O. Horigami

Cryocooler directly cooled 6 T NbTi superconducting magnet system with 180 mmroom temperature bore

Abstract This paper describes a cryocooler cooled NbTi superconducting magnet system. The technical features of this magnet system are a 4K-Gifford-McMahon (GM) refrigerator using magnetic regenerator material and a high-Tc Bi 2 Sr 2 CaCu 2 O y superconducting current lead. The NbTi coil was directly cooled by the 4K-GM refrigerator without liquid helium and it took about 21 hours for the NbTI coil to be cooled from room temperature to below 4 K. The stable magnetic field of 6 T at 3.5 K and the maximum magnetic field of 6.45 T were obtained in the 180 mm room temperature bore.

A cryocooler-cooled 10 T superconducting magnet with 100 mm room temperature bore

A NbTi/Nb/sub 3/Sn superconducting magnet directly cooled by a 4 K cryocooler has been fabricated. It successfully attained a 10 T field at the center of a 100 mm room temperature bore. A 4 K Gifford-McMahon (GM) cryocooler, using Er/sub 3/Ni regenerator, cools the magnet without the use of liquid helium. Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub y/ superconducting power leads were adopted to reduce heat input to the 4 K stage. The coil heat is removed through heat conducting copper cylinders. The coil winding is epoxy-impregnated without a bore tube to stabilize the magnet against quenching. Two diode shunt circuits are placed in the cryostat to protect the coils. A fast ramp of 10 T/20 min, was attained by reducing Nb/sub 3/Sn conductor hysteresis loss and by decreasing the contact resistance between the coil and the conduction cylinder.

A 6 T refrigerator-cooled NbTi superconducting magnet with 180 mm room temperature bore

A 6 T NbTi superconducting magnet, cooled by a 4 K GM (Gifford-McMahon) refrigerator has been developed. Magnetic material, Er/sub 3/Ni, is used as the regenerator to cool the coil to liquid helium temperature. The central field strength in the 180 mm room temperature bore is designed to be 6 T. NbTi coil dimensions are, 215 mm in inner diameter, 279 mm in outer diameter, and 240 mm in axial length. The coil winding is an epoxy-impregnated structure without a bore tube, thereby reducing mechanical disturbances. The coil heat is removed through the outer metal cylinder. A pair of Bi(2212) current leads has been developed to reduce heat leakage into the 4 K level. A cold diode is used to protect the coil even if the oxide lead is burned out. The magnet attained 6.45 T central field at 3.61 K coil-winding temperature.<<ETX>>

A 10 T cryo-cooled superconducting magnet with 100 mm room temperature bore

Abstract A cryo-cooled NbTi/Nb 3 Sn superconducting magnet has been fabricated. It generated 10T field in the center of a 100 mm room temperature bore. A 4K GM refrigerator, using Er 3 Ni regenerator material, cooled the magnet without help of liquid helium. Heat generated in coils is removed through the heat conduction copper cylinder attached on the outer wall of each coil. Bi(2212) superconducting current leads are adopted to reduce heat leakage into the 4K stage. Two diodes shunt circuits were set into the cryostat to protect the coils. Fast ramp, 8T energization within 15 min, was attained by controlling the hysteresis loss for the Nb 3 Sn conductor.

Stress Effects on W/Cu Reinforced Nb3Sn Composite Conductors

Filamentary Nb3Sn wire has become increasingly important as a high-field conductor because of its high upper critical field and critical temperature, in developing large scale magnets such as those used in a fusion reactor, energy storage and electrical machinery. Compound Nb3Sn, however, has a disadvantage in that superconducting properties of Nb3Sn are rapidly degraded by tensile and bend stresses1,2,3 resulting from reeling and coiling the material and, by electromagnetic force generated when a coil is energized and by thermal contraction stresses due to cooling.

Anisotropic transport critical current density and its field dependence in a Bi2Sr2CaCu2Ox single crystal

Abstract The authors observed an anisotropy in the transport critical current density, J ct , at 77 K and 500 G, for a Bi 2 Sr 2 CaCu 2 O x single crystal. The transport critical current density, J ct , along the c-axis for a field parallel to the a-axis, with the Lorentz force parallel to the b-axis, was about three times larger than that for the field parallel to the b-axis, with the Lorentz force parallel to the a-axis. This shows the possibility that a bismuth atom modulation structure, existing only along the b-axis, may act as an effective pinning site. J ct with the Lorentz force perpendicular to the BiO insulating layers was about 10 times larger than that with the force along the layers, indicating that the BiO layers also serve as pinning centres. A correlation was observed between the decrease in J ct and the field dependence rate, with J ct ∝ B − n (where n = 0.8–1.3). This is attributed to the pronounced flux creep effect in this material.

Influences of tensile and bending strains on the critical current in multifilamentary Nb3Sn composites processed by Nb tube method

Abstract The critical current degradation of multifilamentary Nb 3 Sn superconducting composites strained mechanically at room temperature has been investigated. The experimental results show that the effects of the Nb 3 Sn layer thickness and the specimen wire structure, such as monolithic and stranded cable on critical current degradation, are appreciable for the specimens strained by bending stress, but are not for specimens strained by tensile stress. The results of the critical current degradation by tensile strain were discussed, based on the stress-strain characteristics of the composites. It was clarified that the critical strain in the case of applying tensile and bending stresses simultaneously at room temperature lay around the line which was drawn from the point of the critical tensile strain to that of the critical bending strain, when the ordinate was tensile strain and the abscissa was bending strain.

Cryocooler-cooled Bi-2223 High-T c Superconducting Magnet in Persistent Current Mode

A cryocooler-cooled high-T c superconducting (HTS) magnet was fabricated and persistent-current operation was carried out. The decay behavior of the magnetic field was strongly affected by flux- flow resistance and joint resistance. The influence of the flux creep was found to be relatively small in this experiment, but to be large considering the MRI application. However, the overshoot method of the coil operation was effective to reduce the flux-creep rate lower than the MRI requirement.

High Tensile Bi-2223 Ag-MgO Sheathed Oxide Superconductor

Ag-Mg(Ni)O sheathed mono-core and multifilamentary tapes have been fabricated to improve the mechanical property of powder-in-tube method (Bi,Pb)2Sr2Ca2Cu3Ox, Bi-2223, conductor. Fine Mg(Ni)0 precipitates in the sheath increased the 0.2% yield strength of the conductor up to 190MPa, which is comparable to the commercial Nb3Sn conductor. The Ag-MgNiO sheathed multifilamentary conductor exhibited a good strain tolerance: critical current density, Jc, did not decreased much up to 0.2% of bending strain and retained 80% of the zero strain Jc value at 0.5% of bending strain. The dependence of Jc on magnetic field was measured at 4.2K. The Ag-MgO sheathed mono-core tape made by pressing showed Jc value of 105 A/cm2 at 4.2K and 13T. The Ag-MgNiO sheathed multifilamentary tape made by rolling exhibited Jc of 2.6×104 A/cm2 at 4.2K and 12T. The present results of high yield strength, good strain tolerance and high values of Jc are enough to the application to the high field magnet.