高尾 智明

<jats:title>Abstract</jats:title> <jats:p>This paper describes the first persistent-mode medium magnetic field (400 MHz; 9.39 T) nuclear magnetic resonance (NMR) magnet which uses superconducting joints between high-temperature superconductors (HTSs). As the ultimate goal, we aim to develop a high-resolution 1.3 GHz (30.5 T) NMR magnet operated in the persistent-mode. The magnet requires superconducting joints between HTSs and those between an HTS and a low-temperature superconductor (LTS). Towards this goal, we have been developing persistent-mode HTS inner coils to be operated in a 400 MHz (9.39 T) NMR magnet and here we present the first prototype inner coil wound with a single piece (RE = rare earth)Ba<jats:sub>2</jats:sub>Cu<jats:sub>3</jats:sub>O<jats:sub>7−<jats:italic>x</jats:italic> </jats:sub> (REBCO) conductor. The coil and a REBCO persistent current switch are connected with intermediate grown superconducting joints with high critical currents in external magnetic fields. To evaluate the performance of the joints in an ultimately stable and homogeneous magnetic field, the coil is operated in the persistent-mode, generating 0.1 T, in a 9.3 T background magnetic field of a persistent-mode LTS outer coil. The magnetic field drift over two years of the 400 MHz LTS/REBCO NMR magnet is as small as ∼1 ppm, giving high-resolution NMR spectra. The magnetic field drift rate over the second year was 0.03 × 10<jats:sup>−3</jats:sup> ppm h<jats:sup>−1</jats:sup>, which is more than three orders of magnitude smaller than that required for an NMR magnet, demonstrating that the superconducting joints function satisfactorily in a high-resolution NMR system. The corresponding joint resistance is inferred to be &lt;10<jats:sup>−14</jats:sup> Ω.</jats:p>

The superconducting Magnetic Energy Storage (SEMS) application still has a great potential to stabilize the utility grid when the uncontrollable power generation from renewable sources increases and power flows change rapidly due to the broad introduction of high-speed response semiconductor switching devices. Along with the development of liquid hydrogen supply chain, the SMES system using MgB₂ conductors also attracts great attention at this point. Although the MgB₂ wires which have critical temperature of around 39 K have been commercially available with more affordable prices, their bending strain sensitivity is an issue to be solved for fabricating large-scale conductors and coils. The experience of constructing a 10-kJ SMES system using Bi2223 tapes and the successful demonstration of compensating very fast electric power fluctuations in the previous project will help us to develop a larger-scale MgB₂ SMES system by investigating conductor and coil design while considering its bending strain sensitivity and mechanism of critical current deterioration to maximize its performance as one of the most promising energy storage devices, following the movement toward a CO₂-free environment.

Our group has developed a coil using MgB2 wires for SMES. In this paper, a prototype coil using a MgB2 Rutherford-Type stranded conductor was fabricated based on react-And-wind (R&amp W) method. In the R&amp W method, a Rutherford-Type conductor in which nine MgB2 wires were wound at pitch of 450 mm around a copper former were manufactured using reacted wires (performed by Columbus Superconductors SpA), and then coiling was performed. The coil was cooled by conduction cooling and the I-V properties were evaluated under magnetic field. As a result, in the R&amp W method, critical current of a coil was degraded, since making a coil by hand would cause the strain beyond the scope of the assumption which was the marginally allowable bending strain. This result suggests the coil for SMES proposed in ASPCS is difficult to react before twisting, and Rutherford-Type conductors should be fabricated before reacting, which means that coil processing should be performed based on the React after making stranded conductors and Wind method, or the wind-And-react method.

