Y.S. Cha

Performance measurements of superconducting current leads with low helium boil-off rates

A performance-measurement facility for current leads has been developed as a part of Argonne National Laboratorys program to develop applications for high-temperature superconductors. The facility measures the rate of helium vapor boil-off due to current-lead heat input to liquid helium and the pressure drop across a current lead for a pair of leads operating at currents up to 100 A. The facilitys major components are a liquid-helium dewar with low background-heat input; a dewar insert that incorporates the current leads and associated instrumentation or connections for flow, pressure, level, temperature, and voltage measurements; and a computer-driven data-acquisition system. Background heat input is low enough so that boil-off rates one-tenth that of an optimized conventional lead can be characterized. The facility has been operated with conventional leads, and with leads incorporating high-temperature superconductors at their cold ends. Details of the facility design, construction, and operating experience are presented.<<ETX>>

Thermodynamic analysis of helium boil-off experiments with pressure variations☆

Abstract A thermodynamic analysis by calorimetric experiments in a system with changing pressure is presented. A general equation is derived for use in calculating the rate of heat loss from measured mass flow rate. The results show that the largest contribution from pressure variation is the sensible heat of liquid helium in a Dewar. A dimensionless parameter that was identified provides an indication of the importance of pressure variation relative to the latent heat of vaporization during an experiment. This dimensionless parameter is a function of system pressure land the thermodynamic properties of helium), rate of change of system pressure, liquid helium inventory in the Dewar and measured mass flow rate. In the special case when the effect of pressure variation is small compared to the latent heat of vaporization, the heat loss rate is the product of the measured mass flow rate and the latent heat of vaporization, multiplied by a correction factor that is a function of the ratio of vapour density to liquid density. This correction factor is significant for helium at pressures near or above 1 atm and should always be included in the calculation.

High-temperature superconducting current leads for micro-SMES application

SMES is being applied on a microscale (1-10 MJ stored energy) to improve electrical power quality. A major portion of the SMES refrigeration load is for cooling conventional (copper, vapor-cooled) current leads that transfer energy between the magnet and the power-conditioning equipment. The lead refrigeration load can be reduced significantly by the use of high-temperature superconductors (HTSs). An HTS current lead suitable for micro-SMES application has been designed. The lower stage of the lead employs HTSs. A transition between the lower stage and the conventional upper-stage lead is heat-intercepted by a cryocooler. Details of the design are presented. Construction and operating experiences are discussed. >

Design of a high-temperature superconductor current lead for electric utility SMES

Current leads that rely on high-temperature superconductors (HTSs) to deliver power to devices operating at liquid helium temperature have the potential to reduce refrigeration requirements to levels significantly below achievable with conventional leads. The design of HTS current leads suitable for use in near-term superconducting magnetic energy storage (SMES) is in progress. The SMES system has an 0.5 MWh energy capacity and a discharge power of 30 MW. Lead-design considerations include safety and reliability, electrical and thermal performance, structural integrity, manufacturability, and cost. Available details of the design, including materials, configuration, performance predictions, are presented.<<ETX>>

An empirical correlation for E(J,T) of a melt-cast-processed BSCCO-2212 superconductor under self field

An empirical correlation is developed for the electrical field strength E(J, T) of a melt-cast processed BSCCO-2212 superconductor. The empirical correlation is based, in part, on the theory of magnetic relaxation and on experimental data at 77 and 87 K. It is developed for temperatures in the range between 77 and 92 K, which is the range of interest for practical devices such as the superconducting fault current limiters. The general form of the correlation may be applicable to other high-T/sub c/ superconductors.

Measurement of critical current and transient characteristics of a high-temperature superconductor tube with a pulsed current supply

The transient response of a melt-cast-processed BSCCO-2212 superconductor tube was investigated by using a pulsed current supply, it was found that (a) the maximum induced current and the excitation current at field penetration increase with the maximum excitation current, and (b) there is a time delay between peak excitation current and peak magnetic field inside the superconductor. These observations can be explained by magnetic diffusion. The AC steady-state critical current of the superconductor was found to compare favorably with that of the pulsed current test when the excitation current is relatively low, but it falls below that of the pulsed current test when the excitation current is relatively high.

Optimization of Pulsed-Current Profile for Magnetizing High

Compared with conventional field cooling (FC) and zero field cooling magnetization methods, pulsed field magnetization (PFM) is a promising way to magnetize high-temperature superconductors (HTS) with the advantage of dramatically decreasing the size and complexity of the electromagnetic charging system. The effects of the amplitude, width, and ramp rate of the pulsed current are reported. Transient responses of the HTS to pulsed magnetic fields are discussed and analyzed. A series of three pulses was found to be adequate to magnetize the HTS monolith with optimal trapped field if proper pulse amplitudes and widths are applied. Experiments verified that the trapped magnetic fields of HTS by PFM were comparable to those obtained by FC magnetization

T_rm C

A one-dimensional heat conduction model is employed to predict burnout of Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub 8/ current lead. The upper end of the lead is assumed to be at 77 K and the lower end is at 4 K. The results show that burnout always occurs at the warmer end of the lead. The lead reaches its burnout temperature in two distinct stages. Initially, the temperature rises slowly when part of the lead is in flux-flow state. As the local temperature reaches the critical temperature, it begins to increase sharply. Burnout time depends strongly on flux-flow resistivity.

Bulk YBCO Superconductors

Shielding current limits and magnetic diffusion characteristics have been measured at 77 K for a set of YBCO single-domain rings. These were fabricated from melt-textured cylindrical YBCO monoliths that were densified to nearly 100%, and then oriented from a single seed. The rings were surrounded by a drive coil that can, under pulse conditions, achieve applied magnetic fields in excess of 1 T and induce currents in excess of 50 kA. Simultaneous magnetic characterization with a Rogowski coil and Hall probe was used to determine the induced current in the sample and the magnetic field in the center of the sample. Magnetic fields trapped in the samples were mapped with a scanning Hall probe. When compared with similar measurements on multidomain c-axis-oriented YBCO rings, the flux penetration is faster and more uniform around the circumference of the ring. The observed critical current density, /spl ap/ 15,000 A/cm/sup 2/ at 77 K, is suitable for application in penetration-type fault current limiters. Separate measurements of the trapped magnetic field and critical current density in the rings are compared with results obtained by analysis of magnetic diffusion characteristics.

Prediction of burnout of a conduction-cooled BSCCO current lead

An analysis was carried out to predict the forced response of the equivalent circuit of an innovative superconducting fault current limiter (FCL). The FCL employs two superconducting coils with differing critical currents wound noninductively. The analysis shows that to reduce the voltage drop under normal operating conditions, the coupling coefficient should be kept fairly high (k>0.90). For a given coil configuration (/spl omega/L=constant), the limiting capability of the device increases with the resistance R of the trigger coil up to a certain value of R, then further increase in R changes very little the limiting capability of the FCL. However, further increase in R can reduce the heat generation rate in the device which will help alleviate the problem of relatively long recovery time. The fault current predicted from estimated values of R compares fairly well with results of an experiment reported in the literature. The discrepancy between the predicted and measured current is due mainly to the uncertainty in the estimated resistance R, because both the purity of Cu and the percentage of Cu in CuNi are not known. By varying the ratio of NbTi-Cu-CuNi in the matrix, the purity of Cu, and the percentage of Cu in CuNi various values of R, can be achieved, which should help to alleviate the problem of excessive Joule heating and recovery time.