Abstract: The filamentary (Nd0.33Sm0.33Gd0.33)(1.18)-Ba2.12Cu3.09Oy (NSG123) superconductors chemically doped with nominal Pt concentrations from 0 to 0.5 at% were fabricated by a solution spinning method. We investigated effects of the amount of Pt addition and cooling rate from partial melting temperature of 1050 degrees C during OCMG processing on superconducting properties and microstructure of filamentary samples. Pure NSG123 filaments cooled at 20 degrees C/h-50 degrees C/h showed the J(c) vale higher than 2.0 x 10(4) A/cm(2) at 77 K and 0 T. Although the sample treated at fast cooling rate of 60 degrees C/h exhibited the J(c) value of about 3.0 x 10(3) A/cm(2) at most, the NSG123 doped with 0.05 at% Pt cooled at 60 degrees C/h showed high J(c) value of 2.0 x 10(4) A/cm(2) at 77 K and 0 T. The field dependence of transport J(c) value was also investigated at 77 K for samples cooled at 60 degrees C/h. The J(c) value of 0.8 x 10(4) A/cm(2) was maintained up to magnetic field of 5 T for the 0.05 at% Pt doped sample. The size of (Nd, Sm, Gd)(2)BaCuO5 particles in NSG123 matrix was reduced from about 3 to 0.5 mu m with increasing Pt concentration from 0 to 0.5 at%. In addition, the connectivity in microstructure was improved with increasing Pt concentration. It was found that both J(c)-B properties and the microstructure for the sample treated at fast cooling were effectively improved by Pt doping.
Abstract: On the progress of the Deuterium-Deuterium (D-D) or Deuterium-Tritium (D-T) burning plasma devices, the importance of neutron irradiation on superconducting magnet materials increases and the data base is desired to design the next generation devices. To carry out the investigations on the effect of neutron irradiation, neutron irradiation fields are required together with post-irradiation test facilities. In these several years, a collaboration network of neutron irradiation effect on superconducting magnet materials has been constructed. 14 MeV neutron irradiation was carried out at Fusion Neutronics Sources (FNS) in Japan Atomic Energy Agency (JAEA) and fission neutron irradiation was performed at JRR-3 in JAEA. After the irradiation, the Nb3Sn, NbTi and Nb3Al samples were sent to High Field Laboratory for Superconducting Materials (HFLSM) in Tohoku University and the superconducting properties were evaluated with 28 T hybrid magnet. Also, the organic insulation materials are considered to be weaker than superconducting materials against neutron irradiation and cyanate ester resin composite was fabricated and tested at the fission reactor. One clear result on Nb3Sn was the property change of Nb3Sn by 14 MeV neutron irradiation over 13 T. The critical current was increased by 1.4 times around 13 T but the increment of the critical current became almost zero at higher magnetic fields and the critical magnetic field of the irradiated sample showed almost the same as non-irradiated one. (C) 2009 Elsevier B.V. All rights reserved.
Abstract: Correlation of phase formation, critical transition temperature T-c, microstructure, and critical current density J(c) with sintering temperature has been studied for acetone doped MgB2/Fe tapes. Sintering was performed at 600-850 degrees C for 1 h in a flowing At atmosphere. High boron substitution by carbon was obtained with increasing the sintering temperature; however, the acetone doped samples synthesized at 800 degrees C contain large size MgB2 grains and more MgO impurities. Incomplete reaction for the acetone doped samples heated at 600 degrees C result in bad intergrain connectivity. At 4.2 K, the best J(c) value was achieved in the acetone doped sample sintered at 700 degrees C, which reached 24,000A/cm(2) at 10 T and 10,000A/cm(2), at 12 T, respectively. Our results indicate that the small grain size and less impurity were also important for the improvement of J(c)-B properties besides the substitutions of B by C. (C) 2008 Elsevier B.V. All rights reserved.
Abstract: To understand the effects of the hot pressing on the pinning behavior for the SiC doped MgB2 tape wire, magnetic field and temperature dependences on a critical current density J(c) and the pinning force have been studied. The MgB2 tapes were fabricated using an in situ powder-in-tube (PIT) method, the hot pressing process with nano-SiC powders and the carbon steel sheath. J(c) of the hot pressing processed wire is enhanced especially at low field compared with that of the no hot-pressed wire. It is suggested that the enhancement of J(c) is caused by the improvement of the MgB2 grain connectivity came from the hot pressing. The pinning analysis using the pinning models of a surface pinning and a normal point pinning lead to that the scaled curve of the pinning force for the SiC-doped MgB2 tape with the hot pressing process showed anisotropy in a lower field. This result suggests that the pinning center of the hot pressing processed MgB2 wire forms into inequi-axial shape, which is different from that in the tape without the hot pressing having the equi-axial pinning center in our previous report. It was also found that the scaled pinning force of the SiC-doped MgB2 tape with the hot pressing is larger than the surface pinning model in a higher field. This suggests that the pinning force was enhanced by the hot pressing in a higher field. (C) 2009 Published by Elsevier B.V.
Abstract: (Nd0.33Sm0.33Gd0.33)(1.18)Ba2.12Cu3.09Oy (NSG123) precursor filaments doped with Al were prepared by a solution spinning method and partially melted at 1050 degrees C for 30 min in flowing 0.1% O-2 + Ar gas. Samples were cooled at various rate of 10-60 degrees C/h, and then oxygenated in pure O-2 gas. It was found that the microstructural and superconducting properties were strongly influenced on the cooling rate from partial melting temperature to 900 degrees C during OCMG processing. Although the sample without Al cooled at 20-50 degrees C/h showed J(c) value higher than 2.0 x 10(4) A/cm(2) at 77 K and 0 T, the sample treated at fast cooling rate of 60 degrees C/h exhibited the J(c) value of about 3.0 x 10(3) A/cm(2) at most. In contrast, the NSG123 doped with 0.05at% Al even at a rate of 60 degrees C/h cooling showed high J(c) value of 1.5 x 10(4) A/cm(2) at 77 K and 0 T, and exhibited a well aligned texture and a dense microstructure. From J(c) measurements in applied magnetic fields, a J(c) value higher than 10(3) A/cm(2) was maintained up to 9 T for 0.05 at% Al doped filament. It was found that a small amount of Al doping into NSG123 was effective on refinement of the size of the NSG211 particle and reduction pores and cracks in microstructure for sample treated even at fast cooling, and thereby increased J(c) values.
Abstract: In order to study the mechanism of the small n-value for Bi2212, E - J characteristics were measured carefully for various kinds of Bi2212 tapes and round wires in high magnetic fields. Some of wires were heat-treated in a magnetic field of 5 T. We found that the n-values of all the samples increase by the in-field heat-treatment at 5 T. In addition, we analyzed the E - J properties by the combination of the percolation model based on the local J(c) distribution and the two directional J(c) distribution model. The improvement of the n-value by the in-field heat treatment can be understood by the change of the aspect-ratio of the grains. Therefore, the aspect-ratio of the grains plays an important role in the current transport influencing not only J(c) but also n-value.
Abstract: The minimum quench energy (MQE) and normal zone propagation velocity (NZPV) of three kinds of Nb3Al superconductors fabricated by the rapid heating, quenching and transformation (RHQT) process were measured under various conditions of applied magnetic field (10-14 T), temperature (7-11 K), and transport current (80-95% of the critical current), while cooled by a cryocooler for developing the over 20-T class cryogen-free magnet. As a result, MQE values were related to the critical current density (J(c)); high MQE was obtained for low J(c). It is assumed that J(c) has a stronger influence on the MQE than specific heat, thermal conductivity, resistivity, and other parameters of the composite superconductor including the matrix and the stabilizer. NZPV was mainly proportional to the transport current density varying with applied field and temperature. The second contribution to NZPV is assumed to be heat capacity depending on the wire configuration.
Abstract: A 7-strand Nb3Sn superconducting cable, which consists of three pre-bent CuNb/Nb3Sn strands, three stainless-steel (SS) strands, and a center SS strand, was fabricated. 2.5 turns of the cable was wound on a 262-mm diameter GFRP bobbin A triplet consisting of three pre-bent CuNb/Nb3Sn strands was also tested for comparison. The reinforcement effect of stainless-steel strands in a cable was investigated to develop a high-strength large-current superconducting cable. The hoop stress test result indicated that the stainless-steel reinforced CuNb/Nb3Sn cable showed better performance than the CuNb/Nb3Sn triplet under the hoop stress, because the stainless-steel strands reduced the hoop stress on Nb3Sn strands. Furthermore, the solder impregnation of the cable, which made the seven strands a monolithic conductor, improved its performance.
Abstract: We report the effect of acetone addition on in situ powder-in-tube processed MgB2/Fe tapes by using acetone as liquid additive. The amount of acetone was varied from 0 to 15 wt.%. We found that a significant J (c)-B enhancement was easily achieved in high field by acetone doping. At 4.2 K, the transport J (c) for the best acetone added tapes (5 wt.%) reached up to 2.4x10(4) A/cm(2) at 10 T. With increasing acetone concentration, the a-axis lattice parameters, B (c2) and B (irr), were not much changed; however, the insulating MgO impurities and pores gradually increased, thereby resulting in the obvious decrease of J (c) values for 10 and 15 wt.% acetone-added samples, especially within low field region.
Abstract: We have been developing high-strength Nb3Sn strand cables to construct a high-field superconducting outsert for a 45 T hybrid magnet with a 25 T water-cooled resistive magnet. Evidence for cold work strengthening of repeated bending treatment (prebending effect) on Nb3Sn strands internally reinforced with CuNb stabilizer, which exhibits significant enhancement of the critical current density, has been found in the superconducting magnet fabrication process using a react-and-wind method. The strand cables were designed by controlling the constituent number of CuNb/Nb3Sn strands with the prebending effect and stainless steel strands, which are expected to have a stress limit 580 MPa at 0.4% strain. In order to design a compact superconducting outsert, high-strength strand cables are adopted in a magnetic field region below 14 T to maintain relatively large engineering current densities (J(e)). In a higher-field region above 14 T, YBa2Cu3O7 (Y123) coated conductors are employed for an insert coil. Using combination of Y123, Nb3Sn and NbTi superconductors, a 20 T superconducting outsert with a room temperature bore of 400 mm consisting of three layers made of Y123, two layers of CuNb/Nb3Sn and two layers of NbTi was designed. The coil parameters are 440 mm inner diameter, 1080 mm outer diameter and 1138 mm coil height. A very compact 20 T superconducting outsert with a stored magnetic energy 72 MJ at an operation current 903 A can be developed for a 45 T hybrid magnet.
Abstract: Transport characteristics of IBAD/CVD-YBa2Cu3O7 (YBCO) coated conductor were measured at 4.2 K under hoop stress. The conductor was fabricated by a multi-stage metal-organic chemical vapor deposition (CVD) method. The YBCO layer was deposited on Hastelloy substrate with PLD-CeO2 and IBAD-Gd2Zr2O7 buffer layers. A 20-mu m silver layer was sputtered as a protective and stabilizing layer. The hoop stress test coils were fabricated by winding the conductor on a 250-mm diameter stainless-steel bobbin by five turns. Two coils, denoted as coils A and B, were fabricated. The Hastelloy substrate located outside for coil A and inside for coil B. Both coils were tested in magnetic field at 4.2 K under hoop stresses. Coil A and B experienced 1028 and 777 MPa at 11 T, 4.2 K. The measured stress-strain curves provided that the Youngâs modulus of the conductor was 190 GPa. The tolerable stress of similar to 1000 MPa and the Youngâs modulus of 190 GPa are consistent with the values obtained by a tensile test. The hoop stress test results indicates that the YBCO coated conductor is promising for application under huge hoop stress.
Abstract: NbTi Cable-In-Conduit-Conductors (CICCâs) are employed to fabricate plasma fusion experimental apparatuses. NbTi is a preferable material in terms of low cost and good manufacturability. In contrast, it shows lower stability compared to materials for high magnetic field applications, e.g. Nb3Sn, due to its small critical temperature. A characteristic feature in NbTi multi-strand cable is so called avalanche-like quench, which is specific instability in a parallel electric circuit consisting of superconducting lines. To evaluate stability of a NbTi cable, we employed induced-current methods. Current is increased and decreased by constant voltage source, i.e. magnetic flux change produced by the bias magnet. We evaluated the quenches and equilibrium condition between heat generation due to flux How voltage and heat transfer to surroundings at critical current. A parameter of experiment was a ramp rate of the bias field that corresponds to electric field in the cable. Of course, the cable was stable with smaller electric field and/or critical current. To survey the influence of heat dissipation during this equilibrium, we changed the surrounding mediums; those are liquid helium, frozen ethanol and ice. Ice is the material with large thermal conductivity, therefore this results in good stability of the cable.