An HTS coil quenches despite of the high quench margin. Main origins of unexpected quench of HTS coils are non-reversible local defects, and training effects as in LTS coils are not observed in HTS coils. Therefore, when an HTS coil is quenched before the required coil performance is met, the coil cannot be reused unless the coil is safely protected from quench damage and its operating conditions are readjusted. This paper studies on the conditions to reuse the coil which experienced a quench and is not damaged by the quench to meet the required performance. The study is conducted based on the temperature and current dependences of the coil stability measures of the maximum allowable defect (MAD) and minimum propagating zone (MPZ).

A quench protection system for a high-temperature superconducting (HTS) coil using Cu tape co-wound with an HTS tape was studied by numerical analysis using a thermal model of a coil winding pack. In normal operation, the voltage across the resistive zone V s in the HTS coil was monitored by measuring the voltage difference between the HTS coil and the co-wound Cu coil. When V s exceeded the quench detection voltage, the HTS coil was disconnected from the power supply and the energy stored in the HTS coil was dumped into a dump resistor. At the same time, the terminals of the co-wound coil were connected to another resistor, and some of the current in the HTS coil was quickly transferred to the co-wound Cu tape coil due to the tight magnetic coupling of both coils. This is expected to decrease the hot-spot temperature due to the quick decrease of HTS coil current and to protect the coil from damage caused by overheating at the hot-spot. The analysis investigated the effectiveness of this method.

The most probable cause of quench damage of REBCO coated wire is localized over-heating at a hot-spot in the coil wire during the quench protection sequence. To reduce the hot-spot temperature, it is effective to increase the thickness of the Cu layer of the coated wire. However, the thick Cu layer deteriorates the overall current density of the wire. The wire with thin Cu layer may be damaged by a quench at low operational current. For a coil of a real application it is important to increase the current density of the winding pack of the coil at operating condition. Increase of the overall current of the wire itself by reducing thickness of the Cu layer is not necessarily to increase the winding pack current density considering quench protection. In the paper, the optimum value of the thickness of the Cu layer is analytically studied based on a thermal model of the winding pack to maximize the winding pack current density, while the coil is safe from the damage caused by quench.

REBCO coated conductors are now available from several industrial manufacturers and are expected to be promising conductors for high-field-magnet applications. Using these conductors, the development of solenoids capable of generating high magnetic fields of 20–30 T is ongoing in major high-field laboratories in the world. In addition, CERN recently launched a conceptual design study for the Future Circular Collider, in which a 20-T dipole magnet is listed as a candidate for the bending magnet of the main ring. However, there has been limited research published on the electrical transport properties of commercially available REBCO conductors in a high-field, low-temperature environment. For magnet designers, the transport properties are of the highest importance in choosing a suitable conductor, and the data form the bases for high-field magnet development. Therefore, in this work, a new sample holder, which allows the measurements of full-width conductors to be carried out relatively easily, was developed, and the transport properties of commercial REBCO conductors from seven manufacturers (AMSC, Fujikura, Shanghai Superconductor, SuNAM, SuperOx, SuperPower, and SWCC Showa) were investigated at 4.2 K in perpendicular fields of up to 18 T. The results show that the Ic values at 4.2 K clearly vary to some extent among these commercial conductors and the higher-current 4-mm-wide conductors have Ic values in the range of 230–305 A at 18 T and in the range of 320–424 A at 12 T.

The influence of the Al layer thickness on the critical current density J(c) of transformation-processed Nb3Al superconductors was investigated. The Al layer thickness in the jerry-roll filaments of precursor wires was varied from 120 to 250 nm; the Nb layer thickness is proportional to the Al thickness, keeping the Nb-Al composition ratio to 3. The rapid heating and quenching (RHQ) operation condition was varied within a range of the conditions to form the ductile body-centered cubic (BCC) Nb-Al phase. The RHQ wires were also followed by an areal reduction process prior to the Nb3Al phase transformation. Thinner Al thickness wires show a steeper increase in J(c) with reduction in the area, whereas thicker Al thickness wires show a slower increase. The best J(c) performance was obtained for the thinnest Al thickness sample. Without areal reduction, there was not much difference in the optimum J(c) with respect to the Al thickness.

