Critical Current Performance Research Articles

Comparative study on the critical current performance of Bi-2223/Ag and YBCO wires in low magnetic fields at liquid nitrogen temperature

A comparative study on the critical current performance of Bi-2223/Ag and YBCO coated conductor wires in low magnetic fields at liquid nitrogen temperature was carried out in this work. Five commercial high temperature superconductor wires from different manufacturers were collected. Their critical currents were measured in magnetic fields, ranging from 0 to 0.4 T. On contrary to the common conception, the Bi-2223/Ag samples had better performance than YBCO coated conductor samples in the magnetic fields parallel to the wide surface of superconducting wires within the experimental scope. We also found similar results by collecting the concerned datum from the published literatures to confirm our measurement results. At the present stage, this fact made that the Bi-2223/Ag wires might be the preferred choice for the applications with mainly low parallel fields involved, unless other considerations were prioritized.

A 1.3 GHz NMR Magnet Design Under High Hoop Stress Condition

NMR magnets using high-T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> superconductors (HTS) to generate high magnetic fields exceeding 25 T are currently being designed by several organizations. In these designs, the HTS is used for the inner coils, and the other coils consist of NbTi and Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn wires. The YBCO wire, which is a typical HTS, has excellent critical current performance over a wide range of magnetic fields and tolerates high tensile stress of up to 700 MPa. These properties make it possible to realize a high-field NMR magnet. In particular, the superior mechanical strength allows for the high-stress criterion of the electromagnetic force to be applied to the design of the magnets. In this study, we show the conceptual design of 1.3 GHz (30.5 T) NMR magnets under the condition of high hoop stress of 500 MPa. To achieve high magnetic field homogeneity in these designs, we propose three magnet design plans that have different arrangements of the compensation coils. We assumed that the magnet would be operated by the driven mode at 4.2 K. We also considered the strong angular dependence of the critical current of the YBCO wires to design the magnet.

Two HTS options for a 600 MHz insert of a 1.3 GHz LTS/HTS NMR magnet: YBCO and BSCCO

In 2008, the Phase 3 program to complete a 1.3GHz (30.5T) NMR magnet started at the Francis Bitter Magnet Laboratory, Massachusetts Institute of Technology. It comprises two sub-phases, 3A and 3B. In Phase 3A, a 600MHz high temperature superconductor (HTS) insert magnet (H600) will be designed, constructed, and operated in the bore of a 500MHz low temperature superconductor (LTS) background magnet. This will be followed by Phase 3B, in which the H600 will be combined with a 700MHz LTS background magnet to complete a 1.3GHz NMR LTS/HTS magnet. This paper presents and discusses design issues for two conductor options for H600: BiSCCO-2223 (Bi2223) and coated-YBCO or its variants, here designated as YBCO. For each conductor option, we focused on the following issues: (1) elastic and thermal properties; (2) critical current vs. field performance; (3) splice and index heat dissipations; (4) mechanical and thermal stresses; and (5) protection.

Numerical simulation of critical current performance in Nb3Sn strand subjected to periodic bending deformation

AbstractIn the ITER Engineering Design Activity (EDA), four NB3Sn model coils were developed and successfully tested. However, it was revealed that the critical current of the conductor degraded with the increase of electromagnetic force. One of the explanations of this phenomenon is a strand bending caused by enormous electromagnetic force. The authors therefore developed a simulation code using the distributed circuit model to investigate dependency of the critical current performance on the periodic bending deformation. The simulation results were in good agreement with the experiments. The dependence of the critical current on the periodic transverse load, temperature, periodic load pitch, thickness of Ta barrier which prevents Cu stabilizer from being contaminated by Sn, twist pitch of the strand, and RRR of the bronze matrix was investigated using the developed code. The results showed that the critical current degraded less with decreasing the pitch of the transverse load and increasing the Ta barrier thickness. It suggests that the shorter cabling pitch and the larger bending stiffness prevent the critical current degradation. © 2010 Wiley Periodicals, Inc. Electr Eng Jpn, 171(3): 7–15, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20923

Irreversibility lines in [formula omitted] thin films with Ba–Nb–O nanorods

We report vortex-Bose-glass-like irreversibility lines (IL) depending on both growth temperature and nominal content of a nanorod material in Er–Ba–Cu–O films containing nanorods. The IL was expanded to higher magnetic fields with decreasing growth temperature (Ts), while Tc decreased. Our findings indicate that Ts in the production process of coated conductors containing nanorods is an important determination factor of critical current performance under high magnetic fields.