Abstract: Filamentary Gd-Ba-Cu-O superconductors doped with nominal Zr concentrations between 0 and 0.5 at% relative to Gd123 were prepared by a solution spinning method. Samples were partially melted in flowing 20% O-2 + Ar atmosphere gas at various partial melting temperatures and oxygenated in pure oxygen gas. High J(c) value of 2 x 10(4) A/cm(2) at 77 K and 0 T were obtained for the Gd123 + 0.1 at% Zr sample optimally processed. In the case of doping levels less than 0.5 at%, T-c values, which were about 92 K with sharp transitions, were hardly influenced by Zr concentration. From transport critical current measurements in magnetic fields, the J(c) value of the pure Gd123 filament sharply decreased with increasing applied field. Gd123 with 0.5 at% Zr doping processed under optimum conditions showed little deterioration of J(c) values in low magnetic fields and J(c) values higher than 10(3) A/cm(2) was maintained up to 14 T. It was also found that J(c)s in high magnetic field and the irreversibility field were improved by a small amount of Zr doping.
Abstract: Precursor filaments with starting composition (Nd-0.33, Sm-0.33, Gd-0.33) : Ba : Cu = 1.18 : 2.12 : 3.09 were prepared by a solution spinning method. Samples were partially melted at temperature range of 1020-1090 degrees C in flowing 0.1%O-2 + Ar, 1%O-2 + Ar and 20%O-2 + Ar and then oxygenated in pure 100%O-2 gas. The T-c value decreased from 91.3 K to 88.8 K with increasing oxygen concentration in atmosphere gas. A transport J(c) value higher than 10(4) A/cm(2) at 77 K and 0 T was showed over a wide temperature range of 1030-1080 degrees C in 0.1%O-2 + Ar and 1040-1090 degrees C in 1%O-2 + Ar. Especially, the 0.1%O-2 + Ar sample partially melted at 1050 degrees C and the 1%O-2 + Ar sample at 1060 degrees C exhibited the highest J(c) value of 2.7 x 10(4) A/cm(2) and 2.5. x 10(4) A/cm(2), respectively. The transport J(c) value of the sample melted in flowing 0.1%O-2 + Ar was maintained higher than 2.4 x 10(3) A/cm(2) at 77 K in applied magnetic fields up to 17 T. On the other hand, the maximum J(c) value at 77 K and 0 T for the sample treated in 20%O-2 + Ar melted at 1080 degrees C showed 5.4 x 10(3) A/cm(2) at most, and the superconductivity was disappeared at magnetic field of 7 T.
Abstract: The tensile strain dependence of the upper critical field B-c2 for Nb3Sn wires was measured using a new apparatus for applying tension at low temperature. The critical current I-c calculated from the strain-dependent B-c2 measurements is the almost consistent with the measured I-c. In the measurements by a new apparatus, the axial and the lateral applied strains were examined by the strain gauges attached onto the wire. The deviatoric strain epsilon(dev) and the hydrostatic strain epsilon(hyd) were estimated by the axial and the lateral strain. We found the deviatoric strain dependence of B-c2 is different between the tensile and compressive strain states. This means that the hydrostatic strain also should be taken into account in order to understand the strain effects on the superconducting properties of Nb3Sn.
Abstract: A superconducting multilayer coil using CuNb/ Nb3Sn wire was fabricated by the pre- bent react- and- wind ( R& W) method. The pre- bent R& W method is a combination of the pre- bending treatment and the conventional R& W process. A 0.8% pre- bending strain was applied by ten fixed pulleys. The coil was tested in magnetic fields of 14, 12, 11 and 10 T at 4.2 K. The coil quench current achieved 100% load ratio of the short sample critical current in the 14 T back- up field. The quench current of the outer section coil achieved 100% of the short sample critical current in the 12 T back- up field. The coil quench currents in various magnetic fields are discussed from a hoop stress and mechanical disturbance point of view.
Abstract: The High Field Laboratory for Superconducting Materials (HFLSM) and the Tsukuba Magnet Laboratory (TML) conducted in collaboration a case study on development of a 50 T-class hybrid magnet. To construct a high magnetic field magnet with compact and energy-saving design as well as with easy operation and maintenance, one has to develop high-strength Nb3Sn strand cables, with maximized superconducting characteristics and which can withstand a large electromagnetic force over 500 MPa. For this purpose, the HFLSM has proposed and investigated the effect of repeated bending treatment (prebending) on Nb3Sn strands internally reinforced with CuNb stabilizer leading to significant enhancement of the critical current density. In this report we present our results on application of the prebending effect to the development of high-strength strand cables. The designed prebent-strand cables are composed of three CuNb/Nb3Sn strands (3 x phi = 1.73 mm) and four stainless steel strands (4 x phi = 1.73 mm). High-strength CuNb/Nb3Sn strand cables have shown a stress limit of 552 MPa at 0.4% strain, and a critical current of I-c = 1000 A at 18.5 T and 2.0 K. For such high-strength strand cables, a 20 T superconducting magnet with a room temperature bore (phi = 400 mm) consisting of five layers made of CuNb/Nb3Sn and two layers of NbTi was designed. The coil parameters are: inner diameter phi = 440 mm, outer diameter phi = 1332 mm, coil height 1321 mm, inductance 350 H and magnetic stored energy 144 MJ at 908 A of the operation current. Winding of the coil was experimentally successfully simulated using dummy 3 + 4 strands cable composed of three Cu strands and 4 stainless steel strands with a similar design to the 3 + 4 strands superconducting cable presented above. The 20 T superconducting coil will be used as a 20 T outsert for a 25 T water-cooled resistive insert to obtain a 45 T hybrid magnet.
Abstract: The effect of the starting composition on the superconducting properties for ternary system (Nd, Sm, Gd)-Ba-Cu-O filaments was systematically investigated. The filaments with the composition of (Nd-0.33,Sm-0.33,Gd-0.33)(1.18)Ba2.12Cu3.09Oy (sample A), (Nd-0.50,Sm-0.25,Gd-0.25)(1.18)Ba2.12Cu3.09Oy (sample B), (Nd-0.25,Sm-0.50,Gd-0.25)(1.18)Ba2.12Cu3.09Oy (sample C) and (Nd-0.25,Sm-0.25,GdO.50)(1.18)Ba2.12Cu3.09Oy (sample D) were prepared by a solution spinning method. All samples partially melted at temperature range of 1030-1080 degrees C in flowing 0.1% O-2 + Ar exhibited J(c) value higher than 10(4) A/cm(2) at 77.3 K and self-field. Among these samples, the best transport J(c) value of 7.2 x 10(4) A/cm(2) was attained in sample B with Nd-rich composition. The field dependence of transport J(c) value was also examined at various temperatures between 77.3 K and 90 K in applied magnetic fields up to 17 T. In the case of samples A. the J(c) value gradually decreased with increasing the applied magnetic field, and the J(c) value of 2.4 x 10(3) A/cm(2) at 77.3 K was maintained up to high magnetic fields of 17 T. On the contrary, the Jc value of sample B decreased with increasing the applied magnetic field and the superconductivity disappeared at about 15 T. It was found that the three element ratio in NSG123 contributed to flux pinning in a different manner and affected in J(c) behavior in magnetic field. (c) 2008 Elsevier B.V. All rights reserved.
Abstract: The prebending strain effect is that the repeated bending load at room temperature enhances superconducting properties of practical Nb3Sn wires. The authors are now investigating the application of the effect to practical superconducting cabling conductors for high field superconducting magnets. Large current capacity and high strength are required for superconducting cabling conductors to make large scale and high field magnets. The superconducting cabling conductor with high strength Nb3Sn wires will be useful for making high field magnets. The prebending strain effect was applied to a cabling technique with Nb3Sn wires. High strength Nb3Sn wires reinforced with CuNb composite and conventional Nb3Sn wires without reinforcement were prepared, which were heat-treated at 943 K for 345.6 ks. Both wires were bent by 10 pulleys to give 0.8% prebending strain. After the prebending treatment the wires were assembled and fabricated 3-strand cables and 7-strand cables. The cables with high strength Nb3Sn wires showed the enhancement of critical currents even after the cabling process. The results imply that the prebending treatment is applicable to the fabrication of cabling conductors for making a high field superconducting magnet.
Abstract: A 30 T superconducting magnet employing Ag/YBa2Cu3O7/Hastelloy (Y123) coated conductors would make precise fundamental research much more accessible. In order to investigate the feasibility of such a magnet, the current stability of the conductor was examined at 30 T for B // c. Liquid helium cooled Y123-coated conductors with copper and silver stabilizing layers were examined for coil quench issues. Instability conditions were studied using a static zero-dimensional model for various copper stabilizing-layer thicknesses. The calculations indicate the existence of a stable sharing current in the copper stabilizer, of about 0.7 A per 1 mu m of copper thickness at 30 Tin liquid helium. This sharing current in the copper stabilizer can be added to the 140% critical current of the Y123 conductor at 30 T for B // c. (C) 2008 The Japan Society of Applied Physics.
Abstract: We set out to design as compactly as possible a 30 T superconducting magnet consisting of an YBa2Cu3O7 (Y123) insert coil and a CuNb-reinforced Nb3Sn (CuNb/Nb3Sn) background coil. The engineering current density J(e) for an Y123-coated conductor tape is larger at 1415 T and 4.2 K than that for a CuNb/Nb3Sn strand. Adopting Y123-coated conductor tapes in the high field region above 14-15 T proves highly effective for the fabrication of a very compact 30 T superconducting magnet. A 30 T, 52 mm-room-temperature bore superconducting magnet can be constructed in the coil grading of a 16T Y123 insert and a 14T CuNb/Nb3Sn background coil, and the stored magnetic energy of 32MJ successfully lies in the 1/3 level in comparison with the 30T nuclear-magnetic-resonance (NMR) superconducting magnet reported previously. (c) 2008 The Japan Society of Applied Physics
Abstract: When the quenching occurs in a superconducting coil, excessive joule heating in normal area may damage the coil. It is necessary to detect quenching in the coil as soon as possible and discharge the magnetic energy stored in the coil. Therefore, we propose a superconducting coil protection system based on an active power method. The system is highly resistant to the noise and does not require cancel voltage taps, so it is useful for both AC and DC coils. We have presented the effectiveness of the system using some test coils cooled in LN2 or LHe. However, we have not discussed the effectiveness of the proposed system for helium-free cryocooled magnets, in which a larger temperature rise occurs after quenching than in liquid-cooled magnets. In this paper, we verify the effectiveness and practicality of the proposed system through coil protection tests for a cryocooled Nb3Al LTS coil. (C) 2008 Elsevier Ltd. All rights reserved.
Abstract: It was reported that Lorentz force caused degradation of critical current in the ITER-TFMC conductor. We have used our novel experimental setup, which utilizes the closed electric circuit concept for critical current and stability measurements of multi-stand superconducting cables. The feature of this setup is mechanical loading applied to the multi-strand cable in the transverse direction. Significant degradation in the critical current of the cable was observed when the average compressive stress was about 20 MPa. This degradation was found irreversible after unloading. We tested the cable with epoxy or ice molds as well. No degradation was observed in the molded cables. We also tested the cable with smaller void fraction. In this case, significant degradation in critical current was observed.
Abstract: Relaxation of the thermal residual strain in Nb3Sn wires is realized through cycles of multiple torsion loadings (âpre-torsionâ) at room temperature. As a consequence, the critical current, I-c, is enhanced. This effect is stronger than previously reported mechanical treatments of multiple bending (âpre-bendingâ). The maximum I-c is attained for complex mechanical treatments of pre-torsion and pre-bending. Complex treatments allow efficient and relatively independent control of residual strain relaxation over all three directions of the wire, resulting in enhancement of I-c towards the theoretical limit of the material. Our findings have an immediate positive impact on the performance of Nb3Sn wires and, hence, on their applications (e.g. react-and-wind coils).
Abstract: The transport critical current density J(c) is one of the most important properties for practical applications of high-T-c superconductors. The J(c) properties of high-T-c superconductors are highly dependent on the microstructural texturing of the superconducting phase. In order to improve the grain alignment of the superconducting phase melt-processing in high magnetic fields is a very effective method based on the anisotropy of magnetic susceptibility. For Bi-based superconducting bulks and tapes, there have been many studies using the in-field melt-processing and large J(c) enhancements due to the texture improvement are observed. In this study, we employed melt-processing in high magnetic fields for the Ag/Bi2212 round wires, which are much superior to thin tapes in fabricating superconducting magnets. Since the J(c) properties of Ag/Bi2212 are very sensitive to the maximum temperature of the heat treatment, we performed the melt-processing under various maximum processing temperatures. During the heat treatment, a magnetic field of 0 T or 5 T was applied using a cryocooled superconducting magnet. For the wires heat-treated under a magnetic field, small increase of the J(c) values were observed. From the results of the detailed analysis of the E - J properties, we found that there are possible microstructural changes due to the in-field process. It is suggested that the in-field heat-treatment is very effective for not only J(c) enhancement but also the n-value improvement for Bi2212 conductors.
