A. Otsuka

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.

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).

NMR Upgrading Project Towards 1.05 GHz

An NMR spectrometer over 1 GHz requires the contribution of high-Tc superconductors(HTS). However, a persistent-mode operation with HTS cannot satisfy the field stability of 0.01 ppm/h at present. This is a great barrier for applying HTS to an NMR magnet. To overcome this problem, a new project was undertaken in Japan in October 2006. In the course of the project, we will develop a highly stabilized power supply, field-compensation methods, and measurement techniques that allow a certain field fluctuation. By integrating them, the feasibility of HTS to NMR will be demonstrated. We performed a long-term operation of a 600 MHz NMR magnet in the driven-mode. Allowable field fluctuation of the existing internal lock system for solution NMR was evaluated by a model experiment. As the next step, the innermost Nb3Sn coil of the 600 MHz NMR magnet will be replaced with a Bi-2223 coil, and the field homogeneity, as well as the field stability, will be evaluated. In the final step of the project, the replacement of the innermost coil of the existing 920 MHz NMR magnet will be planned. The targeting field is 24.7 T (1.05 GHz for 1H NMR resonance frequency). The solid-state NMR on 17O nuclei in a labeled peptide will be demonstrated using a magic angle spinning probe; the probe has a 1H decoupling frequency of 1.05 GHz.

REBCO Layer-Wound Coil Tests Under Electromagnetic Forces in an External Magnetic Field of up to 17.2 T

A nuclear magnetic resonance (NMR) system using a high-temperature superconducting (HTS) magnet and a probe with an HTS radio frequency coil is currently under development. The HTS NMR magnet is expected to reduce the volume occupied by the magnet and to encourage users to install higher field NMR systems. RE-Ba-Cu-O-coated (REBCO-coated) conductors offer the advantages of a higher critical current density than low-temperature superconductors in magnetic fields above 10 T as well as tolerance to high tensile stress. Both of these factors are expected to lead to a reduction in the volume of the magnet. Four REBCO layer-wound test coils were fabricated in order to investigate their properties under electromagnetic forces in an external magnetic field of up to 17. 2 T. The REBCO coils with a practical inner diameter were successfully operated under electromagnetic forces of over 200 MPa in high magnetic fields.

A 1.3 GHz NMR Magnet Design Under High Hoop Stress Condition

NMR magnets using high-Tc superconductors (HTS) to generate high magnetic fields exceeding 25 T are currently being designed by several organizations. In these designs, the HTS is used for the inner coils, and the other coils consist of NbTi and Nb3Sn wires. The YBCO wire, which is a typical HTS, has excellent critical current performance over a wide range of magnetic fields and tolerates high tensile stress of up to 700 MPa. These properties make it possible to realize a high-field NMR magnet. In particular, the superior mechanical strength allows for the high-stress criterion of the electromagnetic force to be applied to the design of the magnets. In this study, we show the conceptual design of 1.3 GHz (30.5 T) NMR magnets under the condition of high hoop stress of 500 MPa. To achieve high magnetic field homogeneity in these designs, we propose three magnet design plans that have different arrangements of the compensation coils. We assumed that the magnet would be operated by the driven mode at 4.2 K. We also considered the strong angular dependence of the critical current of the YBCO wires to design the magnet.

Field Stability of a 600 MHz NMR Magnet in the Driven-Mode Operation

Although high-temperature superconductors (HTS) are very promising for high-field generation over 25 T, it is difficult to apply them to an NMR magnet because of their low index values and the difficulty caused by superconducting joints. The properties of HTS appear to cause poor magnetic field stability in the persistent-mode operation. Therefore, in this study, a high-field NMR magnet including HTS coils will be operated in the driven-mode. In order to evaluate the magnetic field stability in the driven-mode, we modified a 14 Tesla (600 MHz) vertical NMR magnet. With regard to the magnet, persistent switches for axial shim coils (z0, Z1, Z2) as well as the main coil were removed for constant operation with a power supply. In addition, a 1 W Gifford-McMahon (GM) cryocooler at 4 K and HTS current leads were installed in the cryostat to re-condense boiled helium gas. As a result, there was a long-term change of about 6 ppm in the magnetic field stability, but, in the short-term (a few hours), a change of about 2 ppm was observed. By the z0 shim control, in combination with the NMR field measurement, an averaged magnetic field drift of less than 0.0001 ppm/h was achieved.