To promote renewable energy sources, we proposed a new system called the Advanced Superconducting Power Conditioning System (ASPCS), which consists of Superconducting Magnetic Energy Storage-system (SMES), Electrolyzer, and Fuel Cell, and is also combined with a liquid hydrogen station for vehicles. The SMES plays a role to compensate the fast fluctuations generated by the renewable energies. In case of the ASPCS with a capacity of 5 MW, we designed the 50 MJ-class SMES composed of 4 solenoid coils. The winding of the solenoid coils is double pancake and a basic coil is 2 m in diameter and 0.5 m in height. Each SMES coil is wound with MgB2 conductor and indirectly cooled at 20 K by liquid hydrogen flowing through a thermo-siphon cooling system. Pure aluminum strips are inserted between the double-pancake coils and the pure aluminum plates gathering the strips lead to liquid hydrogen pipes. This scheme enables the strips and the plates to transfer the heat load in the coils to the cooling pipes and keep the coils at low temperature. On the other hand, we must consider that the strips generate eddy current loss which is strongly affected by a width of the strips. At the same time as the primary study of the SMES coils, we experimented on the thermo-siphon cooling system and investigated the relationship between the heat load and the heat extraction ability of the cooling system. The experiments showed that the cooling system could proficiently function. The estimation of eddy current loss from the particular cooling aluminum strips for the SMES in the ASPCS is reported with the results of the thermo-siphon driving experiment.

We propose a new electrical power storage and stabilization system, called an Advanced Superconducting Power Conditioning System (ASPCS), which consists of superconducting magnetic energy storage (SMES) and hydrogen energy storage, converged on a liquid hydrogen station for fuel cell vehicles. A small 10-kJ SMES system, in which a BSCCO coil cooled by liquid hydrogen was installed, was developed to create an experimental model of an ASPCS. The SMES coil is conductively cooled by liquid hydrogen flow through a thermo-siphon line under a liquid hydrogen buffer tank. After fabrication of the system, cool-down tests were carried out using liquid hydrogen. The SMES coil was successfully charged up to a nominal current of 200 A. An eddy current loss, which was mainly induced in pure aluminum plates pasted onto each pancake coils for conduction cooling, was also measured.

We have been developing Nb3Al wires processed by rapid heating and quenching for a number of years as promising candidates for use in future high-field accelerator magnets. These wires have better strain and stress tolerances than Nb3Sn wires do, but to meet the demands of future accelerator magnet designs, it is necessary to further improve their performance. In particular, it is necessary to increase their non-copper critical current density in 12-20 T fields. To pursue this goal, we introduced double rapid heating and quenching ( DRHQ) treatment into the fabrication process for Nb3Al wires, and studied the mechanical and superconducting properties of the resulting DRHQ-processed wires. (C) 2015 The Authors. Published by Elsevier B.V.

From the point of view of environment and energy problems, the renewable energies have been attracting attention. However, fluctuating power generation by the renewable energies affects the stability of the power network. Thus, we propose a new electric power storage and stabilization system, Advanced Superconducting Power Conditioning System (ASPCS), in which a Superconducting Magnetic Energy Storage (SMES) and a hydrogen-energy-storage converge on a liquid hydrogen station for fuel cell vehicles. The ASPCS proposes that the SMES coils wound with MgB2 conductor are indirectly cooled by thermosiphon circulation of liquid hydrogen to use its cooling capability. The conceptual design of cooling scheme of the ASPCS is presented. (C) 2013 Elsevier B. V. All rights reserved.