Abstract: We measured directly by neutron diffraction the axial and lateral residual strains for the prebent and the as-reacted CuNb/Nb3Sn wires at room temperature and at 7 K, in order to investigate the change of the residual strain with prebending treatment. In the axial direction of the wire, the residual strain was changed with 0.20% to the tensile side when measured at both temperatures, while in the lateral direction, the change was 0.08% and 0.03% to the compressive side for the measurements at RT and 7 K, respectively. From the obtained data, we estimated the deviatoric strain. At 7 K, the value is 0.40% for the as-reacted wires and it reduces to 0.19% when the prebending is applied with a strain epsilon(pb)=0.8%. These results suggest that the reduction of the residual strain in the axial direction as well as in the lateral direction, i.e., of the deviatoric strain is responsible for the observed enhancement in the superconducting properties of the prebent Nb3Sn wires. In addition, we succeeded in the quantitative evaluation of the deviatoric strain dependence of the upper critical field for the practical Nb3Sn wire. This is very important for the understanding of the general axial strain dependence of the superconducting properties. (c) 2007 American Institute of Physics.
Abstract: Transport current properties of the steady and unsteady operating modes and thermal runaway conditions of the Ag-sheathed Bi2Sr2CaCu2O8 composite superconductors are theoretically studied in DC magnetic fields. It is shown that the temperature of a composite is not equal to the coolant temperature before thermal runaway. As a result, the shape of the voltage-current characteristics of high-T-c superconducting composites has only a positive slope during continuous current charging both before and after thermal runaway. That is why the voltage-current characteristics of high-T-c superconductors do not define the boundary of the thermal runaway. This peculiarity has to be considered during experiments at which the critical or quench currents are defined.
Abstract: Six practical composite wires of Nb3Sn (Furukawa Electric Co, Ltd) with different architectures ( design and reinforcement) were compared from the normal state resistance R, critical temperatures T-c(onset), T-c(offset) and Delta T-c, upper critical field B-c2 ( at 4.2 K) and critical current density J(c) points of view. Wires were as follows: three of near-the-edge reinforcement design with Nb reinforcement of 0, 21, 50 vol% in the CuNb region, and two of central reinforcement design with Nb of 21 and 50 vol%. One wire with near the edge 50% vol Nb reinforcement had a different reinforcement/superconductor ratio. As-reacted wires show very different patterns of R, T-c(onset), T-c(offset), Delta T-c, B-c2 and J(c). For the superconducting parameters this is probably due to different 3D thermal residual strains. Data suggest that the architecture of the as-reacted wire can control residual strain values and distribution. During multiple bending of the wires at room temperature ( named pre-bending), introduction of the reinforcement improves relaxation of the 3D residual strain and especially of the lateral components. As a consequence, B-c2 and J(c) versus pre-bending strain, epsilon(pb), are enhanced to values closer to those of the Nb3Sn in the stress-free state. Relative enhancement of these critical parameters for the reinforced wires is higher than for the reinforcement-free wire. Evolution during pre-bending and maximum attained absolute values of the superconducting parameters can be grouped roughly as a function of near-the-edge or central reinforcement design. Variation of the superconducting parameters suggests that the pre-bent state may depend on the as-reacted one ( which is a function of wire architecture and processing history); when J(c) was low as in the case of as-reacted wires with central reinforcement, during pre-bending, despite a relatively high J(c) enhancement, this parameter attained lower absolute maximum values than for the other wires. Depending on the wire, when pre-bending strain epsilon(pb) = 0.8 - 1%, the enhancement of non-Cu J(c) at 4.2 K in the pre-bent wire is by up to 43.5% at 15 T compared to the as-reacted case. The maximum absolute value of J(c) is obtained for the near-the-edge reinforcement pre-bent wires with 21 and 50% vol Nb. B-c2 of these wires was lower than for the other reinforced wires and higher than for the reinforcement-free wire. Pre-bent wires with near-the-edge reinforcement are identified as new and promising candidates for fabrication of react-and-wind coils with improved performance.
Abstract: Nano-C doped Fe-sheathed MgB2 tapes with different doping levels were fabricated using a powder-in-tube method and an in situ reaction. The effects of nano-C doping on the transition critical temperature, microstructure, critical current density, and flux pinning of MgB2 tapes were studied. Compared to the undoped tapes, critical current density J(c) for all the C-doped samples with different doping level was enhanced by more than an order of magnitude in magnetic fields above 10 T. This result indicates that flux pinning was effectively enhanced by the nano-C doping. Highly dispersed nanoparticles; and the substitutions of B by C are proposed to be responsible for the improvement of J(c) -B properties.
Abstract: We investigated the effect of nanoscale C doping on the critical current density J(c) and irreversibility field B-irr of Fe-sheathed MgB2 tapes prepared by the in situ powder-in-tube method. The tapes were heat treated at 600 -950 degrees C for 1 h. Higher values of J(c) and B-irr were seen for 5 at.% C-doped MgB2 tapes at higher sintering temperatures. The C-doped samples sintered at 950 degrees C showed the highest B-irr, for example at 4.2 K the B-irr reached 22.9 T. In particular, at 20 K, B-irr for the C-doped tape achieved 9 T, which is comparable to the upper critical field of the commercial NbTi at 4.2 K. This role of nanosized C particles can be very beneficial in the fabrication of MgB2 tapes for magnetic resonance imaging applications at 20 K.
Abstract: Pre-bending effects of Nb3Sn composite wires, enhancement in J(c) at the as-cooled condition and J(c) peak, J(cm), against applied tensile strain, are well known in especially Cu-Nb reinforced Cu stabilized Nb3Sn wire. In an attempt to understand the effects, three directional strain analyses using FEM were studied by considering thermally-induced residual strain due to temperature difference between the reaction temperature and the cryogenic temperatures including the pre-bending process at room temperature for three types of Nb3Sn composite wires. To evaluate effects of change in three directional strain, the von Mises strain was introduced. As a result, the minimum von Mises strain against the applied tensile strain corresponded to J(cm); low strain shows J(cm) enhancement by the pre-bending for Cu-Nb reinforced Nb3Sn wire. It was found that there is a direct correlation between the minimum von Mises strain and J(cm) related to the pre-bending.
Abstract: (Nd0.33SM0.33Gd0.33)(1.18)Ba2.12Cu3.09Oy, filaments have been successfully fabricated by a solution spinning method and OCMG processing. We have systematically investigated the relationship between cooling rate from the partial-melting temperature and transport J(c) value for filamentary samples treated in 0. 1%O-2 + Ar. It was found that both the Jc value and the microstructure of samples depended on the cooling rate. The samples partially melted at 1050 degrees C and cooling rate of 10-50 degrees C/h showed J(c) value higher than 104 A/cm(2). The maximum J(c) value of 3.0 x 10(4) A/cm(2) was obtained for 40 degrees C/h sample. The transport J(c) was also measured at 83-90 K in external magnetic fields up to 17 T. The 10 degrees C/h sample exhibited superior J(c) behavior and a J(c) value of 1.1 x 10(3) A/cm(2) at 83 K was maintained up to 17 T, in which fine (Nd,Sm,Gd)(2)BaCuO5 particles about I pm in diameter were homogeneously dispersed. (C) 2007 Elsevier B.V. All rights reserved.
Abstract: The effect of Zr and Zn doping on superconducting properties and microstructures of filamentary Eu-Ba-Cu-O superconductors prepared by a solution spinning method has been investigated. Samples doped with small amounts of Zr and Zn showed relatively high J(c) values of around 2 x 104 A cm(-2) at 77 K and 0 T-c which was two times larger than that of the pure Eu 123 filament. The temperature range for the partial melting in reduced oxygen partial pressure obtained the J(c) value higher than 104 A cm(-2) at 77 K became wider when Zr and Zn were added. From the SEM observation, a small amount of Zr and Zn doping was found to be useful to reduce the size of Eu211 particles, whereas it showed adverse effects on the dispersion of Eu211 particles and compositional homogeneity in a Eu123 matrix. In the case of Zn doping, although sample showed high J(c) value larger than 104 A cm(-2) at magnetic field lower than 2 T-c a J(c) value monotonically decreased at above 3 T and superconductivity was disappeared at 11 T. On the contrary, in spite of the fact that a J(c) value of sample with 0.5 at.% Zr decreased by applying only a magnetic field of 0.2 T-c superconductivity could be maintained up to 17 T at 77 K. (C) 2007 Elsevier B.V. All rights reserved.
Abstract: An 18 T cryocooled superconducting magnet (18T-CSM) with a 52-mm room temperature bore has been successfully constructed. The magnet consists of an outer NbTi coil, which generates 6.2 T, four Nb3Sn coils, which generate 9.3 T and a Bi2223 high-Tc insert coil, which generates 2.5 T. The magnet system is cooled by a GM/JT cryocooler and two single-stage GM cryocoolers. The initial cool down takes 11.5 days. The ramp time up to 18 T is 60 minutes. The innermost layer of the Nb3Sn coils employs an internal-tin processed Nb3Sn wire due to its high critical current density in high magnetic fields. High mechanical strength is required for other three layers because of the large hoop stress of 234 MPa at 18 T. With this view, bronze-processed high-strength Nb3Sn wires reinforced with Cu-NbTi intermetallic compound are employed. The Bi2223 high-Tc insert coil is composed of 25 double pancake coils using Ag-alloy sheathed Bi2223 superconducting tape with stainless steel tape reinforcement. The reinforcement co-winding reduces the hoop stress down to 48 MPa, which is sufficiently applicable to the Bi2223 tape. The magnet is successfully operated up to 18.1 T.
Abstract: We developed a novel critical current and stability experimental setup utilizing a closed electric circuit with a multi-stand superconducting cable. The feature of this setup is transverse mechanical loading implied to the multi-strand cable in the transverse direction. It was reported that Lorentz force caused degradation of critical current in the ITER-TFMC conductor. Furthermore, these phenomena were only observed in the ITER full size conductors with large Lorentz forces under high magnetic field. The advantage of our setup is a critical current measurement with comparable mechanical stress under high magnetic field. Employing an inductive critical current measurement technique, we conducted the experiment with transport current of around 10 kA without any power supply nor current lead. As experimental results, we observed significant degradation due to compressive stress of around 30 MPa. This degradation was found irreversible, when it was unloaded.
Abstract: Two practical Nb3Sn composite wires (short samples) were investigated from the points of view of critical current, I-c, and cracks evolution versus number of bending load cycles. Bending was applied manually at room temperature using a block-form with curved surface corresponding to applied bending strains of 0.5% and 0.8%. It was applied in plane, alternatively in one direction and in the opposite one up to 130 times. We shall use for this mechanical treatment term pre-bending. The wires had approximately the same diameter but a different architecture: one was of conventional type (standard US-Japan wire, Hitachi Cable) and the second one was CuNb reinforced wire (Furukawa Electric Co. Ltd.) with reinforcement located near-the-edge. It was found that critical current maximizes gradually with the number of bendings and the maximum value of I-c, is attained for a number of bending loadings, N-pb, of about 15. Beyond this optimum N-pb, critical current is constant. Optimum N-pb is likely not dependent on the wire, applied field or pre-bending strains. Results suggest that Cu is important in the work hardening process. Work hardening during pre-bending is imposing limitations in application of this technically convenient mechanical treatment for release of the residual strain and further enhancement of I-c. Semi-quantitative analysis of the electron microscopy observations shows that for our experimental conditions the density of cracks is approximately constant being in agreement with described I-c, behavior. Other possible consequences of our results are discussed. Among the most interesting issues is possibility of the 3D independent control of yielding and hence of residual strain release. Several new practical ideas that will need future confirmation are proposed: e.g. application of loading-unloading cycles of torsion treatment in combination with tensile or pre-bending treatments might be useful.
Abstract: An application of the prebending effect to a cabling process using Nb3Sn strands was demonstrated. The prebending effect is the enhancement effect of superconducting properties due to the repeated prebending treatment for practical bronze-route Nb3Sn wires. CuNb/Nb3Sn and Cu/Nb3Sn strands were applied prebending treatment using 10 fixed pulleys with 0.8% prebending strain. Four kinds of triplets, i.e., prebent CuNb/Nb3Sn, no-prebent CuNb/Nb3Sn, prebent Cu/Nb3Sn and no-prebent Cu/Nb3Sn triplets were fabricated. Critical currents were measured for the four kinds of triplets in magnetic fields up to 11 T at 4.2 K. The obvious critical current enhancement due to the prebending effect was maintained for the prebent CuNb/Nb3Sn triplet. The results imply that the prebending treatment for high-strength Nb3Sn strands is applicable to the cable conductor fabrication.