Analysis of current and voltage distribution in the first Japanese qualification sample of an ITER TF conductor

The critical currents of the Japanese ITER TF conductors made through an internal-tin and bronze process were preliminarily estimated to be about 5.7 K, which approximates the design value of 5.7 K, and about 6.1 K, respectively, at 68 kA and 11.8 T using a short conductor sample. To investigate the influence of the current distribution in the sample conductor, a simulation was performed using a lumped circuit model of a cable and static electrical field model for jackets. The simulation results show that a large, non-uniform current distribution is established due to magneto-resistance of the copper in the joint and an imbalance of contact resistance of the strands to the copper and by poor soldering between the copper shoes, results which make a precise evaluation of the critical current performance difficult. The analytical results indicate that the current sharing temperature of the internal-tin and bronze process conductors is expected to be 6.0 K and 6.7 K, respectively, when the current distribution is uniform. In addition, solder filling of the joints makes the current distribution uniform due to the normal resistance in the high field zone, and the current sharing temperature can be estimated as almost the same as when the current distribution is uniform. The other possible solution is to use a thin copper plate with a low RRR to reduce the influence of magneto-resistance and any non-uniformity in contact resistance. Conductor performance is under-estimated in this case because the non-uniform current distribution still remains. However, the reduction in the estimated current sharing temperature is expected to be a few hundred milli-Kelvins, which seems acceptable as a margin in a qualification trial.

Characterization of Long-Length, MOCVD-Derived REBCO Coated Conductors

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> A leading approach to the fabrication of long-length, high-performance <formula formulatype="inline"><tex Notation="TeX">${\rm REBa}_{2}{\hbox {Cu}}_{3}{\hbox {O}}_{7}$</tex></formula> (REBCO) coated conductor is by metal-organic chemical vapor deposition (MOCVD) of REBCO on buffered templates. Templates are produced by ion beam assisted deposition of textured MgO onto polished metal substrates. The overall performance of MOCVD coated conductors achieved to date is impressive, but further improvement is desired. We have used a coordinated set of characterization techniques to identify the underlying causes for critical current (Ic) performance variations in long-length MOCVD conductors. Using electron microscopy and Raman spectroscopy, we studied tape specimens from specially designed experiments performed in SuperPower's MOCVD manufacturing equipment with its six-track “helix” tape path. We find that in multi-pass depositions used to produce thicker REBCO films, the REBCO phase uniformity and texture quality in the first pass play key roles in pass-to-pass microstructure evolution, with nucleation of second phase particles in the first layer promoting misoriented grains that propagate through subsequent layers. These misoriented grains, many growing in close proximity with second phase particles, present current-blocking obstacles that limit Ic performance. Our results show that achieving more uniform deposition in the very first deposited layer plays a critical role that in turn leads to reduced misoriented grain content and REBCO lattice disorder in the second and subsequent layers of the REBCO film. </para>

$I_{\rm c}$ Measurements on ${\rm Nb}_{3}{\rm Sn}$ Strands Extracted From ITER CICC Prototypes

The unexpectedly low critical current performance of some ITER prototype cable-in-conduit conductors (CICC) measured in the SULTAN test facility during the last two years led to the question whether it arises from a transverse strain in the strands caused by the transverse Lorentz force or from a damage of filaments in the strands. Since the former process is in principle a reversible effect while the latter is not, measurements of the critical current of ~8 cm long strand sections extracted systematically from prototype samples were performed to see if and by how much the critical current dropped relative to the one of the witness sample. Strand witness samples had been mounted on ITER reference barrels, heat treated together with the conductor samples for the SULTAN tests and their I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> measured. Although the short sample lengths caused experimental difficulties and the number of measured strands permitted only a limited statistical analysis, the general trend of the conductor performance in SULTAN samples-high and stable (i.e. without cyclic load degradation) T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">cs</sub> or not-is reflected in the I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> measurements on extracted strands.