We are planning to demonstrate a quadrupole magnet with magnetic mirror structure by using 0.7-mm Nb3Al strands. As feasibility studies for this program, we investigated an influence of the diameter reduction of Cu-stabilized Nb3Al strands from 1.0 to 0.7 mm. Wire breakages and Cu separations did not happen with applying the cold die-drawing. The non-Cu J(c) and n-values of the 0.7 mm strands did not degrade and could keep the same performance of the 1.0 mm strands. Although irregular deformations of Nb-Al filaments slightly occurred, magnetization properties of the 0.7 mm strands are almost the same as those of the 1.0 mm strands. Ta interfilament matrix of the 0.7 mm strands was also effective to improve the low field instability at 4.2 K. In addition, 27 strands Rutherford cable has been made by using the 0.7 mm F1 strand without any troubles. All of 27 extracted strands taken from the F1 cable showed very uniform I-c performance at 4.2 K.

This project aims to establish key hard technologies for large-scale high-temperature superconducting (HTS) rotating machines for a ship propulsion system. Using HTS technology, compact, right weighted and highly efficient ship propulsion motors can be realized. The objectives of the project are R&D of robust and reliable HTS field coils with low loss,and compact cooling systems integrated to a rotor and design technology for a 20 MW-class ship propulsion motor. This project is now in the first year of the stage III; stage II was completed in the last Japanese fiscal year. This report reviews the R&D results in stage II and the plans in stage III.

Degradation of the epoxy impregnated YBCO coil performance is due to tensile radial stress concentration on the outer edge of the conductor during cool down. This stress acts as a cleavage stress and opens the conductor edge, fracturing the YBCO layer. The fracture propagates to another edge of the conductor, resulting in degradation of the coil performance. Degradation of the epoxy impregnated YBCO coil is eliminated, if we use a polyimide- electrodeposited YBCO-coated conductor: tensile radial stress concentration on the outer edge of the conductor is reduced due to plastic deformation of the ductile polyimide. Polyimide electrodeposition onto the YBCO-coated conductor is reliable, uniform, easy to apply, and can be extended to larger YBCO magnets, removing the risk of coil degradation. © 2012 Elsevier B.V. All rights reserved.

We have been developing Nb3Al high-field accelerator magnets with the aim of upgrading the luminosity of the Large Hadron Collider. In this program, four kinds of Ta-matrix Nb3Al strands with slightly different cross-sectional structures (K1, K2, K3 and K4 strands) were designed and fabricated. The non-copper J(c) values of these strandswere 700-800 A/mm(2) at 15 T and 4.2K. The cabling work of these strands was performed at Fermilab, and the superconducting properties of the cables were studied using the extracted strands from the cables. In this paper, we report on the fabrication experience for the strands and cables. The results obtained from this study are also presented.

YBCO-coated conductors are promising for NMR magnets. Their axial strength is up to 700 MPa and this allows for dramatically smaller high current density magnets. One of the major problems of YBCO-coated conductor coils is screening current induced in the coil during charging. The screening current reduces the central magnetic field by &lt 20% and causes the central magnetic field to drift with time. These are crucial problems for NMR magnets. We investigated measures to counter the field reduction and the drift for YBCO-coated conductor coils, including current cycle and temperature cycle. Experimental results for a small YBCO double pancake coil at 77 K are compared to numerical simulations. The magnitude of the drift increases with coil current. The drift is effectively suppressed by current sweep reversal, but the field reduction is not. A demagnetizing method suppresses both problems although it requires large overshooting in coil current and magnetic field. For a cryocooler cooled YBCO magnet, both problems can be suppressed by current sweep reversal in combination with temperature cycle. © 2012 American Institute of Physics.

Mechanical loss in superconducting coils is one of the important parameters for the stability of high temperature superconducting coils (HTS coil). We prepared three kinds of impregnations (no impregnation, ionic liquid grease), measured the AC loss of the Bi-2223 tape under DC magnetic field in liquid nitrogen and quantitatively estimated the relation between the impregnation and losses. As a result, the loss can be reduced about 40% by using ionic liquid compared with no impregnation. © 2012 IEEE.