Abstract: Magnetic field dependencies of the I-c of the IBAD/CVD-YBCO short tape sample and its small coil sample were measured in high fields, up to 18 T at 77 K. Compared with the I-c of the tape sample, the I-c of the coil sample at 0.1 mu V/cm showed the same tendency in high fields. If YBCO tape is applied to a high-field coil application, the application should be operated at a temperature which is lower than 77 K. Using long CVD-YBCO tape, six stacked pancake coils were fabricated. Various current tests were conducted using one of these stacked coils. In AC current tests, thermal stability of the YBCO coil was estimated. When the peak values of AC current were 1.2 times higher than the maximum DC current in a thermal stable state, I-dcmax, and the average electric field of the coil at the first triangular wave was about 10 times higher than 1 mu V/cm criterion, the peak values of the built-up voltage did not tend to increase even after the 99th triangular wave current, and thermal run-away in the coil was not observed. In DC current with overlapped pulse current tests, the maximum peak current of the coil in a thermal stable state was obtained as a function of DC current and sweep time. It was 1.3 times higher than I-c and 1.4 times higher than I-dcmax in a test condition. These results indicate that the YBCO coil has high potential in short-time, over-current operations at high temperatures. In cases where built-up voltages did not disappear, they began to increase just after the coil currents reverted to the initial DC currents. It was found that DC current influenced the increasing speed of built-up voltages once the pulse current had decreased to zero.
Abstract: Six Nb3Sn composite wires with different architectures (âcentral and near-the-edge reinforcementâ) were repeatedly in-plane bent at room temperature (in-plane âpre-bendingâ). Breakage behaviour was revealed from scanning electron microscopy observations by semi-quantitative analysis of the filament crack formation and evolution. Cracks are formed in the transversal and longitudinal directions. Transversal cracks show some tolerance to the applied bending strain due to the fact that filaments are composite materials; residual Nb core can arrest development of a partial transversal crack initiated in the Nb3Sn outer part of the filament. Together with the density of cracks C and the evolution of this parameter with pre-bending strain, epsilon(pb), in different regions of the wire, R-epsilon(pb) curves are important to understand breakage behaviour of the wires. R is the ratio (number of full transversal cracks)/(number of full transversal cracks + number of partial transversal cracks). Parameters C and R allow us to reveal and satisfactorily understand the wire architecture-breakage-critical current decay relationship when pre-bending treatment is applied. As a consequence, breakage criteria necessary to minimize I-c decay were defined and the positive influence of the reinforcement in preventing breakage was observed. It was also found that, in this regard, more Nb in the CuNb reinforcement, for the investigated wires, is better, if the heat treatment for the wire synthesis is performed at 670 degrees C for 96 h. A different heat treatment, 650 degrees C for 240 h, is less efficient in preventing filament breakage. Our results suggest the possibility of control and improvement of the breakage susceptibility of the filaments in the wires and, hence, of the bending I-c decay, through the wise design of the wire architecture (i.e. by correlating design with the choice of composing materials and heat treatments).
Abstract: MoSi2-doped MgB2 tapes with different doping levels were prepared through the in situ powder-in-tube method using Fe as the sheath material. The effect of MoSi2 doping on the MgB2/Fe tapes was investigated. It was found that the highest J(C) value was achieved in the 2.5 at.%-doped samples, more than a factor of 4 higher than the undoped tapes at 4.2 K, 10 T, then further increasing the doping ratio caused a reduction of J(C). Moreover, all doped tapes exhibited improved magnetic field dependence of J(C). The mechanism of J(C) enhancement by MoSi2 doping is also discussed.
Abstract: Nanocarbon-doped Fe-sheathed MgB2 tapes with different doping levels were prepared by the in situ powder-in-tube method. Compared to the undoped tapes, J(c) for all the C-doped samples was enhanced by more than an order of magnitude in magnetic fields above 9 T. At 4.2 K, the transport J(c) for the 5 at. % doped tapes reached 1.85x10(4) A/cm(2) at 10 T and 2.8x10(3) A/cm(2) at 14 T, respectively. Moreover, the critical temperature for the doped tapes decreased slightly. Transmission electron microscopy showed a number of intragranular dislocations and the dispersed nanoparticles embedded within MgB2 grains induced by the C doping. The mechanism for the enhancement of flux pinning is also discussed. These results indicate that powder-in-tube-processed MgB2 tape is very promising for high-field applications.
Abstract: The thermal runaway conditions of the composite Bi2Sr2CaCu2O8 superconductor cooled by liquid helium or liquid hydrogen are compared. The study based on the static analysis of thermoelectric modes was made when the volume fraction of the superconductor in a composite was varied. Some specific trends underlying the onset of thermal runaway in superconducting composites cooled by liquid coolants are discussed. It is stated that the operating modes of superconducting composites may be characterized by stable states during which the current-carrying capacity of a superconductor is not effectively used even with a high amount of superconductor in the composite. These states are possible due to the corresponding temperature variation of the resistivities of the matrix and the superconductor in the high operating temperature range. They have to be considered in experiments when the critical current of a superconductor is determined or when the optimal stable operating modes of the current-carrying elements based on the Bi2Sr2CaCu2O8 superconductor, which is cooled by liquid coolant, are defined.
Abstract: Fe-sheathed MgB2 tapes were prepared by the in situ powder-in-tube technique using nanometre Si/N/C, SiC whiskers and SiC as doping materials, respectively. The doping effect on phase composition, microstructure and critical current properties was investigated. Heat treatment was performed at 650 degrees C for 1 h under an argon gas atmosphere. All the doped tapes were found to have significantly enhanced critical current density J(C) at 4.2 K in magnetic fields up to 14 T compared with their undoped counterparts. Moreover, the tapes doped with nano-SiC had the best pinning performance, while the SiC whiskers and Si/N/C powders showed a similar improved field dependence of J(C) compared to undoped samples. At 4.2 K and 10 T, J(C) for the nano-SiC doped samples increased by a factor of 32. Even for Si/N/C doped tapes, a 16-fold improvement in the magnetic field J(C) was observed. It is inferred that the different chemical properties of the Si and C elements in SiC, SiC whiskers and Si/N/C led to the J(C)-B difference.
Abstract: We investigated the effect of ZrSi2 and SiC doping on the microstructure, critical current density J(c) and flux pinning of Fe-sheathed MgB2 tapes prepared by the in situ powder-in-tube method. Heat treatment was performed at a low temperature of 650 degrees C for 1 h. The phases, microstructure and flux pinning were characterized by means of x-ray diffraction, scanning electron microscope, magnetic and transport property measurements. It was found that the tapes doped with nanoscale SiC had the best pinning performance, while the ZrSi2 powder showed a similar improved field dependence of J(c) compared with undoped samples. J(c) values for the SiC doped samples were enhanced by two orders of magnitude at 4.2 K in magnetic fields above 8 T. At 4.2 K and 10 T, the J(c) reached similar to 1.5 x 10(4) A cm(-2). Moreover, the critical temperature for the doped tapes decreased slightly ( < 1.2 K). Microstructural analysis shows that very good grain connections or/and grain refinement were obtained for the doped tapes. The mechanism of the enhancement of the flux pinning is also discussed.
Abstract: Superconductors carrying currents larger than several 10 kA are required for magnetic confinement fusion systems. A cable-in-conduit-conductor (CICC), which has about 1000 of twisted thin superconducting strands in a ridged conduit, is a promising candidate for such large and high magnetic field superconducting system. A specific phenomenon in the CICC is so called avalanche-like quench (AQ). When some strands become resistive due to current saturation at critical current and/or a mechanical disturbance, the current re-distributes from the strands into neighboring ones, which are electro-magnetically coupled more strongly than the others. This causes current saturation in the neighboring strands. Finally, the whole cable becomes normal. We call this phenomenon an avalanche-like quench. Small multi-strand cables were tested by induced-current methods. Quench behaviors of both Nb3Sn and NbTi cables are observed and compared. In the experiment, for the Nb3Sn cable, the critical current measurement was stably carried out at high field without quench. In contrast, for the NbTi cable, we observed a number of AQs during inductive energization. Consequently, we suspected that the reason is based on the difference of critical temperature (T-C) between Nb3Sn and NbTi. The difference of TC is several Kelvins, however, their macroscopic behaviors differ significantly. (c) 2006 Elsevier B.V. All rights reserved.
Abstract: The critical current, upper critical field and critical temperature of bronze route Nb3Sn commercial wires are enhanced by applying a repeated bending strain at room temperature, i.e., âprebending strainâ. In order to investigate the prebending effects from a viewpoint of a residual strain, axial and lateral residual strains were evaluated directly by neutron diffraction at room temperature. We found that the axial residual strain changes from -0.10% to 0.02% but the lateral one is unchanged by applying a prebending strain of 0.5% for an ordinary bronze route (Nb, Ti)(3)Sn wires without reinforcement. Hence, in the case of the ordinary Nb3Sn wires without reinforcement, the prebending treatment modifies only the axial residual strain states independently to the lateral one, although it may depend on the wire structure. The critical current properties under the axial tensile strain suggest that the axial residual strain is reduced by about 0.11% but the radial residual strain unchanged by the prebending treatment of 0.5%. This is consistent with the results of the neutron diffraction.
Abstract: To demonstrate the applicability of the prebending strain effect to the R&W coil winding process, we have developed the prebent react-and-wind (R&W) process, which is the combination of the conventional R&W process and the prebending treatment. The superconducting and mechanical characteristics were investigated for CuNb/Nb3Sn superconducting wires which were picked up from the prebent R&W coil winding process. The results were compared with that of CuNb/Nb3Sn wires which were prebent manually using a prebending former. The prebending treatment using pulleys enhanced I-c more than the manual prebending treatment. This is explained qualitatively by a difference of the prebending strain distribution. The prebending strain distribution is plain symmetric and axis symmetric in cases of the manual prebending and the pulleyed prebending, respectively. The axis symmetrically distributed prebending strain reduces the residual strain more than the plain symmetrical one. The mechanical property was also enhanced. The slope of the stress-strain curve for the pulleyed prebent samples were steeper than that of as-heat-treated sample. The maximum I-c of the I-c-strain curve was enhanced and the strain value corresponding to the maximum I-c shifted to smaller strain by the pulleyed prebending treatment. Though in the large stress region over 200 MPa, the I-c-stress curves of pulleyed prebent samples and as-heat-treated sample were in good agreement, suggesting that the I-c enhancement effect by the prebending treatment was not beneficial, in the small stress region, it is quite beneficial.
Abstract: E - J properties of the high-T, materials are related to the distribution of the local critical current density J(c). From the previous results on Bi2212 thick films and practical Ag-sheathed Bi2212 tapes, we found that the distribution of the local J. is much affected by microstructures, and the local J(c) distribution becomes broad and asymmetric for well aligned plate-like Bi2212 grains. However, from a viewpoint of microstructures, the origin of the asymmetric J(c) distribution remains unclear. In order to understand the relationship between the J(c) distribution of Bi2212 materials and microstructure, we propose a âtwo directional J(c) distribution modelâ. In this model, the difference of the local J(c) distributions for various kinds of Bi2212 samples was explained in terms of the two different directional current flows and the aspect ratio of the grains. Moreover, the possibility of the small n-value even for high J(c) materials in Bi2212 is also shown.
Abstract: The worldâs first cryogen-free hybrid magnet, which was developed at the High Field Laboratory for Superconducting Materials in order to realize an easy-operational magnet system with no use of liquid helium and nitrogen, has achieved 22.7 T in a 52 mm room temperature bore. After this success, we started to construct a new cryogen-free 30 T-class hybrid magnet, consisting of an outer wide-bore cryogen-free 11 T superconducting magnet and an inner water-cooled 19 T resistive magnet. Up to now, the NbTi outer section coil and the Nb3Sn inner one of a wide-bore cryogen-free superconducting magnet has generated individual central fields of 5.3 T at 350 A and 5.8 T at 303 A, respectively in a 360 mm room temperature bore. The wide-bore cryogen-free superconducting magnet was energized up to 9.5 T as a total background field. In hybrid magnet mode the system was operated up to an 8.5 T background field form the cryogen-free superconducting magnet, because a cooling problem was encountered with the innermost coil bobbin during ramping the Bitter magnet. As a result, the cryogen-free hybrid magnet generated 27.5 T in a 32 mm room temperature bore.
Abstract: To understand the underlying physical trends of the current instability in a composite high-T-c superconductor, the limiting margin of its current-carrying capacity is derived in dc magnetic fields in the framework of the macroscopic continuum approximation. A static zero-dimensional model was used to formulate the peculiarities of the nonisothermal electric field distribution in a composite in the fully penetrated current states. The power and exponential equations describing the E(J) dependences of a superconductor are used. The boundary of the allowable stable values of the electric field, current, and temperature are investigated using qualitative and quantitative models. Permissible stable values of the electric field and current, which might be lower (subcritical states) or higher (overcritical states) than those determined by the critical voltage criterion, are discussed. It is stated that the subcritical quenching electric states are more probable in the operating regimes, which are observed in the high magnetic field. The overcritical stable quantities of the electric field exist, for example, if the superconducting composite has a relatively small volume fraction of the superconductor in a composite. In the meantime, the stable current modes may be both subcritical and overcritical when the permissible value of the electric field is overcritical. As a consequence of these features, an unavoidable increase in temperature of the composite superconductor occurs before its transition to the normal state. The latter depends on a broad shape of the E(J) dependence of high-T-c superconductor and the current sharing between the superconducting core and the matrix. In the limiting case, a stable value of the composite temperature may equal the critical temperature of the superconductor. For such operating states, the criterion of the complete thermal stability condition is written taking into consideration the nonlinear character of the E(J) dependence. Simultaneously, an allowable change in temperature of the superconducting composite leads to the thermal degradation of its current-carrying capacity. It depends on the critical current density of the superconductor at bath temperature, amount of a superconductor, and cross section of a composite under fixed cooling conditions. In particular, it is shown that the currents corresponding to the instability onset do not increase proportionally with relevant increase of the superconductorâs amount. The estimates presented have general character and may be used to verify the operating states of low-T-c superconducting composite. (c) 2006 American Institute of Physics.