Strong influence of the YBa2Cu3O7 grain size on critical current densities of thickYBa2Cu3O7 layers made by a metal–organic deposition process

We correlate the critical current performance of thick film second-generationYBa2Cu3O7 (YBCO) coated conductors, made by metal–organic deposition,with specimen grain size. The transport critical current densities,JcT, of these conductors were observed to be inversely proportional to YBCO grain size in the5–40 µm size range. Analysis of magnetic hysteresis curves of the conductors and transmissionelectron microscopy data from a large YBCO grain revealed that the reducedJcT in tapes with large YBCO grain size is due to the accumulation of secondary phaseprecipitates within the grains near the periphery of, and in, the grain boundaries.Grain size is determined to be one of the important factors in determiningJcT of YBCO coated conductors made by the metal–organic processes.

Numerical Simulation of Critical Current Degradation of $\hbox{Nb}_{3}\hbox{Sn}$ Strand in CIC Conductor

The critical current of the ITER model coil degraded with an increase in the electromagnetic force. Experiments in which a periodic transverse load was artificially applied to a single coil strand showed this degradation. However, since these experiments were carried out under a certain condition, a general relation between the transverse load and the critical current was not obtained. Therefore, we have developed a numerical simulation code to investigate this general relation. The results of the numerical simulations are in good agreement with the experiments. This indicates that our numerical code is valid for the evaluation of the critical current performance of strands in CIC conductor. Next, the general dependence of the critical current degradation on Ta barrier thickness and resistivity of bronze was evaluated by using the developed simulation code. The results of the calculation showed that the Ta barrier thickness affects the critical current characteristics because of the large flexural rigidity of Ta barrier. On the other hand, the resistivity of bronze and the twist pitch of the filament rarely affect the critical current degradation since current transfers between filaments do not occur within the practical resistivity range of bronze.

Numerical Simulation of Critical Current Performance in Nb3Sn Strand Subjected to Periodic Bending Deformation

In the ITER Engineering Design Activity (EDA), four Nb3Sn model coils were developed and successfully tested. However, it was revealed that the critical current of the conductor degraded with the increase of electromagnetic force. One of the explanations of this phenomenon is a strand bending caused by enormous electromagnetic force. The authors therefore developed a simulation code using the distributed circuit model to investigate dependency of the critical current performance on the periodic bending deformation. The simulation results were in good agreement with the experiments. The dependence of the critical current on the periodic transverse load, temperature, periodic load pitch, thickness of Ta barrier which prevents Cu stabilizer to be contaminated by Sn, twist pitch of the strand and RRR of the bronze matrix was investigated using the developed code. The results showed that the critical current degraded less with decreasing the pitch of the transverse load and increasing the Ta barrier thickness. It suggests that the shorter cabling pitch and the larger bending stiffness prevent the critical current degradation.

Processing of Long-Length YBCO Coated Conductors Based on Stainless Steel Tapes

Recent progress is reported in the development of long lengths (up to 100 m) of YBCO coated conductors with good mechanical stability and improved critical current homogeneity. A non-magnetic CrNi stainless steel tape, 0.1 mm thick, is employed as a substrate tape. Prior to deposition, the tape is coated with yttria stabilized zirconia in the form of a bi-axially textured buffer layer, using a novel "alternating beam assisted deposition" technique. After deposition of a ceria cap layer onto the buffer, YBCO films are deposited employing a high-rate pulsed laser deposition method that allows higher critical current densities and incurs a lower YBCO loss (~10%) than other deposition techniques. The last processing step consists of coating the with a thin layer of silver or gold and a 2-40 mum thick copper shunt layer. In 40 m long tapes with a 1 mum thick YBCO film (at 77 K and zero external field), critical current, J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> , up to 235 A per cm-width were obtained. Higher critical currents, up to 574 A per cm, were achieved in short 0.2-7 m sections corresponding to current densities, J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> , of up to 4.4 MA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . The critical current homogeneity and performance of our improved coated conductors under applied temperature cycling, multiple quench-recovery cycles, and repeated bending are reported.

Bi Addition Effect on $J_{\rm c}$-B Performance of Powder-in-Tube Processed ${\hbox{MgB}}_{2}$ Tapes

MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> tapes with 5 at.% Bi addition were fabricated through the in situ powder-in-tube method. Heat treatment was carried out at 700degC and 800degC for 1 h. The phase, microstructure, superconductivity and flux pinning ability were characterized by XRD, SEM, magnetic and transport measurements. The critical current density J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> and J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> -B performance for the doped and undoped samples are also given. It is found that nano-Bi doping decreased the magnetic field sensitivity of J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> , indicating an improvement of flux pinning. At 4.2 K, 10 T, J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> values of the Bi doped tapes enhanced about 2.5 times compared to that of undoped samples. The mechanism for improved J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> -B performance of Bi-doped MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> tapes is investigated.

