Tomoaki Takao

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 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.

Study of practical applications of HTS synchronous Machines

A small research and study committee, composed of corporations in the industrial/governmental/academic fields, was established, and a feasibility study of HTS synchronous machines (HTSMs) was carried out. Promising application fields for HTSMs were considered and their applicability was studied. Fundamental designs of these HTSMs were then carried out using a specially developed design program, and their merits and demerits were clarified. As key components, HTS field coils and refrigeration systems were examined closely, and technological problems related to their development were studied.

Application of SMES and Fuel Cell System Combined With Liquid Hydrogen Vehicle Station to Renewable Energy Control

It is an urgent issue to reduce global carbon-dioxide in the world, and hence the renewable energy, that is environmentally friendly, should be supplied as a large amount of the electric power. Since installation of a large amount of the fluctuating renewable energy, such as wind turbine and photovoltaic, will cause the power utility network unstable, we propose an advanced superconducting power conditioning system (ASPCS) that is composed of Electrolyzer-Hydrogen-FC and SMES cooled with liquid hydrogen from a station for vehicles. The ASPCS has a function of compensating the fluctuating renewable energy with SMES that has quick response and large I/O power, and with that has moderate response and large capacity. The SMES is wound with superconductor with a critical temperature of 39 K from an economical point of view, because it is cooled with through a thermo-siphon system to keep safety against a flammable gas. The ASPCS effectively fulfills a power balance by applying a statistical prediction method of Kalman filter algorithm. The capacity of SMES is optimized by using the trend prediction for a number of wind power data. The overall electric efficiency of the ASPCS is evaluated for a typical wind generator.

Thermal Conductivity of High-Strength Polyethylene Fiber and Applications for Cryogenic Use

The local temperature rise of the tape is one of instabilities of the conduction-cooled high temperature superconducting (HTS) coils. To prevent the HTS tape from locally raising a temperature, high thermal conductive fiber reinforced plastic was applied to coil bobbin or spacer for heat drain from HTS tape. The thermal conductivity of ramie fibers increases by increasing orientation of molecular chains with drawing in water, and decreases by chain scission with γ-rays irradiation or by bridge points in molecular chains with vapor-phase-formaldehyde treatments. Thermal conductivity of high strength ultra-high-molecular-weight (UHMW) polyethylene (PE) fiber increases lineally in proportion to tensile modulus and decreases by molecular chain scissions with γ-rays irradiation. This result suggested the contribution of the long extended molecular chains due to high molecular weight on the high thermal conductivity of high strength UHMW PE fiber. Thermal conductivity of high strength UHMW PE fiber reinforced plastics in parallel to fiber direction is proportional to the cross sectional ratio of reinforcement oriented in the conduction direction. Heat drain effect of high strength UHMW PE fiber reinforced plastic from HTS tape is higher than that of glass fiber reinforced plastic (GFRP) and lower than that of aluminum nitride (AlN). In the case of HTS coil, the thermal stability wound on coil bobbin made of high strength UHMW PE fiber reinforced plastic is good as that of AlN, and better than that of GFRP.

Compressive strain and critical current properties of Nb/sub 3/Sn conductors reinforced by center-arranged and distributed Ta

We have fabricated two kinds of Nb/sub 3/Sn superconductors having the same diameter; one superconductor is reinforced by center-gathered tantalum (Ta) in a cross-section of the conductor, and the other is reinforced by distributed Ta in the cross-section. We applied compressive force to the conductors, and measured degradation of the conductors. When the conductors strain due to the compressive force to the conductors was zero, a critical current (I/sub c/) of the distributed Ta conductor was larger than that of the center-gathered Ta conductor. However, an I/sub c/ of the distributed-Ta type conductor rapidly decreased, and when the compressive strain became approximately 5 or 6%, the I/sub c/ of the center-gathered Ta conductor exceeded that of the distributed-Ta type. It was experimentally shown that the center-gathered Ta conductor was more tolerant than the distributed Ta conductor about the compressive strain to the conductor.

Effect of coil current sweep cycle and temperature change cycle on the screening current-induced magnetic field for Ybco-coated conductor coils

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 <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 YBCOcoated 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 prob...

Influence of Bending and Torsion Strains on Critical Currents in YBCO Coated Conductors

A YBCO coated conductor is expected to be a next generation high performance superconductor. To design superconducting machines using YBCO conductors, evaluation of not only superconducting properties but also mechanical properties is very important. Hence, we applied bending and torsion to the YBCO conductor and measured degradation of the conductor against the strain. To compare with the YBCOs data, a Bi2223 conductor was also tested under the same experimental condition. From the bending test, degradation of the Bi conductor was faster than that of the YBCO conductor and the critical current gradually decreased with increasing bending strain. The YBCO conductor kept better performance than the Bi conductor, and degradation suddenly occurred. And when the superconducting layer in the YBCO conductor was subjected to compressive strain in the bending test, the critical current hardly decreased at approximately one percent of the bending strain.

Influence of thermal expansion of coil bobbins made of DFRP and GFRP on quench characteristics of superconducting coils

We have excited two types of superconducting coils having same specifications. One is made of DFRP (Dyneema(R) fiber reinforced plastic) bobbins which expands during cooling process from room temperature to liquid helium temperature. The other is made of GFRP (glass fiber reinforced plastic) bobbins which contract during the cooling process. Firstly, we have numerically estimated changes of winding tensions of the two types of the coils by the cooling down. And then, training behaviors of the DFRP and GFRP coils wound with the same winding tension at room temperature were measured. Influence of quench currents in the training quenches and the number of quenches to reach to the maximum currents on the winding tensions at liquid helium temperature is systematically discussed.