Abstract: We have found that superconducting properties such as a critical current for bronze route Nb3Sn superconducting wires were enhanced by prebending strain (epsilon(pb)), which is the repeated bending strain at room temperature. In this study, four kinds of react and wind (R&W) processed CuNb reinforced bronze route Nb3Sn coils with epsilon(pb) = 1.0, 0.8, 0.5 and 0% were prepared. We investigated the effect of prebending strain for the coils. In the electromagnetic compressive stress condition, a critical current (I-c) of the R&W processed coil was enhanced by the prebending strain. These I-c values are larger than those of a short sample wire without prebending strain. In the hoop stress condition, the I-c of four coils revealed a similar value. Therefore, it is considered that the I-c was limited by the large tensile stress for all coils in the hoop stress states. These results suggest that the I-c enhanced by the effect of prebending strain is applicable for the R&W coil design without degradation in large stress and strain states.
Abstract: MgB2 bulk samples were synthesized by the solid-reaction process in high magnetic fields. The effect of magnetic fields on the microstructure and superconducting properties of MgB2 tapes has been investigated by using X-ray diffraction, scanning electron microscope and magnetic measurements. Under the application of an 8 T magnetic field, the critical transition temperature, T-c, remained unchanged, and the critical current density, J(c) was enhanced by a factor of more than 2 for both temperatures of similar to 5 K and 20 K. The experimental results suggested that the high magnetic field is effective for improving J(c) of MgB2 superconductors. (c) 2006 Elsevier B.V. All rights reserved.
Abstract: Fundamental examinations were carried out of high strength bronze-processed Nb3Sn Wires with Ta-reinforced filaments. A long Nb3SD wire with reinforced filaments was also trial-manufactured to test its practicality and its properties were measured. It is expected that Nb3Sn wires with high strength and high critical current can be realized by reinforcing the filaments with Ta. Our series of examinations and the trial-manufacture revealed the high potential usefulness of the wire in practical applications such as in superconducting magnets fabricated using the R&W method.
Abstract: It is reported that the repeated bending loads at room temperature for Nb3Sn wires, which show the strain dependence of superconducting properties, cause the large enhancement of a critical current, an upper critical field and a critical temperature. Those phenomena are so called âprebending effectâ. It is considered that the enhancement effect is associated with the reduction of the residual strain state for Nb3Sn wires. In order to apply the prebending treatment to a react-and-wind processed Nb3Sn superconducting magnet, repeatedly bent Nb3Sn wires with bending strains up to 1.0% were fabricated by using pulleys, and were confirmed to demonstrate the large enhancement of the critical current in spite of the bending strain of 0.5%. From these experimental results, we intended to fabricate the high strength Nb3Sn strands applying the prebending effect for the cabling conductor. As a first step, we prepared triplet cables using prebent CuNb/Nb3Sn and Cu/Nb3Sn strands and evaluated critical currents and n-values of each strand. The prebent Nb3Sn strands show the large enhancement of critical current I-c even after the cabling process. Hence, it is expected that the prebent high strength Nb3Sn strands may be useful for the cabling conductors. (c) 2006 Elsevier B.V. All rights reserved.
Abstract: We have successfully developed a cryocooled superconducting magnet which generates the worldâs highest magnetic field of 18.1 T. This conduction-cooled superconducting magnet has a 52-mm room temperature bore. The magnet consists of three sections; a high-T-c superconducting insert coil, four Nb3Sn coils, and an outer NbTi coil. Each section was energized by an individual power supply. The superconducting coils are cooled conductively by a Gifford-McMahon-Joule-Thomson (GM-JT) cryocooler with a cooling capacity of 4.3 W at 4.3 K. The ramp-up time to 18 T is 60 min. The high-T-c insert coil is composed of 25 double- pancake coils using Ag/(Bi,Pb)(2)Sr2Ca2Cu3O10 (Bi2223) high-T-c superconducting tape with stainless steel tape reinforcement. The reinforcement co-winding reduces the effective hoop stress down to 48 MPa, and, hence, Bi2223 tape can be used. The innermost of the Nb3Sn section is made of an internal-tin-processed Nb3Sn wire because this wire can withstand high critical current density in high magnetic fields. Other three coils require high mechanical strength. They experience hoop stress of 230 MPa, and in this case, bronze-processed high-strength Nb3Sn wires were used. The magnet has a distinctive feature, that is the high-T, insert coil is replaceable. It indicates that the magnet can be used as a 15.5 T back-up magnet for a new insert coil. A Y123 coil is a candidate for the replacement. (c) 2006 Elsevier B.V. All rights reserved.
Abstract: Superconducting properties of high-strength Nb(3)Su wires with Th-reinforced filaments were evaluated at various temperatures and magnetic fields. Mechanical properties were also investigated at 4.2 K. The critical current was measured at temperatures up to 14 K and in magnetic fields up to 15 T. The data were well scaled by using empirical scaling laws. The stress-strain curve showed that the stress corresponding to 0.5% strain was 284 MPa. The value is twice as large as that of conventional Nb3Sn wires, indicating that the high-strength Th reinforcement enhances the mechanical property of the wire. Furthermore, the wire was sufficiently ductile. To evaluate the thermal stability, minimum quench energy and normal zone propagation velocity were measured under a cryocooled condition. The Ta reinforced Nb3Sn wire shows relatively high stability and large propagation velocity. The results suggest that the Th reinforced Nb3Sn wire has sufficient applicability for cryocooled superconducting magnet windings.
Abstract: It is known that current sharing effects of the stabilizer are limited in cryocooling conditions being different from poolboiling. Therefore we have studied stability under it, focusing on minimum quench energy (MQE) of Nb3Sn superconductors under various conditions of the applied magnetic field, temperature, and the transport current, for various volume fractions and configurations of Cu stabilizer. From the experimental results, the larger the external Cu volume fraction, the higher the MQE was obtained. All MQE data were summarized by only one line in characteristics of normalized MQE and normalized B (B/B-c2). The obtained MQE behavior was to be characterized by only one empirical equation. It is thought that the obtained formula is useful for a standard of designing the Nb3Sn superconductor and its coil under cryocooled condition.
Abstract: A 19 T cryocooled superconducting magnet with a 52 mm room temperature bore is currently under construction. The magnet consists of an outer NbTi section, middle Nb3Sn section and innermost high-T-c section., The coils are exposed to an electromagnetic hoop stress following the magnetic field generation. In this 19 T cryocooled superconducting magnet, the high-T-c coil windings are Bi-2223 tapes with a low matrix ratio. Because of this, the mechanical characteristics, i.e., stress-strain characteristics, stress/strain dependence of the critical current and bending strain dependence of the critical current, were explored for the Bi-2223 tape. Furthermore, the small coil was wound with the superconducting tape. It was tested under magnetic fields to examine the stress-strain characteristics caused by the electromagnetic hoop stress.
Abstract: In order to develop a react-and-wind (R&W) processed Nb3Sn superconducting magnet, we investigated the influence on the superconducting properties due to the prebending treatment. Since a reacted Nb3Sn superconducting wire reveals a very sensitive response from stress and strain, we focus on the prebending treatment which is repeatedly applied to the Nb3Sn wire through several pulleys in the R&W coil winding process. We found that the prebending treatment doubles the 1, value at 20 T and 4.2 K for bronze route Nb3Sn wires. This I., enhancement effect comes from the T-c and 13,2 enhancement. It turned out that T-c increases from 17.4 to 17.9 K and as a result B-c2 also increases from 23.7 to 25.2 T at 4.5 K for bronze route multifilamentary Nb3Sn wires prebent at bending strain of 1.0% and at 5 repeated times.
Abstract: We designed and have been fabricating a conduction-cooled 19 T superconducting magnet consisting of a NbTi coil, Nb3Sn coils and a Bi2223 insert coil. The insert coil is made up of 25 double-pancake coils, which are wound with the conductors composed of an Ag-sheathed Bi2223 tape and a stainless steel tape for reinforcement. The inner diameter and the outer diameter of each double-pancake coil are 090 mm and 0180 mm, respectively. In order to investigate the mechanical and superconducting properties of the double-pancake coils, a model coil consisting of two double-pancake coils was fabricated and excited in liquid helium in the background fields up to 11 T and hoop stresses were applied to the model coil. It was found that the superconducting properties of the model coil did not change even after the hoop stress application of 52.7 MPa, which was exceeding the hoop stress applied in the 19 T superconducting magnet. The Bi2223 insert coil will be cooled by a GM-JT cryocooler to approximately 4.5 K through aluminum plates. Thermal stability analysis is very important especially for the conduction-cooled HTS coils operated at lower temperature since thermal runaway may cause the burnouts of the HTS coils. In this paper the analysis results are also discussed.
Abstract: E-J properties of the high-T, superconductors are related to the distribution of the flux pinning strength and the local critical current density J(c1). From the previous result of Bi2212 thick films, we found that the distribution of the local J(c) is much affected by microstructures, and the local J(c) distribution becomes broad and asymmetric for well aligned plate-like Bi2212 grains. These microstructures are similar to the case of practical Ag sheathed Bi2212 tapes. In this study, we performed E-J measurements of practical Ag sheathed Bi2212 tapes at various temperatures and magnetic fields, and the local Jc distribution was investigated.
Abstract: We have found that prebending treatment, which is repeated bending load at room temperature, greatly enhances the critical current, upper critical field and critical temperature of practical Nb3Sn superconducting wires. In this paper, we focus on the application of the prebending strain effect to practical superconducting coils fabricated by a react-and-wind method. To demonstrate the prebending strain effect on the react-and-wind coil, we prepared two kinds of CuNb/Nb3Sn superconducting monolayer coils. For one of the coils, the superconducting wire of 1.0 mm diameter was repeatedly bent by using ten fixed pulleys before the winding process, resulting in a prebending strain value of 1.0%. The final winding diameter of both coils was 200 mm and the number of turns was 49. In the compressive stress condition, the critical current of the coil with 1.0% prebending strain was 296 A at I I T, which was larger than that of the coil with 0% prebending strain. These values were much larger than the critical current of the witness sample. The results indicate that the prebending treatment enhanced the coil critical current. In the case of the hoop stress condition, the coil critical currents were approximately 230 A at I I T for both coils. The maximum electromagnetic hoop stress was calculated to be 360 MPa. The short sample tensile test results qualitatively explained the critical current deterioration of the coil. Furthermore, the 360 MPa hoop stress did not deteriorate the critical current irreversibly.
Abstract: Control and optimization of the residual strain are one of the most important issues in the development of Nb3Sn superconducting magnets. We found that the repeated bending loads at room temperature change the prestrain of Nb3Sn wires and result in the enhancement of J(c), B-c2 and T-c. We call this repeated bending strain âprebending strainâ. In order to understand the prebending effect, superconducting properties were measured as functions of temperature, field, axial tensile stress and strain. In addition, the three-dimensional strain state was also evaluated by the neutron diffraction. Those obtained results strongly suggest that the prebending process changes the radial residual strain as well as the axial one independently. Hence, it is considered that the prebending effect is effective for the control and optimization of the three-dimensional strain state on the react and wind process.
Abstract: The effect of nanoscale SiC addition on the field dependence of J(c) for MgB2 superconducting wire by suspension spinning was examined to enhance the flux pinning. The suspension spinning of commercially available MgB2 and SiC powders was examined to fabricate a long superconducting MgB2 wire by using the spinning medium of poly(vinyl alcohol) (PVA) or polyacrylonitrile (PAN). The as-drawn filaments were uniaxially pressed under 20 MPa at 200 degrees C for 8 h to remove volatile components and connect the MgB2 grains. The filamentary samples were enveloped by an iron sheet with a pellet of mixed powder of Mg and B to prevent Mg loss, and vacuum-sealed in a fused quartz tube and sintered. The field dependence of J(c) for the samples was examined at 4.2 K in magnetic fields up to 14 T. The J(c) of the sample was strongly dependent on the spinning medium and sintering condition. Addition of 5 at% SiC in the sample spun by PVA medium attained the maximum J(c) value more than 1000 A/cm(2) at 4.2 K by applying the field of 14 T.
Abstract: Fe-sheathed tapes of MgB2 with addition of SiC have been prepared by the powder-in-tube (PIT) method using a powder mixture mechanically milled in an argon or hydrogen atmosphere. The reactivity of the component powders in the mixtures, Mg, B and SiC, was significantly enhanced by mechanical milling, so that MgB, formed faster than in tapes fabricated from a hand-milled mixture. At the same time the number of impurity phases was higher, and the impurity phases of Mg2Si and Fe2B were observed only in these tapes. Mechanical milling also induced a lower quality of MgB2, especially for tapes produced from powders milled in a hydrogen atmosphere, with a significant influence on the values of the critical temperature T-C and critical field, J(C). (c) 2005 Published by Elsevier B.V.