Since it is an urgent issue to reduce the global Carbon-dioxide in the world, renewable energy should be supplied as a large amount of the electric power. However, if a large amount of fluctuating renewable energy becomes more than adjustable amount of a utility grid capacity, instabilities such as frequency deviation might occur. We propose a system that is composed of SMES and FC-H-2-Electrolyzer and also installed adjacent to Liquid Hydrogen station to cool down the SMES. Since the SMES has potentials of quick response and large I/O power, and Fuel Cell has potentials of slow response and steady power supplied from a large amount of hydrogen, we combine both storage devices and apply them to suppress the fluctuating power. We convert the fluctuating power to the constant power by using a developed prediction technology of Kalman filter to predict a trend of the fluctuating power. While the trend power should be supplied by FC or absorbed by the electrolyzer to produce hydrogen, the power difference between the renewable power and the trend power should be stored by the SMES. We simulate the power balance and analyze the required SMES capacity, design the concept of the SMES, and propose an operation algorithm for the SMES to estimate the electric efficiency of the system. It is found that the electric efficiency of the ASPCS can become greater than that of a pumped hydro-machine. (C) 2012 Published by Elsevier Selection and/or peer-review under responsibility of ISS Program Committee

The conductors for plasma equilibrium field (EF) coils of JT-60SA are NbTi cable-in-conduit (CIC) conductor with stainless steel 316L jacket. The production of superconductors for actual EF coils started from February 2010. Nine superconductors with 444m in length were produced up to July 2010. More than 300 welding of jackets were performed. Six nonconformities were found by inspections as go gauge, visual inspection and X-ray test. In order to shorten the manufacturing time schedule, helium leak test was conducted at once after connecting the long length jacket not just after the welding. The maximum force to pull the cable into jacket was about 7.6 kN on average. The mass flow rates of 9 conductors showed almost same values indicating that there are no blockages in the conductors. The measured current sharing temperature agreed with the expectation values from strand performance indicating that no degradation was caused by production process. The coupling time constants of conductors ranged from 80 to 90 ms which are much smaller than the design value of 200 ms. (C) 2011 Elsevier B.V. All rights reserved.

Cleavage strength for an YBCO-coated conductor at 77 K was investigated with a model experiment. The nominal cleavage strength for an YBCO-coated conductor is extremely low, typically 0.5 MPa. This low nominal cleavage strength is due to stress concentration on a small part of the YBCO-coated conductor in cleavage fracture. Debonding by the cleavage stress occurs at the interface between the buffer layer and the Hastelloy substrate. The nominal cleavage strength for a slit edge of the conductor is 2.5-times lower than that for the original edge of the conductor cracks and micro-peel existing over the slit edge reduce the cleavage strength for the slit edge. Cleavage stress and peel stress should be avoided in coil winding, as they easily delaminate the YBCO-coated conductor, resulting in substantial degradation of coil performance. These problems are especially important for epoxy impregnated YBCO-coated conductor coils. It appears that effect of cleavage stress and peel stress are mostly negligible for paraffin impregnated YBCO-coated conductor coils or dry wound YBCO-coated conductor coils. © 2011 Elsevier B.V. All rights reserved.

We made small superconducting coils and measured the total AC loss under the AC and DC external magnetic fields. The coil winding conditions are that 1, 5, and 10 N of the winding tensions and bobbin materials are glass fiber reinforced plastic (GFRP) and Dyneema fiber reinforced plastic (DFRP). The GFRP and the DFRP have thermal strain properties of contraction and expansion respectively with cooldown to cryogenic temperature. Mechanical losses were estimated from the measured data. According to the estimated mechanical losses, the loss of the coil whose bobbin material is the DFRP was small because the DFRP bobbin expanded and hence the winding was tightly fixed. Approximate 10 to 13% of the AC loss can be reduced by only changing the bobbin material from the GFRP to the DFRP. © 2010 IEEE.

