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A numerical model is developed to calculate the total alternating-current (ac) loss of Cu-stabilized YBa2Cu3O7-δ (YBCO) coated conductors with a ferromagnetic substrate when carrying an alternating transport current in an ac background magnetic field. The time evolutions of current and magnetic field distributions along the width of the conductor are calculated by solving the one-dimensional Poisson equation for the magnetic vector potential. In addition to the ac loss dissipated in the superconducting layer, the ferromagnetic loss in the substrate and eddy current loss in the Cu stabilizer are also modeled. In the calculations, the superconducting voltage-current behavior is assumed to follow a power law. The model is capable of addressing other practical aspects of YBCO conductors, including the field dependence of the critical current density, and n value, and nonuniformities in the conductor. The magnetic shielding of the ferromagnetic substrate, however, was ignored in the calculations. Numerical results are compared with analytical and experimental results for some special cases. [ABSTRACT FROM AUTHOR]

The effect of phase difference between transport current and perpendicular applied magnetic field, Δφ, on total ac loss in multifilamentary, silver-sheathed (Bi,Pb)2Sr2Ca2Cu3Ox (Bi-2223) superconducting tapes was calculated numerically using Brandt’s equation [Phys. Rev. B 54, 4246 (1996)] and the results were compared with experimental results obtained by the calorimetric method. In the numerical calculations, the superconducting transition was modeled by E-J characteristics represented by a power law. The dependence of critical current density on the n value was taken into account. The dependence of critical current density and n value on applied magnetic field was obtained from experimental data. It was observed that Δφ has a significant effect on the loss behavior of Bi-2223 tapes and depends strongly on the relative values of transport current and applied magnetic field. The effects of Δφ on the individual components of transport loss and magnetization loss were also calculated. Good agreement between the calculated and experimental results is demonstrated. [ABSTRACT FROM AUTHOR]

A noncontact superconducting transition temperature (Tc) measurement system using a Hall probe array was devised to measure Tc of high-temperature superconductor (HTS) coated conductors (CCs) without physical contact to the surface, thereby avoiding contact with electrodes, which can cause damage to the conductor surface. The variations of the Hall voltage (V) distribution, which comes from the distribution of magnetic field on the surface of the CC, under a fixed applied field at different temperatures were used to estimate Tc. The measured Tc of the HTS GdBCO CC made by SuNAM was approximately 93.2 K by the noncontact Tc method, as compared to approximately 93.5 K by the conventional four-probe-contact method in a commercial PPMS. Thus, the Tc obtained by this system is comparable with that from the transport four-probe method. In addition, the noncontact Tc measurement system devised and used in this study is simple because it does not need comparison and conversion with values measured by contact methods. [ABSTRACT FROM PUBLISHER]

The development of high field superconducting magnets using high temperature superconductors (HTSs) is progressing for high energy physics, nuclear magnetic resonance and energy storage applications. Yet the key issue of quench protection remains unresolved, primarily due to the slow normal zone propagation velocity (NZPV) in HTS magnets. High magnetic field may affect the quench behavior through two opposing effects: increased NZPV may result due to reduced critical temperature and current sharing temperature, but decreased NZPV may result due to reduced critical current density and thus operating current. At present it is unclear which effect dominates. Here, a series of quench experiments at high magnetic field on multilayer wind-and-react Bi2Sr2CaCu2Ox (Bi2212) coils addresses this question. The two- and three-dimensional quench behavior is investigated in a magnetic field up to 20 T at 4.2 K. With increasing magnetic field, the minimum quench energy decreases significantly. The NZPV also decreases with magnetic field, but only up to about 8 T. For magnetic fields above 8 T, the NZPV is independent of magnetic field up to at least 20 T. Thus, at low field the NZPV is dominated by the decreasing critical current density, whereas at higher magnetic field the competing effects of decreasing critical current density and decreasing temperature margin offset each other. [ABSTRACT FROM AUTHOR]

YBa2Cu3O7-x (YBCO) coated conductors (CCs) are now capable of carrying very high transport critical current density Jc over a broad range of magnetic field and temperature space, and as a result, they are receiving significant interest for a wide range of applications. While many of these applications take advantage of the high-temperature performance of YBCO CCs, because the YBCO CC is typically produced on a high-strength substrate and carries very high Jc at very high magnetic field, there is now growing interest in using YBCO CCs at 4.2 K to generate very high magnetic fields. The transition from high-field conductor to high-field superconducting magnet, however, requires that some challenging issues be addressed. One of the most important challenges remaining is to better understand the stability and quench behavior at 4.2 K, so that an effective quench protection system can be developed. Here, we report on measurements of the stability and quench behavior of short YBCO CC at 4.2 K by inducing a quench via a heat pulse from a heater mounted on the conductor surface. Through gradually increasing heater pulse amplitude, the transition from stable to unstable (i.e., recovery to quench) is observed through voltage and temperature measurements. Using these data, the minimum quench energy (MQE) and normal zone propagation velocity (NZPV) are determined. It is found that, for the same fraction of critical current (I/Ic), YBCO CCs have similar MQE and NZPV as Ag-alloy-clad Bi2Sr2CaCu2Ox wires and significantly higher MQE and lower NZPV than those of MgB2 round wires of similar Ic, (4.2 K). Furthermore, the voltage and temperature versus time data are correlated to better understand the quench onset behavior at 4.2 K. It is determined that a normal temperature gradient exists .from the CC surface to the YBCO layer within the conductor, as well as a directly measured longitudinal temperature gradient. After the heater pulse has ended but while the transport current continues, the temperature gradient along the length becomes dominant. Nevertheless, voltage and temperature measurements remain problematic for quench detection in large magnets because of the slow longitudinal propagation velocity. Thus, new approaches to quench detection and/or protection of high-field YBCO magnets are needed. PACS number(s): 84.71. Mn [ABSTRACT FROM AUTHOR]