Abstract: We prepared filamentary 0.1 at% Zr-doped (Nd0.33Eu0.38Gd0.28)Ba2Cu3Ox superconductors by solution spinning and partial melting in flowing 0-1% O-2 + Ar gas. The transport critical-current density (J(c)) was measured at 77 K in applied magnetic fields up to 14 T by rotating the sample along a direction perpendicular to the filament length. Anisotropic behavior of the field dependence of J(c) was detected by applying a field of more than 4 T. The J(c) values for the Zr-doped sample were higher than those for the undoped sample on the application of fields of up to 11 T. The J(c) values measured at the optimized angle for the doped sample were more than 10(5) A/cm(2) at 77 K by applying fields of up to 6 T. Transmission electron microscopy (TEM) of the sample was performed using an ultra high-voltage TEM to clarify the pinning centers. Intergrowth of 124 phase and stacking faults in the oriented 123 matrix were observed both for the doped and undoped samples. Zr doping resulted in the fluctuation of the structure with a short disorder range of 10-30 nm. A modulated structure and fine twin planes crossing each other were partly observed for the doped sample. The presence of such small-scale disorder could improve flux pinning in the middle field region. (c) 2005 Elsevier B.V. All rights reserved.
Abstract: Carbon nanohorn (CNH) doping in MgB2 wire prepared by suspension spinning was studied to enhance flux pinning. The mixed powders of MgB2 and CNH with nominal composition of (MgB2)(0.95)C-0.05 were suspended in a mixed poly(vinyl alcohol) solution. The as-drawn filaments were hot-pressed under 20 MPa at 200 degrees C for 8 h. The samples were enveloped by an iron sheet with a pellet of mixed powder of Mg and B and vacuum-sealed in a fused quartz tube, and then sintered at the temperature ranging from 885 degrees C to 900 degrees C for 2-3 h. The superconductivity of the samples was studied by electrical resistivity and SQUID method. Serious deterioration of T, was not observed for the doped sample. The CNH doping resulted in enhancement of the transport J(C) values at 4.2 K in high fields ranging from 6 T to 10 T by sintering at 885 degrees C for 2 h. The superconductivity at 4.2 K was maintained by applying the field of 14 T for the doped sample sintered at 900 degrees C for 3 h. (c) 2005 Elsevier B.V. All rights reserved.
Abstract: Lorentz force is applied to coil windings when the magnet is charged. The coil windings are subjected to huge hoop tensile stress and transverse compressive stress in both the radial and axial directions due to the Lorentz force. The higher the magnetic field and the larger the bore, the larger the Lorentz force applied to the coil windings. Thus, not only the hoop stress, but also transverse compressive stress should be taken into account when designing the magnet. In this paper, we focus on transverse compressive stress, and the influence of this stress on critical current is explored for a standard Nb3Sn wire and a Cu-NbTi-reinforced Nb3Sn wire. It was confirmed that CuNbTi sufficiently reinforced the superconducting wire, not only for tensile stress, but also for transverse compressive stress. The effect of reinforcement arrangement in the cross section was also investigated. In the case that Cu-NbTi reinforcement is arranged at the center of the wire, there was less deterioration in superconducting property than in the case of the reinforcement being arranged in the outer part of the wire. (c) 2005 Elsevier Ltd. All rights reserved.
Abstract: It is recognized that the strain in Nb3Sn conductors is a serious problem, because it suppresses superconducting properties. We have proposed that prebending treatment, which means repeated bending loads, is one of the ways to control the strain state. The upper critical field and the critical temperature were examined for the prebent. Nb3Sn wires. We found that the enhancement of the upper critical field from 23.7 T to 25.2 T at 4.5 K and the enhancement of the critical temperature from 17.4 K to 17.9 K were achieved for the CuNb-reinforced Nb3Sn wire by applying the prebending strain. Since these values are similar to those of the strain-free Nb3Sn bulk, the strain state of the wires is definitely optimized by the prebending treatment.
Abstract: The magnet technology at the High-Field Laboratory for Superconducting Materials (HFLSM) contributed to the success of the worldâs first practical cryogen-free superconducting magnet. A new project of a cryogen-free 19 T superconducting magnet with a 52mm room temperature experimental bore has started. A B(i)2Sr(2)Ca(2)Cu(3)O(10) high-temperature superconducting insert coil is to be employed. In the magnetic field application for materials development, magnetic levitation has been demonstrated using a cryogen-free hybrid magnet. Up to now, the cryogen-free 21.5 T hybrid magnet is easily operated for a container less melting experiment. In addition, a cryogen-free 30 T hybrid magnet is now being constructed at the HFLSM. (C) 2004 Published by Elsevier B.V.
Abstract: We present the magnetic-field dependence of terahertz (THz)-radiation power from femtosecond-laser-irradiated InAs with various surface orientations. Under 800 nm optical excitation, the magnetic field that provides the maximum THz-radiation power is found to be affected by the surface orientation, and InAs (111) exhibits it at lower magnetic fields than that of the other surfaces. In contrast, under 1560 nm excitation, the dependence on the surface orientation almost disappeared, and saturation is observed at a much smaller magnetic field than that in the 800 nm excitation case. Additionally, from the results of magnetic-field dependence up to 14 T, the shift of the peak in the THz-radiation spectrum toward lower frequency is confirmed, depending on the magnetic field applied, which is possibly induced by the emergence of a magnetoplasma effect. (C) 2004 American Institute of Physics.
Abstract: This paper presents acoustic emission (AE) signals induced from the Central Solenoid (CS) model coil of the International Thermonuclear Experimental Reactor (ITER) program. Envelopes of the AE signals were quasi-continuously measured with high-time resolution of 100 mus using six multi-channel AE sensors in order to detect the disturbances in the CS model coil during the process of the series of direct current (DC) operations. The AE signals were considered to be originally induced by motion of superconducting cables in cable-in-conduit (CIC) conductors and local motion of the conductors, judging from the close correlation between the AE signals and voltage spikes in the coil, especially during the virgin current operation. The multi-channel measurements provide us with information about the distribution of disturbances that could be detected acoustically by the AE sensors installed at each point of the CS model coil. The observation of AE envelopes with high-time resolution showed that the disturbances at each location of the CS model coil decreased by repeatedly charging-up the coil, judging from instantaneous AE levels, AE energy and the number of AE events. Direct measurements of the number of AE events that were carried out at another point on the coil confirmed that the disturbance dependence on the number of operations was similar to that mentioned above. The transfer function methods using one pair of AE sensors enabled us to analyze changes that might occur in either the coil structure or in the disturbance in frequency region during the repeating of the charging-up processes of the coil. (C) 2003 Elsevier Ltd. All rights reserved.
Abstract: Several types of reinforced Nb3Sn wires have been developed to prevent reduction of superconducting properties by applying a strong electromagnetic force. To fabricate a cryocooled magnet using those reinforced wires, we experimentally measured the minimum quench energy (MQE) under cryocooled conditions of some reinforced Nb3Sn wires. As a result, it became clear that thermal stability expressed as MQE was controlled by the temperature margin between the temperature of the operating condition and the transition temperature from superconductivity to normal. Using the FEM analysis, it was realized that the cause of the decline in thermal stability for the reinforced wires was the low thermal conductivity of the reinforced materials. (C) 2004 Elsevier Ltd. All rights reserved.
Abstract: Nb3Sn superconducting wires reinforced with Cu-Ni/Nb-Ti composite have been developed. Nb3Sn wires reinforced with Cu-Ni/Nb-Ti showed good mechanical and electrical properties. In this study, 1.2 mm-diameter (Nb, Ti)(3) Sri superconducting wire of 13 km in length reinforced with Cu-Ni/Nb-Ti was successfully fabricated. Moreover, the effect of copper fraction to the strength of the wire was studied.
Abstract: In high temperature superconductors, a low resistivity state is quite stable from a viewpoint of power application. When a low resistivity state instead of the superconductivity with zero resistance is concentrated on, a current-carrying limitation before thermal runaway has to be investigated for high temperature superconductors. We have examined the thermal runaway parameters related to the beat transfer coefficient and the matrix resistivity for Ag-sheathed Bi2Sr2CaCu2O8 tape. The calculated parameters were evaluated in comparison with the experimental results obtained under the cryocooling condition.
Abstract: In order to investigate the E-J properties in a practical current region, we performed the E-J measurement for a PAIR processed Bi2212 short sample using a pulsed current power supply. We could measure the E-J properties in a wide range of electric fields from 0.05 to 100 muV cm(-1) at transport currents up to a few tens of amperes without the thermal effect due to the Joule heating in a cryocooling condition.
Abstract: In order to evaluate the effect of the microstructure on E-J properties, the E-J properties for Bi2212 thick films prepared under three different conditions were measured. The measured E-J properties were analyzed based on the percolation model, and the local J(c) (J(cl)) distribution was evaluated. The distribution of the local J(c) was much affected by its micro structures. For the well aligned sample with large plate-like grains, which is similar to Ag-sheathed Bi2212 tape, the distribution of the local J(c) at percolation transition field B-GL was found to be asymmetric and reached the high J(cl) region. (C) 2004 Elsevier B.V. All rights reserved.
Abstract: We have studied a fabrication of filamentary ternary RE123 superconductors by changing the RE element for pinning enhancement. In this paper, the J(c) of the filamentary (Eu, Gd, Ho)123 superconductors prepared by solution spinning and partial melting under various atmospheres was examined. As the oxygen partial pressure decreased, the optimum melting temperature decreased. The T-c of the samples melted in flowing 0.1%O-2 + Ar and 1%O-2 + Ar was around 92 K and the T-c for the samples melted in flowing 20%O-2 + Ar was low such as 83 K. The transport J(c) values for the samples were measured at 77 K in applied magnetic fields up to 14 T by rotating the sample in a direction of the filament diameter. Anisotropic behavior of the field dependence of Jc along the filament diameter was detected. The highest J(c) value of 4100 A/cm(2) at 77 K and 14 T was detected for the sample melted in flowing 0.1%O-2 + Ar. By the rotating the angle for 90degrees, the superconductivity at 77 K was disappeared by applying the field more than 8 T. (C) 2004 Elsevier B.V. All rights reserved.
Abstract: To understand the basic mechanisms of the thermal runaway phenomenon, the limiting margin of the current instability, which may spontaneously occur in composite high-T-C superconductors like multifilament Bi-based wire or tape, is derived under DC magnetic field. The current sharing and allowable temperature rise effects were considered. A static zero-dimensional model was utilized to describe the basic formulae dealing with the peculiarities of the non-isothermal change of superconducting composite voltage-current characteristic. The boundary of allowable stable values of the temperature, electric field and current are derived analytically. It was shown that permissible values of the current and electric field might be higher than those determined by use of the standard critical current criterion. In consequence of this feature, the noticeable allowable temperature rise of the composite superconductor before its transition to the normal state may be seen. The criterion for complete thermal stability condition is written describing the state when temperature of the composite equals critical temperature of a superconductor and the transport current flows stably only in matrix. The performed analysis also proves the existence of value of the volume fraction of a superconductor in composite at which its current-carrying capacity has minimum. These peculiarities are due to the stable current redistribution between superconductor and stabilizing matrix. Therefore, the current sharing not only leads to the matrix/superconductor ratio effect on the stable operating characteristics of the composite high-T-C superconductors but also becomes important in the adequate description of quench process in the high-T-C superconducting magnets. (C) 2004 Elsevier B.V. All rights reserved.
Abstract: The current-carrying capacity of the Ag/Bi2Sr2CaCu2O8 multifilament conductor is studied in detail as a function of the heat transfer coefficient, resistivity of the matrix, and volume fraction of the superconductor. The thermal runaway parameters corresponding to the irreversible superconducting-to-normal transition are derived analytically under the aspect of possible finite temperature rise of the composite conductor before thermal runaway. The static analytical model determines the thermal runaway parameters. The power law describes the voltage-current characteristic of a superconductor with a linear temperature dependence of the critical current (linear approximation). The performed analysis reveals that the allowable magnitude of the current and electric field before the thermal runaway may be higher than those determined by the criterion E-c = 10(-6) V cm(-1) in many practical cases. The condition of the stable state for the over-critical current is formulated and the peculiarities of such operation regimes are discussed. It is shown that the essential stationary overheating of the superconductor may occur before the thermal runaway. The minimum value of the current at which the thermal runaway starts is found if the volume fraction of superconductor in the composite is changed.