Research and development of Nb3Al strands and cables for a high field accelerator magnet is ongoing under the framework of the CERN-KEK collaboration. In this program, new Ta-matrix Nb3Al strands were developed and their mechanical properties and superconducting properties were studied. The non-Cu J(c) values of these strands were 750 similar to 800 A/mm(2) at 15 T and 4.2 K. Using these strands, test fabrication of 27-strand Rutherford cable was carried out in collaboration with NIMS and Fermilab. The properties of the strands extracted from the cable were examined and it was found that there was no degradation of the superconducting properties of the strands. In this paper, we report the fabrication of the strands and the cable in brief and present some of the results obtained by studying their properties.

In order to investigate the local structure in the CuO2 plane of the La-based lhigh-T-c cuprate La2-xBaxCuO4, we have carried out measurements of Cu-63-NMR on single crystals with x=0.08 and 0.10 at low temperatures down to 4 K. In both samples, the local structure deduced from observed NMR spectra agreed with the averaged structure. In high-temperature region around 300K, detailed investigation of NMR line width has shown the existence of the local structure different from the averaged structure; that is, CuO6 octahedra tilt from the c-axis toward the random direction by approximately 1.4 deg.

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.

Rapid heating, quenching and transformation (RHQT)-processed Nb3Al wires possess better strain tolerance than Nb3Sn wires. However, RHQT-processed Nb3Al wires so far developed show lower non-copper critical current density (J(c)) than recently developed high-J(c) Nb3Sn wires in the field range of 12-15 T. Therefore, the development of Nb3Al wires having enhancing J(c) in this field range is a major concern for accelerator magnet designers. Among a number of fabrication parameters that might affect J(c) properties, we analysed the effect of thickness of Al and Nb foils in the jelly-roll-shaped filaments embedded in the precursor wires. Mechanical and superconducting properties of these wires with various Al-foil thicknesses in the filaments were investigated.

S-33 nuclear magnetic resonance (NMR) spectroscopy is limited by inherently low NMR sensitivity because of the quadrupolar moment and low gyromagnetic ratio of the S-33 nucleus. We have developed a 10 mm S-33 cryogenic NMR probe, which is operated at 9-26 K with a cold preamplifier and a cold rf switch operated at 60 K. The S-33 NMR sensitivity of the cryogenic probe is as large as 9.8 times that of a conventional 5 mm broadband NMR probe. The S-33 cryogenic probe was applied to biological samples such as human urine, bile, chondroitin sulfate, and scallop tissue. We demonstrated that the system can detect and determine sulfur compounds having SO42- anions and -SO3- groups using the S-33 cryogenic probe, as the S-33 nuclei in these groups are in highly symmetric environments. The NMR signals for other common sulfur compounds such as cysteine are still undetectable by the S-33 cryogenic probe, as the S-33 nuclei in these compounds are in asymmetric environments. If we shorten the rf pulse width or decrease the rf coil diameter, we should be able to detect the NMR signals for these compounds. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3424853]

Rapid heating/quenching and transformation (RHQT)-processed Nb3Al wires possess better strain tolerance than Nb3Sn wires and exhibit similar high-field properties. Therefore, Nb3Al wires might be promising candidates for use in future high-field accelerator magnets. For this reason, we have been developing RHQT-processed Nb3Al wires for a number of years. During this development, magnetization measurements on several samples have been carried out as a function of either temperature or the magnetic field. This paper presents some of the results of the magnetization measurements performed.

The 20^th Power and Energy Society Annual Conference was held on August 18-20, 2009 at Shibaura Institute of Technology. The total number of technical papers was 352, and technical sessions were 47 (46 oral sessions and 1 poster session). An invited lecture, a panel discussion, technical exhibitions and two technical tours were organized. All events were very well attended and the final enrollment attained to 881 registrations. The conference was successfully closed by the great contribution of all participants. The outline of the conference is reported in this article.

