potential to act as an ultra-compact super-high field NMR magnet. As a first step, we developed the worlds first NMR magnet comprising an inner REBCO coil and outer low-temperature superconducting (LTS) coils. The magnet was successfully charged without degradation and mainly operated at 400MHz (9.39T). Technical problems for the NMR magnet due to screening current in the REBCO coil were clarified and solved as follows: (i) A remarkable temporal drift of the central magnetic field was suppressed by a current sweep reversal method utilizing ∼10% of the peak current. (ii) A Z2 field error harmonic of the main coil cannot be compensated by an outer correction coil and therefore an additional ferromagnetic shim was used. (iii) Large tesseral harmonics emerged that could not be corrected by cryoshim coils. Due to those harmonics, the resolution and sensitivity of NMR spectra are ten-fold lower than those for a conventional LTS NMR magnet. As a result, a HSQC spectrum could be achieved for a protein sample, while a NOESY spectrum could not be obtained. An ultra-compact 1.2GHz NMR magnet could be realized if we effectively take advantage of REBCO conductors, although this will require further research to suppress the effect of the screening current.

HTS-NMR: Present Status and Future Plan

Using high-Tc superconductors (HTS) is considered to be the only solution to dramatically increase the highest fields of NMR magnets because of their high critical fields. However, it is not easy to apply HTS to an NMR spectrometer (HTS-NMR) because a persistent-mode operation with HTS cannot satisfy the field stability of 0.01 ppm/h at present. To overcome this problem, we are now developing an HTS-NMR spectrometer in a driven-mode operation. As the first step, a layer-wound coil was fabricated with bronze-reinforced Bi-2223 conductors. Instead of the Nb3Sn coil, the Bi-2223 coil was installed as the innermost part of an existing NMR magnet. The magnet operated at a field of 11.7 T with a highly stabilized power supply. NMR measurements were carried out, and it was demonstrated that the quality of the multi-dimensional NMR spectra on the protein was equivalent to that obtained with a persistent-mode system. The next step will be to demonstrate its usefulness as a high-field NMR system. The upgrade of the 920 MHz NMR system installed at the Tsukuba Magnet Laboratory is underway. Its innermost coil is scheduled to be replaced by a Bi-2223 layer-wound coil for 2010. Its target field is 24.2 T (1.03 GHz).

Towards beyond-1 GHz solution NMR: internal 2H lock operation in an external current mode.

We have commenced a project to develop a beyond-1 GHz solution NMR spectrometer using a HTS coil. Due to a small residual resistance present in the HTS conductor and joint resistance between conductors, a stable persistent current sufficient for NMR measurements is unlikely. Therefore, a current has to be supplied to the HTS coil from an external power supply. The ripple of an external power supply causes a field fluctuation which must be stabilized. In this study we show results of NMR measurements using a 500-600 MHz NMR in such an external current mode: the field fluctuations are stabilized by an internal 2H lock. The field fluctuation from the external power supply comprises a major field fluctuation component at low frequencies, 0.003-0.005 Hz, and superimposed minor field ripples at 2 Hz and 50 Hz. The former limits the time interval of the internal 2H lock, while the latter generates sidebands in the NMR spectrum. Sideband and baseline noise are controlled by appropriate selection of the feedback loop parameters of the lock. The quality of the 1D-solution NMR spectra observed in external current mode is equivalent to that obtained in persistent current mode. However, if the feedback loop time is as short as the gradient pulse width, refocusing of the NMR signal is lost and NMR peaks disappear. The 2D-NOESY and the 2D-HSQC spectra of ubiquitin in an external current mode have been acquired. The quality of the 2D spectra is equivalent to those obtained in persistent current mode; i.e. the internal 2H lock operates stably over an experimental time interval of 40-50 min. To realize a beyond-1 GHz NMR spectrometer, further investigations must be made of (i) the long term stability of a DC power supply, (ii) the enhancement of the compensation field limit for the internal 2H lock, (iii) the extension of the helium refill time interval, and (iv) a method to correct the field homogeneity in the external current mode.

1020 MHz single-channel proton fast magic angle spinning solid-state NMR spectroscopy

This study reports a first successful demonstration of a single channel proton 3D and 2D high-throughput ultrafast magic angle spinning (MAS) solid-state NMR techniques in an ultra-high magnetic field (1020MHz) NMR spectrometer comprised of HTS/LTS magnet. High spectral resolution is well demonstrated.