Abstract: We have started a new project to develop a cryogen-free 19 T superconducting magnet with a 52-mm room temperature bore. The magnet is designed to be consisting of three sections, i.e., the innermost high-T-c superconducting coil section, the middle Nb3Sn section and the outer NbTi section. The electromagnetic hoop stress is applied to the superconducting wire due to the magnetic field generation. It is described to the products of magnetic field, transport current density, and winding radius. The maximum electromagnetic stress is estimated to exceed 200 MPa in the middle Nb3Sn section and 50 Pa in the high-T-c section. It is important to explore the mechanical characteristics to design the magnet. In this study, mechanical properties, i.e., stress-strain characteristics and stress dependence of the critical current, were investigated for oxide superconducting tapes.
Abstract: Terahertz (THz) radiation from femtosecond-laser-irradiated InAs (100) surface is investigated. It is found that THz-radiation spectrum exhibits two inter-related phenomena in a strong magnetic field under the Voigt configuration. The peak shift of THz-radiation spectrum toward lower frequency is observed with increasing magnetic field. Additionally, THz-radiation spectrum is found to posses a periodic structure owing to the interference of THz-radiation pulses from the front and back surfaces of the InAs substrate. The physical origin of the latter phenomenon is discussed by considering the magneto-plasma effect, which affects both real and imaginary parts of refractive index for THz-radiation propagating in a direction perpendicular to the magnetic field.
Abstract: We investigated the influence of the repeated bending strain on the critical current properties. The bending strain was applied to the Ti added high strength CuNb/Nb3Sn wires at room temperature and released. We define this loading and unloading bending strain at room temperature as a âpre-bendingâ treatment. We found that the critical current I-c is much enhanced by applying the pre-bending treatment. The maximum enhancement of I-c due to the pre-bending treatment is about twice at 17 T, for instance. The stress dependence of I-c shows that I-c is improved in low stress states by pre-bending treatment. This phenomenon can be described well by the uniaxial strain analysis on the basis of the strain distribution. In addition, the increase of the maximum critical current of the strain dependent I-c is also observed. This may be related to the reduction of the radial/tangential prestrain.
Abstract: We are now developing a 30 T hybrid magnet utilizing a cryocooler-cooled superconducting magnet wound with highly strengthened (Nb, Ti)(3)Sn. Diameter of the room temperature bore of the superconducting magnet is 360 mm and it generates 11.1 T. Water cooled resistive insert magnet generates 18.9 T, thus the hybrid magnet generates a central field of 30.0 T. The (Nb, Ti)(3)Sn multifilamentary wires are strengthened by Cu/NbTi composite which volume ratio in conductor is about 35%. The reinforcing Cu/NbTi composite changes to CuTi intermetallic compounds during heat treatment for reaction of (Nb, Ti)(3)Sn phase formation. The Nb3Sn coil with inner diameter of 400 mm will be fabricated by wind and react method with Cu/NbTi reinforced (Nb, Ti)(3)Sn wires. The innermost section of Nb3Sn coil is wound with a wire which diameter is 1.85 mm and next second section is wound with a wire diameter of 1.8 mm. The Nb3Sn coil is operated at 303 A and generates 5.8 T. The NbTi coil is wound with NbTi wires of 2.0 mm and 1.6 mm diameters. The NbTi coil generates 5.3 T at an operating current of 350 A. The maximum hoop stress is under 220 MPa for Nb3Sn coil and 200 MPa for NbTi coil.
Abstract: This paper describes a design of a cryocooler-cooled 19 T superconducting magnet. The technical features of the magnet are a Bi2223 insert coil composed of 25 double pancake coils, Nb3Sn coils using a Nb3Sn wire reinforced with Nb-Ti-Cu compound, and a cooling structure using two types of cryocoolers. Coil protection from quenching was confirmed by numerical analysis. A preliminary experiment was carried out in order to investigate the influence of the bending strain upon a maximum permissible hoop stress of Ag-sheathed Bi2223 tape conductor.
Abstract: In order to solve a problem of a large amount of liquid helium supply for a wide bore superconducting magnet of a hybrid magnet, we intend to construct the first cryogen-free 23 T hybrid magnet, consisting of an outer wide bore cryogen-free superconducting magnet and an inner water-cooled resistive magnet. Up to now, the wide bore cryogen-free superconducting magnet has been tested to generate a central field of 6.0 T in a 360 mm room temperature bore. The worldâs first cryogen-free hybrid magnet achieved 21.5 T in a 52 mm room temperature bore, combining with a 15.5 T water-cooled Bitter magnet. As a result, the cryogen-free hybrid magnet no longer needs a troublesome handling time for liquid helium transfer, and therefore the available machine time is highly increased. The improvement of the maximum magnetic field generation toward our designed value of 23 T is being carried out. Furthermore, we have started a new construction project of a cryogen-free 30 T hybrid magnet. Since the magnetic force field BdB/dz of 2274 T-2/m is obtained by 21.5 T, the cryogen-free hybrid magnet can provide a large magnetic force field enough to levitate diamagnetic materials. For new processing of materials development, a YAG laser furnace was installed into the cryogen-free 23 T hybrid magnet. A container-less melting for paraffin was examined by controlling a thermocapillary convection. We succeeded in fabricating a magnetic field oriented ball in magnetic levitation.
Abstract: In order to study magnetic levitation easily, we develop a cryo-cooled large magnetic field gradient superconducting magnet as the insert magnet for a large bore (220 mm cylindrical room temperature bore) cryo-cooled superconducting magnet. The insert magnet consists of a modified cusp coil and generates large magnetic field gradient. Combined with the background magnet, this magnet system realizes the stable magnetic levitation for the diamagnetic materials. The magnet system is designed to generate 1400 T-2/m for the magnetic levitation of water. As a result, this cryo-cooled magnet system quenched from the insert magnet, but could achieve nearly 1050 T-2/m. As a performance test, a bismuth tip was levitated using the newly developed cryo-cooled superconducting magnet.
Abstract: We report on the terahertz radiation from femtosecond-laser-irradiated indium arsenide in high magnetic fields up to 14 T. It is found that the radiation power exhibits anomalous magnetic-field dependence, including saturation, decrease, and recovery up to 14 T. Moreover, the radiation spectrum possesses a clear periodic structure over 6 T. (C) 2003 American Institute of Physics.
Abstract: The influence of a repeated bending strain at room temperature on the critical current densitiy of bronze route Nb3Sn wires was investigated. We found that the critical current I-c is significantly increased by applying a repeated bending load (pre-bending treatment) at room temperature. The maximum enhancement of I-c was approximately twice at 16 T for CuNb-reinforced Nb-3 Sn wires by a 0.5% prebending strain and the critical current density J(c) became about 528 A/mm(2) at 4.2 K and 16 T. A comparison of the stress dependent I, values obtained before and after applying prebending treatments for CuNb/Nb-3 Sri wires indicates that not only the stress dependence of I-c but also the maximum I-c at the axial strain-free state are improved. The improvement of the stress dependent, is in agreement with the prediction made based on the calculation using the uniaxial stress/strain distribution model. Moreover, the increase in the maximum I-c may be related to the release of stress/strain states along the radial direction through prebending treatments.
Abstract: The development of a cryogen-free hybrid magnet with no use of liquid helium was intended. A cryogen-free hybrid magnet is composed of an outer conduction-cooled superconducting magnet using GM-cryocoolers and an inner water-cooled resistive magnet. As a first step, we constructed a cryogen-free NbTi superconducting magnet with a 360 mm room temperature bore, which generated 4.5 T at a magnet center. The thermal stability concept of the critical current margin is adopted for magnet design. A 20 T static magnetic field and a magnetic force field corresponding to 2030 T-2/M by combining with the 15.5 T water-cooled magnet were obtained. A container-less melting experiment was carried out using a newly developed 20 T cryogen-free hybrid magnet. A great interest is focused on Marangoni convection in container-less melting. Thermocapillary convection known well as Marangoni is being investigated in the non-gravity state due to high magnetic fields, using the worldâs first cryogen-free hybrid magnet. (C) 2002 Elsevier Science B.V. All rights reserved.
Abstract: The magnetic-field dependence of THz-radiation power from InAs surface is investigated by using a hybrid magnet, which is capable of providing a magnetic field up to 28 T. It is found that THi-radiation power saturates at approximately 3 T and also at 13 T. The maximum THz-radiation power with a high-frequency component spectrum is observed at 3 T. This result leads to the conclusion that a magnetic field of 3 T is optimum for the enhancement of THz-radiation power. Additionally, the THz-radiation spectrum exhibits a periodic structure at magnetic fields above 12 T. This can be attributed to the change in dielectric constant induced by the strong magnetic field resulting in the interference of THz-radiation pulses from the front and back surfaces of the InAs substrate.
Abstract: Transport characteristics of Bi-2212 wire in flux-flow state were experimentally investigated under a cryocooled condition in high magnetic fields up to 14 T at various temperatures. The heat balance affected the thermal stability of oxide superconductors, and the oxide superconductor with large current capacity and large n value quenched at the current that was smaller than its critical. current.
Abstract: We are now constructing a cryogenfree 23 T hybrid magnet at the High Field Laboratory for Superconducting Materials, Institute for Materials Research, Tohoku University. At present, an outer section coil employing NbTi multifilamentary Wires for a cryogen,free superconducting magnet of the hybrid magnet wag combined with an inner 15.5 T water-cooled resistive magnet, and was tested as the worldâs first cryogenfree hybrid magnet., The NbTi coil With 491 mm inner diameter and 584 mm outer diameter generated 4.59 T at 198 A, and the central magnetic field of 20.0 T was generated in a 52 mm room temperature experimental bore. The magnetic force field of 2030 T-2/m was obtained, and a piece of paraffin was levitated at 1200 T-2/m. Using a CO2 laser combined with the cryogenfree hybrid magnet, a containerless melting experiment in magnetic levitation was demonstrated easily for paraffin.
Abstract: Evaluation method of critical current (I-c) and current sharing temperature (T-cs) is proposed and applied to the experiment of ITER Center Solenoid (ITER-CS) Model Coil Insert, which is a Nb3Sn superconducting coil. Voltage behavior related to normal state transition of conductors during I-c or T-cs measurement is not yet well understood especially in case of such a large cable with more than one thousand strands as ITER-CS Insert, because magnetic field, which makes large effect on superconducting property, is not constant inside of a cable. From the detailed analysis on the voltage behavior of coils, it is found that integral of electric field averaged over conductor cross section along strand-longitudinal direction is equal to the voltage which is measured by voltage taps during a coil test. It is because twist pitch of a cable is less than the range of longitudinal field variation in case of a large-cable-conductor coil. This evaluation method can estimate voltage behavior and predict I-c and T-cs values, which are important parameters for the design of a large-cable-conductor coil, based on the property of strands composing the conductor.
Abstract: Superconducting magnets wound with conventional Cu/Nb3Sn are designed considering a stress design criterion of 150 MPa. If the highly strengthened CuNb/Nb3Sn wire is used instead of the conventional CuNb/Nb3Sn wire, the stress design criterion of 250 MPa is applicable. Furthermore, the high strength enables us to make the Nb3Sn superconducting coil using a react and wind method. In order to confirm the stress design criterion of 250 MPa in a CuNb/Nb3Sn coil fabricated by a react and wind method, stress-strain and quench characteristics of the coil were investigated under high electromagnetic stress. The hoop stress of 279 MPa was applied and the measured strain was 0.34%. The quench currents were 50-60% of the critical current for the short sample, and their relevant deterioration of the quench current strongly influenced the large hoop stress. It was confirmed that the design criterion of 250 MPa is adequate for the CuNb/Nb3Sn superconducting coil using the react and wind method.
Abstract: The Central Solenoid Model Coil (CSMC) was designed and built from 1993 to 1999 by an ITER collaboration between the U.S. and Japan, with contributions from the European Union and the Russian Federation. The main goal of the project was to establish the superconducting magnet technology necessary for a large-scale fusion experimental reactor. Three heavily instrumented insert coils were built to cover a wide operational space for testing. The CS Insert, built by Japan, was tested in April-August of 2000. The TF Insert, built by Russian Federation, will be tested in the fall of 2001. The NbAl Insert, built by Japan, will be tested in 2002. The testing takes place in the CSMC Test Facility at the Japan Atomic Energy Research Institute, Naka, Japan. The CSMC was charged successfully without training to its design current of 46 kA to produce 13 T in the magnet bore. The stored energy at 46 kA was 640 MJ. This paper presents the main results of the CSMC and the CS Insert testing-magnet critical parameters, ac losses, joint performance, quench characteristics and some results of the post-test analysis.
Abstract: In order to settle problems requiring a large amount of liquid helium and limiting the operation time for a wide bore superconducting magnet of a hybrid magnet, a cryogen-free 23 T hybrid magnet is being constructed at the High Field Laboratory for Superconducting Materials for the first time. An outer compact superconducting magnet is wound with highly strengthened CuNb/Nb3Sn multifilamentary wires and is refrigerated conductively by GM-cryocoolers. The maximum stress value of 210 MPa was designed for the CuNb/Nb3Sn coil. The cryogen-free super-conducting magnet will be operated using dual power supplies independently, and has potential to generate central fields of 4.59 T at 198 A for the outer section NbTi coil and 3.41 T at 145 A for the inner section CuNb/Nb3Sn coil. When the cryogen-free 7.5 T superconducting magnet with a 360 mm room temperature bore is combined with an inner 15.5 T water-cooled resistive magnet, a cryogen-free hybrid magnet will achieve 23.0 T in a 52 mm room temperature experimental bore.