The effect of current sweep reversal on the temporal drift in magnetic field intensity for a Bi-2223 solenoid was investigated by experiment and using numerical simulation. Current sweep reversal, by as small as 1% of the peak current, was found to stabilize the drift in magnetic field intensity for a Bi-2223 tape solenoid. The field drift was due to flux creep in the Bi-2223 tape and the current sweep reversal formed a barrier for flux entrance at the upper and lower surface of the conductor, preventing flux creep. With a current reversal of several% of the peak current, the barrier formation extended over half of the solenoid and the magnetic field intensity became constant with time. The current sweep reversal technique should prove useful to stabilize an ultra-high field low/high-temperature superconducting nuclear magnetic resonance magnet operated at frequencies (field intensities) beyond 1 GHz (23.5 T). © 2009 Elsevier B.V. All rights reserved.

The screening current-induced magnetic field in the (Bi,Pb)2Sr2Ca2Cu3O x (Bi-2223) insert coil proposed for a beyond 1 GHz nuclear magnetic resonance (NMR) spectrometer may generate a long-term field drift, resulting in a loss of field-frequency lock operation and an inability to make high resolution NMR measurements. The measured screening current-induced magnetic field of a Bi-2223 double-pancake coil exhibits a hysteresis effect at 4.2 K that is reproduced by a numerical simulation based on a finite thickness rectangular superconductor bar model. The screening current-induced field at the coil center is of opposite polarity to that generated by the coil current, and thus the apparent field intensity shows a positive drift with time. On the contrary, the field at a coil end is of the same polarity as the coil field, and the apparent field intensity decreases with time. If we wait for ∼1000 h after coil excitation, the field drift rate approaches the field decay rate of the persistent current of 10-8 h-1, suitable for a long-term NMR measurement in a beyond 1 GHz NMR spectrometer. © 2009 Elsevier B.V. All rights reserved.

In the development of RHQT-processed Nb3Al wires for high-field accelerator magnet applications, one of the major objectives is to increase the non-copper critical current density in the field range of 12-15 T. To pursue this goal, a low-matrix-ratio wire was fabricated, and the effects of the RHQ conditions and the area reduction after the RHQ treatment on the properties of the wire were investigated. This paper describes the details and results of this study.

We have developed a novel critical current and stability measurement experimental setup, which utilizes a closed electric circuit with a multi-strand superconducting cable. The feature of this setup is mechanical loading applied to the multi-strand cable in the transverse direction. It was reported that Lorentz forces caused degradation in the critical current of the ITER-TFMC conductor. Furthermore, these phenomena were mainly observed in the ITER full-size conductors with large Lorentz forces under high magnetic fields. The advantage of our setup is critical current measurement under mechanical stresses comparable to those in the full-size conductor under high magnetic fields. By employing an inductive critical current measurement technique, we conducted an experiment with a transport current of about 10 kA without any power supply or current leads. In our experiments, we observed significant degradation in critical currents due to a compressive stress of about 30 MPa. We applied an innovative technique to mitigate the critical current degradation in mechanically loaded Nb₃Sn superconducting multi-strand cables. We molded one such cable with ice and tested it. No degradation occurred in the icemolded cable. In addition, stability was also ensured due to the large thermal conductivity of ice. Thus, we have successfully mitigated the degradation in the critical current of the Nb₃Sn conductor by ice molding.

We have developed Nb3Al wires with a low Nb matrix ratio in order to attain a high critical current density. Test wires were prepared with different fabrication parameters, mainly the Nb matrix ratio, RHQ current and n-values of these wires were measured. Our newly developed wire with a Nb matrix ratio of 0.6 was almost the same RHQ heat-treatement effect and area reduction effect as those of samples with different Nb matrix ratios. However, the critical current density of our wire was lower than that of these samples. In order to determine the reason for this low critical current density, T-c measurements and tensile strain tests were also performed. In this paper, we report the characteristics of the sample with a Nb matirx ratio of 0.6 and compare them with those of the other samples.