A versatile experimental facility was designed and set up to measure transport ac losses, magnetization ac losses, and total ac losses in high-temperature superconductors at variable temperatures. Several sets of measurements were carried out in the temperature range of 35 K to 100 K. Sample temperature during the measurements could be controlled within ±0.5 K of set temperature. Temperature dependence of transport losses reflects variation of critical current density of the tapes with temperature. Temperature dependence of magnetization losses exhibits an interesting behavior with a peak, whose position shifts to lower temperatures as the magnetic field is increased. Experimental data of ac losses at various temperatures are compared with those calculated using numerical methods. Generally, the simulated results reproduce well the experimental data. [ABSTRACT FROM AUTHOR]

Bi2Sr2CaCu2Ox/AgMg conductors are potentially important for many applications up to 20 K, including magnets for cryogen-free magnetic resonance imaging and high field nuclear magnetic resonance research. One promising approach to increased critical current density is partial-melt processing in the presence of a magnetic field which has been shown to enhance c-axis texturing of wide, thin tape conductors. Here, we report on low aspect ratio rectangular conductors processed in an 8 T magnetic field. The magnetic field is applied during different stages of the heat treatment process. The conductors are electrically characterized using four-point critical current measurements as a function of magnetic field and magnetic field orientation relative to the conductor. The superconductive transition and magnetization hysteresis are measured using a SQUID magnetometer. The microstructures are characterized using scanning electron microscopy and energy dispersive spectroscopy and analyzed using digital image processing. It is found that the presence of a magnetic field during split melt processing enhances the electrical transport and magnetic behavior, but that the anisotropy is not consistently affected. The magnetic field also affects development of interfilamentary Bi2212 bridges, and that this depends on the initial shape of the Bi2212 filament. At least two behaviors are identified; one impacts the oxide phase assemblage and the other impacts textured growth. [ABSTRACT FROM AUTHOR]

High-field superconducting solenoids have proven themselves to be of great value to scientific research in a number of fields, including chemistry, physics and biology. Present-day magnets take advantage of the high-field properties of Nb3 Sn, but the high-field limits of this conductor are nearly reached and so a new conductor and magnet technology is necessary for superconducting magnets beyond 25 T. Twenty years after the initial discovery of superconductivity at high temperatures in complex oxides, a number of high temperature superconductor (HTS) based conductors are available in sufficient lengths to develop high-field superconducting magnets. In this paper, present day HTS conductor and magnet technologies are discussed. HTS conductors have demonstrated the ability to carry very large critical current densities at magnetic fields of 45 T, and two insert coil demonstrations have surpassed the 25 T barrier. There are, however, many challenges to the implementation of HTS conductors in high-field magnets, including coil manufacturing, electromechanical behavior and quench protection. These issues are discussed and a view to the future is provided. [ABSTRACT FROM AUTHOR]

The design of a versatile ac loss and stability characterization facility for high temperature superconducting materials suitable for variable temperature measurements is described. A non-metallic vessel inside a transverse field double-helix magnet acts as the measurement chamber. A cryocooler cools the samples to a target measurement temperature between 35 and 80 K. The facility is suitable for measurements on samples as long as 15 cm with ac transport current and ac background magnetic field, both at variable frequency. The facility allows for sample rotation to vary its orientation with respect to the magnetic field. Initial temperature measurements showed that the sample could be maintained at uniform temperature to ±0.5 K. AC loss measurements performed on a precursor setup, identical in concept, confirmed the suitability of the double-helix magnet design and cryocooler based cooling arrangement. [ABSTRACT FROM AUTHOR]

The COHMAG report of the National Academy of Sciences provides an assessment of high field magnet science and technology in the United States, and identifies possible new initiatives for future high field magnets. Regarding NMR, the report identifies high resolution NMR spectrometers in the range of 30 T/1.3 GHz as having great scientific interest. This paper discusses technology issues associated with 30 T superconducting magnets. The enabling technology is HTS conductors and coil technology. Significant issues remain for application of HTS conductors to large coils, including long length quality, heat treatment, anisotropy and strain dependence. The outer LTS coil sections are large, with high stress and stored energy. Structural issues, including high strength conductor and reinforcement, and quench protection issues are significant. The status of technology required for a 30 T NMR magnet is examined. A preliminary conceptual design of the 30 T magnet is presented. [ABSTRACT FROM AUTHOR]

In most power applications, the superconducting tapes carry AC transport currents and are exposed to applied AC or/and DC fields with different orientations. Because of the anisotropy of high temperature superconductors, AC losses depend on both the strength and the orientation of the magnetic field. To achieve high current carrying capacity, several tapes are often stacked. Thus an estimation of AC losses in stacked tapes is necessary for the design of superconducting devices. Here, AC transport losses in single, double- and triple-stacked Bi2223/Ag tapes were measured under different magnetic fields. The measurements were carried out at 77 K with the field angle between the tape surface and the direction of magnetic field varied from 0° to 90°. The experimental results on the angular dependence of the AC losses are reported and analyzed. [ABSTRACT FROM AUTHOR]