Abstract: Quench properties of Bi2Sr2CaCu2O8/AgMg tape were experimentally investigated under a cryocooled condition in high magnetic fields. The critical current (I-c), quench current (I-c, and minimum quench energy (MQE) were measured at various initial temperatures of 4.2 K, 10 K, and 20 K and at magnetic fields of 7 T and 9 T. MQE was the minimum disturbance energy enough to make thermal quench and was evaluated from the input energy by a resistive heater. It was found that MQE exhibited the minimum value around 20 K. The temperature margin, difference between the initial temperature and the thermal runaway temperature, decreased monotonously with increase at the operating temperature. Since the temperature dependence of MQE did not agree with that of temperature margin, we focused on the internal energy calculated by integrating specific heat of the sample. The temperature dependence of the internal energy was in agreement with that of MQE qualitatively.
Abstract: In order to investigate the stress-strain and superconducting properties for a react and wind method under large electromagnetic stress states, a test coil with a large bore was fabricated using the highly strengthened CuNb/(Nb, Ti)(3)Sn wire. We found that the analysis on the basis of the bending and tensile strains successfully explains the obtained mechanical properties. The operation currents could be applied, until the electromagnetic stress and strain became about 340 MPa and 0.34%, respectively. This is mainly due to not only the strong mechanical property of CuNb/(Nb, Ti)3Sn wire but also the compression stress induced by the thermal contraction. As a result of the bending and tensile strain analysis for the critical current, the ratio of the quench current to the critical current was about 86% at 11 T and then decreased with decreasing magnetic fields.
Abstract: The worldâs largest pulsed superconducting coil was successfully tested by charging up to 13 T and 46 kA with a stored energy of 640 MJ. The ITER central solenoid (CS) model coil and CS insert coil were developed and fabricated through an international collaboration, and their cooldown and charging tests were successfully carried out by. international test and operation teams. In pulsed charging tests. where the original goal was 0.4 T/s up to 13 T. the CS model coil and the CS insert coil achieved ramp rates to 13 T of 0.6 T/s and 1.2 T/s, respectively. In addition, the CS insert coil was charged and discharged 10 003 times in the 13 T background field of the CS model coil and Ilo degradation of the operational temperature margin directly coming from this cyclic operation was observed. These test results fulfilled ail the goals of CS model coil development by confirming the validity of the engineering design and demonstrating that the ITER coils can now be constructed with confidence.
Abstract: A 60kA HTS current lead has been designed for large fusion magnets such as ITER magnet. The actual refrigeration input power required to cool the current lead is specified to be reduced to one third that of the conventional copper lead. The HTS part of the 60 kA lead consists of 48 units installed with cylindrical array into the enter surface of a stainless steel tube with a diameter of 145 mm, Each unit is composed of six Bi2223/Ag-10at%Au tapes, and its cross-sectional dimension is 6.5 mm x 2.7 mm. The HTS part is cooled by conduction, and the warm and cold end temperature conditions of the HTS part are 50 K and 4.5 K, respectively. The copper part is cooled by helium gas, a flow rate of 3.9 g/s and the inlet temperature of 35 kA. The 60-kA lead has been designed in consideration of safety under the long discharge time condition of ITER-TF coil with a detection time of 2 sec. and a discharge time constant of 15 sec. For the purpose of verifying the reliability of the design for the lung discharge time, one unit sample has been fabricated and tested. The result indicates that the maximum temperature rise of the HTS part is less than 150 k for the ITER like-discharge from 1.25 kA. corresponding to 60 kA of the full lead with 48 units.
Abstract: The Inner and Outer modules of the Central Solenoid Model Coil (CSMC) were built by US and Japanese home teams in collaboration with European and Russian teams to demonstrate the feasibility of a superconducting Central Solenoid for ITER and other large tokamak reactors. The CSMC mass is about 120 t, OD is about 3.6 m and the stored energy is 640 MJ at 46 kA and peak field of 13 T. Testing of the CSMC and the CS Insert took place at the Japan Atomic Energy Research Institute (JAERI) from mid March until mid August 2000. This paper presents the main results of the tests performed.
Abstract: A 13 T-46 kA Nb3Al conductor with steel jacket has been developed in the framework of ITER-EDA. The thermal strain will appear on the Nb3Al strands after heat treatment, when the steel jacket is used because of difference in their thermal expansion. The conductor critical current performance is degraded by this process. The critical current performance of this conductor was then studied to verify no degradation during the fabrication and small deterioration in the critical current performance even when the steel jacket is used. The thermal strain is evaluated to be < 0.4% for the developed Nb3Al conductor. The critical current deterioration for this case is < 10%. Also, the critical current of the conductor without the thermal strain shows good agreement with that calculated by the critical current of a strand multiplied by its number, 1152. Consequently, no degradation during the conductor fabrication process can be verified. Moreover, the critical current is expected to be more than 90 kA for the thermal strain of 0.4%, resulting in a large margin against the nominal current, 46 kA. <(c)> 2001 Elsevier Science B.V. All rights reserved.
Abstract: The Inner and Outer modules of the Central Solenoid Model Coil (CSMC) were built by US and Japanese home teams in collaboration with European and Russian teams to demonstrate the feasibility of a superconducting Central Solenoid for ITER and other large tokamak reactors, The CSMC mass is about 120 t, OD is about 3.6 m and the stored energy is 640 MJ at 46 kA and peak field of 13 T, Testing of the CSMC and the CS Insert took place at Japan Atomic Energy Research Institute (JAERI) from mill March until mid August 2000. This paper presents the main results of the tests performed.
Abstract: The CSMC is a solenoid consisting of 18 concentric layers, wound with Nb3Sn cable-in-conduit conductor, jacketed with Incoloy. The Japanese HT and USHT were in charge of the manufacturing, and the coil is being tested at JAERI, Naka (Japan). The European Home Team is involved in the testing program, which includes electromagnetic characteristics of the CSMC, and of the CS Insert. Two effects influence strongly the electromagnetic properties: (i) connections introduce a non uniform current distribution among the strands, (ii) the performance of the strands are dependent on the strain of the Nb3Sn filaments. The current distribution calculated in the CS Insert takes into account magnetic field and strain applied on the conductor, the joint effect and the current transfer between strands. The model developed at CEA, uses an equivalent electric network composed of resistive joints, superconducting lengths and interstrand resistances and provides the voltage-current characteristics, which can be directly compared to the experimental results.
Abstract: The ITER-CS model coil was fabricated and its performance test was successfully carried out. The assembling technique by using indium wires was developed for the terminal joints. The short samples were fabricated to establish the technique and their joint resistance was low enough ( similar to 4 n Omega at 4.5 T, 50 kA). This technique was applied to the assembling of seven joints in the CS model coils and the assembling was successfully completed. It has been confirmed in the performance test that the resistance of these joints is lower than 5 n Omega. According to these results, it can be concluded that the developed assembling technique by using indium wires is applicable and useful to the terminal joints of superconducting coils with the large-current conductors. (C) 2001 Elsevier science B.V. All rights reserved.
Abstract: The largest pulsed superconducting coils ever built, the Central Solenoid (CS) Model Coil and Central Solenoid Insert Coil were successfully developed and tested by international collaboration under the R&D activity of the International Thermonuclear Experimental Reactor (ITER), demonstrating and validating the engineering design criteria of the ITER Central Solenoid coil, The typical achievement is to charge the coil up to the operation current of 46 kA, and the maximum magnetic field to 13 T with a swift rump rate of 0.6 T/s without quench, The typical stored energy of the coil reached during the tests was 640 MJ that is 21 times larger than any other superconducting pulsed coils ever built. The test have shown that the high current cable in conduit conductor technology is indeed applicable to the ITER coils and could accomplish all the requirements of current sharing temperature, AC losses, ramp rate limitation, quench behavior and 10 000-cycle operation. (C) 2001 Elsevier Science B.V. All rights reserved.
Abstract: The fabrication of the outer module has been completed, which is one of the Central Solenoid (CS) model coils being developed as the most important R\&D for ITER construction. The inner and outer diameters are 2.8 m and 3.6 m, respectively. The weight is 63 tons. The conductor is a Nb3Sn cable-in-conduit conductor with a square Incoloy 908 conduit. The coil that consists of 8 layers was fabricated with a solenoid winding with two conductors-in-hand. The outer module will be connected with the inner module coil in series and contribute to the generation of 13 T at 46 kA. The test will be started in November 1999.
Abstract: The Central Solenoid (CS) model coil program is in progress with an international collaboration under the frame of the ITER-EDA. The purpose of the CS insert coil is to test the performance of the ITER-CS conductor. The CS insert coil is installed in the bore of the CS model coil and tested at a magnetic flux density of 13T. The installation work is underway with the inner and outer module of the CS model coil. The superconducting characteristics of the CS conductor, the critical current and the current sharing temperature are evaluated under the operating load. The AC loss characteristics of the conductor are also evaluated under pulsed magnetic field. The fabrication of the CS insert coil was completed on May 1999. The winding tools and the results of the winding of CS insert coil are reported. The heat treatment for Nb3Sn processing was performed successfully with no SAGBO (Stress Accelerated Grain Boundary Oxidation). The procedure of the heat treatment is also reported.
Abstract: The conductor joint is one of the key technologies for superconducting coils. A butt type joint has been successfully developed for the ITER magnets. The 46kA Nb3Sn conductors are connected by the diffusion bonding technique in vacuum, after the reaction of Nb3Sn. The advantage of this joint is low losses against pulse field, because the compacted part is very small compared with other types of joint. 15 butt joints have already been fabricated in the ITER CS Model Coil. According to the test results of the full-size conductor samples, these butt joints will be operated stably in the pulse operation, because the temperature increase due to ac losses and Joule heating by joint resistance is very small and the joint has sufficiently high temperature margin.
Abstract: As research and development of central solenoids for fusion reactors, a 13 T, 640 MJ superconducting pulsed coil has been constructed. The coil has a 1.6m inner diameter, a 3.6m outer diameter and a 1.7m height, which is composed of a inner module and an outer one. The coil has been fabricated with layer winding method in order to realize compact machine. A forced cooled Nb3Sn cable-in-conduit conductor with a rectangular incoloy conduit with a circular hole has been used for the coil windings. The coil is designed to be charged up to 46 kA corresponding to 13 T with a ramp rate of 0.4 T/s and discharged to 0 kA with a time constant of 10.8 s, which corresponds to the maximum ramp rate of 1.2 T/s. The ac loss in the coil is expected to be about 10 kW during operation with a ramp rate of 0.4 T/s.
Abstract: The Central Solenoid (CS) Model Coil - Outer Module being fabricated to demonstrate the justification of the CS design for the ITER, was almost completed except for epoxy impregnation to concrete whole layers. All the wound and heat treated layers have been assembled symmetrically with the insulation on the same axis, and for layer-to-layer joints the newly developed butt joint, has been installed.
Abstract: The flow reduction of forced flow superconducting coil with a cable in conduit (CIC) conductor has been studied for AC losses due to pulsed operation. In this paper, the flow reduction by rapid heating is described for forced flow superconducting coil with a CIC conductor. The phenomenon of flow reduction of the forced flow coil has been applied for coil quench detection and has been developed by Japan Atomic Energy Research Institute (JAERI). It is named the âfluid methodâ and essential technology for quench detection of large-scale forced flow superconducting coil as fusion magnets and superconducting magnetic energy storage (SMES) coil. In the fluid method, the inlet flow reduction is caused by Joule heating on the normal zone of superconducting coil. The fluid method has no electric noise in its detection. This is an advantage for pulsed operation in comparison with other electrical quench detection systems. On the other hand, there are no quantitative considerations between the inlet flow reduction and Joule heating of the coil inside in previous studies. The flow reduction for the quench detection has been determined by the operation experience of forced flow superconducting coil. The purpose of this paper is an estimation of the relation between the inlet flow reduction and Joule heating at coil quench. First, the inlet flow reduction was obtained by experiment in which the sample has an inductive heater for the quench emulation. Second, the evaluation model was proposed and its model showed good agreement between the inlet flow reduction and heat generation of the coil inside by rapid heating as coil quench. (C) 2000 Elsevier Science Ltd. All rights reserved.
Abstract: Magnetic stability of AC superconducting wires was investigated experimentally. In a long superconducting wire, self-field cannot diffuse sufficiently, and it has been thought that flux jumping occurs when the thickness of the saturated region is larger than some threshold value in multifllamentary superconductors. In this paper, we measured a current sweep rate dependence of quench current of the AC superconducting wire. The experimental results indicated that AC superconducting wires were magnetically stable and could carry the critical current at the sweep rate that was 100 times higher than a threshold value which was estimated from one-dimensional diffusion equation.