Computerised optimisation of bronze route Nb3Sn conductors

The bronze process for the fabrication of niobium-tin superconductors is a mature and successful technology, but the optimisation of these conductors still requires a timeconsuming trial-and-error approach. The choice of heat treatment influences not only the amount of Nb3Sn formed, but also uniformity across the wire cross-section and the grain size of the A15 phase, and consequently the critical current performance. The heat treatment for a given application must also consider the coil's thermal mass and the capabilities of the furnace. The key to optimisation is knowing the state of A15 phase formation as a function of time and temperature during heat treatment. A combination of reaction-diffusion modelling by finite difference methods and resistivity calculation has been used to develop a system for the optimisation of bronze route superconductors. Given the wire geometry, the radial variation of Nb3Sn layer thickness and matrix composition can be calculated as a function of time and temperature. A computer program has been written to perform these calculations automatically, and rapidly estimate the optimal heat treatment for the parameters specified by the user. This approach has been applied to European Advanced Superconductors NS6000 conductors and, to test its effectiveness, rapid Jc(B) measurements were performed using a Cryo-BI-Pulse system. Improved critical current performance has been observed in low magnetic fields compared to isothermal samples, but further work is required to optimise high magnetic field performance.

Development of advanced Nb3Al superconductors for a fusion demo plant

A Nb3Al superconductor inherently has the outstanding features of a high critical field and an excellent strain tolerance in critical current performance. The Japan Atomic Energy Research Institute developed the world's first large Nb3Al coil using jelly-roll processed Nb3Al strands. This coil was successfully operated up to the nominal current and field of 46 kA and 13 T, respectively. The test results demonstrated that a Nb3Al conductor is suitable for application in high field, large magnets, such as the toroidal field (TF) coils in a fusion reactor. In parallel with this work, a Nb3Al strand having a high critical current at high field has been developed by the National Institute of Materials Science. This strand cannot incorporate enough copper stabilizer, resulting in poor stability, since it is heat-treated at a much higher temperature than the melting temperature of copper. In order to find a solution to this issue, we performed an analytical study, and it showed that the externally incorporated copper after the high temperature heat treatment is effective for stabilization when the electric conductance and heat transfer coefficient between the Nb3Al strand and the external copper are more than 10 MS m−1 and 10 kW m−2 K−1, respectively. Since they seem to be attained by using present conductor technologies, the development of a TF coil operated at a high field around 16 T seems promising.

Improved critical current properties observed inMgB2 bulks synthesized by low-temperature solid-state reaction

MgB2 bulks were synthesized by the solid-state reaction of Mg and B at600 °C and their superconducting properties were compared with samples heated at850 °C. The samplesheated at 600 °C exhibited improved critical current properties up to high fieldsat 20 K. Poor crystallinity is found to contribute enhancement ofHc2,Hirr and Jc at high fields. On the other hand, the strongly grain connectednetwork structure and smaller grain size are responsible for highJc at low fields.Improved Jc up to 3.93 × 105 A cm−2 and a high μ0Hirr of T, as for undoped MgB2 bulks, guarantees that low-temperature sintering is a promising way to fabricateMgB2 conductors with high critical current performance.

Enhanced critical current properties observed inNa2CO3-dopedMgB2

A significant improvement of the critical current properties inMgB2 bulk has been attained by sodium carbonate doping. A series ofMg1−2xB2(Na2CO3)x bulksamples with x = 0–0.1 were prepared by solid-state reaction in sealed stainless tubes. Both the critical current density,Jc, andirreversibility field, Hirr, at 20 K were systematically improved up tox = 0.055 and decreased monotonically by excess doping, while theirTcs were continuously decreased with an increase ofx. The samplewith x = 0.055, having aslightly decreased Tc of 37.6 K, recorded the best critical current performance at 20 K withJc of3.8 × 105 A cm−2 in self-fieldand μ0Hirr of approximately 6 T. Both small particles of MgO and a carbon-containing local region,Mg(B,C)2, are believed to act as effective pinning centres, resulting in anenhancement of the flux pinning force. In addition, the coherence lengthξ ofthe MgB2 was dramatically shortened by sodium carbonate doping, and consequently theμ0Hc 2 was enhanced to approximately 29 T at the highest doping level ofx = 0.10.

