Kyoji Zaitsu

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).

Bi-2223 Innermost Coil for 1.03 GHz NMR Magnet

Because of their high critical fields, high-Tc superconductors (HTS) are considered to be the only solution to dramatically increase the highest fields of NMR magnets. We have successfully demonstrated that a 500 MHz HTS/LTS NMR system with a Bi-2223 innermost coil could be used for solution NMR in a driven-mode operation. As the next step, the upgrade of the 920 MHz NMR system installed at the Tsukuba Magnet Laboratory is underway. The innermost Nb3Sn coil has been replaced by a Bi-2223 coil. The coil was fabricated as a layer-wound coil using five Bi-2223 conductors reinforced with bronze tapes. It was connected in series with the outer Nb3Sn and NbTi coils. The magnet is expected to generate a field of 24.2 T (1.03 GHz of 1H resonance frequency) at an operating current of 244.4 A. The test using the Bi-2223 coil and the outer Nb3Sn coils in combination was successfully carried out. The coil has been installed in the 1.03 GHz NMR magnet. Its cooling and operation are scheduled to take place within Fiscal Year 2010.

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.

The Characteristics of Bi-2223/Ag Conductor for High Field Application

For the development of high magnetic field application using high temperature superconductor (HTS), Bi-2223/Ag tape is one of the promising materials because of its high current capacity and upper critical field. However, Bi-2223/Ag tape shows very strong anisotropy and significant deterioration of critical current in the magnetic field. To explore the possibility of HTS in high field applications, we evaluated the critical current and index value of Bi-2223/Ag conductor in high magnetic field. The Bi-2223/Ag tape was characterized as a function of increasing and decreasing field. The temperature dependence was also examined lower than 4.2 K. All measurements were carried out with hybrid magnet system up to 30 T and the experimental results are reported.

Development of Nb 3 Sn Superconducting Wires for High-field Magnets

Synopsis: Research and development activities and some recent results related to Nb3Sn superconducting wires produced by Kobe Steel, Ltd. and Japan Superconductor Technology Inc. (JASTEC) are introduced. An outline of the activities is described from a historical point of view. Improvements in the characteristics (i.e., critical current density (Jc), n-value and mechanical properties) of bronze-processed Nb3Sn wires are reviewed. Finally, the status of development for the Ta-Sn powder-in-tube (TS-PIT) process newly proposed by Tachikawa is described.

Influence of Wire Parameters on Critical Current Versus Strain Characteristics of Bronze Processed

In order to develop bronze processed Nb₃Sn wire for the ITER CS coil operating under higher compressive strain, the influence of various parameters of wires such as filament diameter, barrier materials, barrier thickness, heat treatment pattern and Ti addition on critical current (I_c) versus intrinsic strain ε_ν(-1.0% <; ε_ν <; +0.1%) characteristics was investigated. The change of these parameters brought significant changes to superconducting properties such as I_c and n-value. In spite of different wire parameters, the strain dependency of normalized I_c was almost the same, except that a Ti addition affects the upper critical field B_c2. This result suggests that assuming the same Ti-addition level, Nb₃Sn wire with higher performance at a certain ε_ν would exhibit higher performance at any ε_ν in the compressive regime. Based on the result, bronze processed Nb₃Sn wires with non-Cu critical current density more than 1100 A/mm² at 12 T, 4.2 K, zero applied strain have been successfully developed for the CS coil.

{\hbox {Nb}}_{3}{\hbox {Sn}}

Nb/sub 3/Sn multi-filamentary superconducting wires prepared by Ta-Sn powder in tube (TS-PIT) process have been developed. To develop the TS-PIT wire with applicable scale, a two stage approach was examined. At the first stage, two kinds of test wires with single and 19 filaments were fabricated by using 10 kg class billets. The critical current density (Jc) of the single filament wire was 167 A/mm/sup 2/ at 21 T and 4.2 K. Based on results of the first stage developments 50 kg class scale wire with 54 filaments was progressed in the following stage. This wire showed Jc of 136 A/mm/sup 2/ at 21 T, 4.2 K. Beside Jc, RRR and 0.2% yield strength were estimated. From these estimations, it was clear that the wire in the second stage has efficient applicable properties to the high field magnet. In both stage, microscopic analysis were followed for samples and found that over 50% in the sheath area reacted uniformly and the grain size was extremely fine. Through this development, TS-PIT processed multi-filament Nb/sub 3/Sn superconducting wire has been developed to applicable quality and scale.

Superconducting Wires

In this study, we investigated in field properties of high-Tc superconductor (HTS) tapes electrically as well as mechanically. Metallic reinforced Bi-2223/Ag superconductor tapes were used in this work. The critical current (Ic), which is highly dependent on the magnetic field, was measured using fields applied parallel and perpendicular to the tape surface. The conductor was exposed to fields of up to 28 T at 4.2 K. In order to confirm the performance of tapes in practical NMR operation, Ic was also measured around 2 K. The conductor was wound on a large diameter bobbin and mechanical properties were then measured as the coil current was increased. Finally, we verified the conductor properties from coil Ic and maximum stress/strain of it.