Drift Compensation of 600 MHz NMR Magnet

Although high-temperature superconductors (HTS) are very promising for high-field generation over 25 T, it is difficult to apply them to an NMR magnet because of their low index values and the difficulty caused by superconducting joints. We have developed a drift compensation technique to apply HTS to a high-resolution NMR using a 14 T (600 MHz) vertical NMR magnet. The magnet had a poor magnetic field stability of about -0.7 ppm/hour, so a drift compensation unit based upon a flux pump method was added. The unit consisted of nested inner (secondary) and outer (primary) coils. The inner coil was connected in series to the main coil circuits, and the outer coil was connected to the auxiliary power supply to sweep the output current very slowly. While the current of the outer coil was changed at an adequate sweep rate by the power supply, the current was induced by inductive coupling in the inner coil. The induced current canceled out the decay of the main coil current that caused the poor drift of the magnet. With the drift compensation unit, the magnetic field drift was improved to less than 0.0001 ppm/hour for 3 days at 14 T. This period was long enough for one NMR measurement.

Evaluation of the Screening Current in a 1.3 GHz NMR Magnet Using ReBCO

NMR magnets using high-Tc superconductors (HTS) are currently by several organizations designed to generate high fields exceeding 25 T. In such designs, the HTS are used for the inner coils, and the outer coils consist of NbTi and Nb3Sn wires. A ReBCO wire with a Hastelloy substrate has excellent critical current performance over wide range of magnetic fields and tolerates high tensile stress up to 700 MPa. We have previously shown conceptual designs in which the main coils are assumed to use ReBCO wires. The designs suggest the potential to realize much more compact and lightweight magnets than conventional magnets with low-Tc superconductors (LTS) and HTS. On the other hand, the problem of screening current induced by the perpendicular field remains to be solved. In this study, we evaluate numerically the amount of the magnetic field generated by the screening current in ReBCO coils. Compensation coils for improving field homogeneity were found to reduce the shielding current by several percentage points. The improved coil arrangement is reported considering the shielding current effect.

HTS Magnetic Field Damper for Short-Term Fluctuations in the Driven-Mode

Although high-temperature superconductors (HTS) are very promising for high-field generation over 25 Tesla, it is difficult to apply them to an NMR magnet because of their low index values and the difficulty caused by superconducting joints. The properties of HTS appear to cause poor magnetic field stability in the persistent-mode operation. Therefore, in this study, a high-field NMR magnet including HTS coils will be operated in the driven-mode. We evaluated the magnetic field drift of a 14 Tesla (600 MHz) NMR magnet in the driven-mode. The magnetic field stability in several hours was about 2 ppm. However, in a shorter-term, magnetic field fluctuations with a main period of approximately 0.01 Hz remained. These fluctuations are unfavorable for a precise NMR measurement. In order to reduce the field fluctuation, we tested the damper coil by making a closed-loop circuit using Bi-2223 tapes. The damper coil was cooled by a single-stage GM cryocooler in the magnetic field. The inside magnetic field of the damper coil was measured with an NMR Teslameter. The shielding current in the damper coil canceled out the field fluctuation, which indicates that the damper coil worked effectively.

Operation of Wax-Impregnated GdBCO Layer-Wound Coil Using Cryocoolers

RE-Ba-Cu-O (REBCO) high-temperature superconducting conductors have the advantage of high critical current density in a magnetic field above 10 T as well as tolerance to high tensile stress. REBCO high-temperature superconducting coils are expected to be applied to superconducting magnets using cryocoolers. While epoxy-resin-impregnated superconducting coils are considered to be appropriate for superconducting magnets using cryocoolers because the impregnation provides sufficient thermal conductivity, the degradation of epoxy-resin-impregnated REBCO coils due to extremely low nominal cleavage strength is a serious problem. This problem is negligible for wax (paraffin)-impregnated REBCO coils. A wax-impregnated GdBCO layer-wound coil using cryocoolers was successfully operated at a current between