Uranium Doping and Thermal Neutron Irradiation Flux Pinning Effects in MgB&amp;lt;tex&amp;gt;$_2$&amp;lt;/tex&amp;gt;

The U/n method is a well-established means of improving flux pinning and critical current performance in cuprate superconductors. The method involves the doping of the superconductor with 235U followed by irradiation with thermal neutrons to promote fission. The resultant columnar damage tracks produced by the energetic fission products pin flux vortices and improve critical current performance in magnetic fields. No such improvement could be observed when the U/n method was applied to MgB2 superconductor. No fission tracks could be observed in TEM, even for samples that were irradiated at the highest fluence. Gamma-ray spectroscopy indicated that fission had occurred in the expected way.

３ｄ遷移金属置換Ｂｉ（Ｐｂ）２２１２単結晶の磁束ピンニング特性

A large amount of lead doping to Bi2212 superconductors is effective in decreasing their high electromagnetic anisotropy and introduces effective pinning sites, such as lamella structure interfaces due to compositional fluctuation of lead ions, resulting in greatly enhanced flux pinning properties up to high temperatures. Particularly in the high-Tc superconducting materials with lowered anisotropy, further improvement of critical current properties can be expected by doping a small amount of impurity elements, which generate locally weak superconducting regions. Based on this background, we have attempted to enhance flux pinning strength of Bi(Pb)2212 single crystals by 3d metal doping in the present study. Crystal boules with starting compositions of Bi1.6Pb0.6Sr1.8Ca(Cu1-xMx)2Oy (M = Fe, Co, Ni : x = 0, 0.001, 0.002, 0.005, 0.02) were grown by the floating zone method. Tc's of the single crystals annealed to be the carrier overdoped state were dramatically decreased with an increase of metal doping levels. Critical current performance of the 3d metal-doped Bi(Pb)2212 crystals was quite poor at high temperatures near Tc, The crystal with x= 0.001, 0.002, 0.005, however, showed improved Jc properties accompanied by larger second peak effects in their magnetization hysteresis loops when compared with the 3d metal free Bi(Pb)2212 crystal below 50 K. On the other hand, magneto-optical measurements revealed that anisotropy in the in-plane Jc properties (Jca > Jcb ) was systematically suppressed with an increase of the doping level x. These results suggested that a very small amount of 3d metal doping is an essentially effective method to enhance pinning strength of Bi-based superconductors due to their generation of point-defect-like pinning centers. 3d doping, however, does decrease the pinning strength at the lamella structure interfaces.

Evaluation of strain applied to strands in a 13 T–46 kA Nb3Al cable-in-conduit conductor

A 13 T–46 kA Nb3Al coil, a Nb3Al insert, has been developed to demonstrate the applicability of the react-and-wind technique to toroidal field (TF) coils of a fusion reactor, such as ITER. It is estimated that a conductor is subjected to ≈0.4% bending strain after heat treatment when the react-and-wind method is applied to the TF coil fabrication. Therefore, 0.4% bending strain was artificially applied to the Nb3Al insert conductor after the heat treatment. Since a stainless steel conduit was used in this conductor, the strands are subjected to the compressive strains due to thermal stress. The effective strains applied to the strands are estimated by comparing the critical current test results of the Nb3Al insert at 10–13 T and calculation. Those due to thermal stress and conductor bending were evaluated to be ≈−0.4% and ≈0%, respectively. The effective strain by the conductor bending is sufficiently small so as not to affect the critical current performance of the Nb3Al conductor. In addition, the evaluated strain of the Nb3Al conductor is compared with those of same scale Nb3Sn conductors, in which there is an unexpected strain, which is proportional to the electromagnetic force. Such strain was not observed in the Nb3Al conductor. One of the explanations is higher rigidity of the Nb3Al strand than the Nb3Sn one. This shows that a Nb3Al conductor is suitable for application to large magnets, such as the TF coil of the fusion machine, which experiences large electromagnetic force in the conductor.