Thomas R. Askew

Transient response of a high-temperature superconductor tube to pulsed magnetic fields.

Abstract The transient response of a melt-cast-processed BSCCO-2212 superconductor tube under a pulsed magnetic field was investigated by using a pulsed current source driving an external copper solenoid. The induced current in the superconductor tube was measured by a Rogowski coil and the penetration field was measured by a Hall probe inside the tube. It was found that (a) the maximum induced current I s (max) and the excitation current at field penetration NI * increase with the maximum excitation current NI(max); (b) there is a time delay between peak NI and peak H in tests with relatively low NI(max), and this time delay diminishes in tests with relatively large NI(max); (c) the induced current I s does not remain constant and begins to decrease immediately after its peak has been reached while the excitation current NI is still increasing; and (d) the decay of the induced current I s exhibited different behavior between tests with relatively low and high NI(max), the former showed a gradual decay while the latter showed a rapid decay initially and then a nearly constant value for an extended period of time. All of these observations can be explained by magnetic diffusion and a postulated transition from flux-creep to flux-flow behavior. The constant value of induced current after its rapid decay can be interpreted as the steady-state critical current of the superconductor tube.

χ″ peaks, flux pinning, and the irreversibility line in high-temperature superconductors

Abstract Physical processes that produce various peaks in the adsorption component χ″ of the AC susceptibility of high-temperature superconductors are described. It is shown that for good YBa 2 Cu 3 O 7− x crystals twinning planes are the main source of flux pinning and the primary determinant of the reported irreversibility line (IL) position in the H–T plane. In other situations, particularly ones involving weaker pinning, the apparent IL is found to lie much lower in the H–T plane. Ambiguous results from experiments designed to produce pinning sites by irradiation are discussed in the context to this new observation.

Dynamic measurement of flux flow resistivity in YBa/sub 2/Cu/sub 3/O/sub 7/ wires

Polycrystalline samples of YBa/sub 2/Cu/sub 3/O/sub 7/ have been pushed deep into the flux flow regime using submillisecond feedback-controlled current ramps. Thermal stability has been maintained in the 65-93 K range in spite of current densities that exceed J/sub c/ by a factor of four or more. Linear I-V relations are observed, and the measured flux flow resistivity shows a simple dependence on the microstructure of the samples. The resistivity of unoriented ceramic samples shows essentially no dependence on magnetic field between 0.05 T and 0.5 T, and shows a slight dependence on temperature that mirrors the linear behavior of the normal state resistivity down to about 72 K. Below 72 K the flux flow resistivity begins to rise with dropping temperature. In contrast, directionally solidified samples show a much lower flux flow resistivity with almost no temperature dependence. A strong field dependence appears in this latter case, one that is reminiscent of the Bardeen-Stephen flux flow resistivity of low T/sub c/ materials.<<ETX>>

Nonlinear magnetic diffusion in single-domain YBCO cylinders under pulsed field conditions

A step-function increase in magnetic field has been applied to various single-domain Y-Ba-Cu-O rings and disks at 77 K. The magnetic field in the center of these structures exhibits an exponential approach to an equilibrium value, as predicted by simple diffusion theory. The measured time constants are in the millisecond range and show nonlinear behavior, as predicted in recently advanced theories of magnetic diffusion under flux flow conditions. Measurements of trapped magnetic field, critical current density, and current-voltage (I-V) characteristics are compared to results obtained by measurement of magnetic diffusion. The single-domain structures are intended for use in penetration-type fault current limiters, an application where the magnetic diffusion time constants have a strong influence on device design.

Transient response of 50 KiloAmp Y-Ba-Cu-O rings and ring pairs to pulsed magnetic fields

Shielding current limits and magnetic diffusion characteristics have been measured at 77 K in large-grain YBCO rings with oriented microstructures. The samples are surrounded by a drive coil that can achieve magnetic fields in excess of 1 Tesla and induce currents in excess of 50 kA when driven by a current pulse of a few msec duration. Simultaneous magnetic measurements with a Rogowski coil and a Hall probe allow determination of the induced current in the sample and the field in the center of the sample. These measurements show that field penetration occurs in a complex way that includes delays and transient effects caused by magnetic diffusion and sample heating. Dramatic threshold effects are observed that are probably related to a creep-flow transition coupled with local heating effects. Geometric effects are investigated using a single drive coil and a pair of YBCO rings with various spacings. A test geometry equivalent to a simple penetration-type inductive fault current limiter is used; the experimental results are therefore of interest for design and characterization of these devices.

Demagnetization of Trapped Field in High

This paper examines the demagnetization of a trapped magnetic field in a high TC superconducting (HTS) monolith using a single, short, high-amplitude field pulse. With a single optimized pulse, the peak trapped field can be reduced to one-third of the maximum saturated value. Alternatively, the spatial average of the trapped field could be fixed very close to zero. After demagnetization, the trapped field has both positive and negative values at various locations in the HTS monolith. Transient responses of the HTS to pulsed fields with different amplitudes are also reported and discussed. It does not appear that the residual trapped field can be reduced to zero throughout the monolith using pulsed field demagnetization methods in a fixed sample/coil geometry. Beans critical state model is used to explain the measured equilibrium field distributions. Transient response of the HTS to an externally applied field is qualitatively explained by magnetic diffusion. Drive circuit characteristics and design feasibility issues are also addressed

T_{C}

The relatively low values of critical current density (J/sub C/) in bulk high T/sub C/ materials are a problem for many applications, but these materials are now ending use in developmental fault current limiters of various design. Intended primarily for AC power applications, these devices rely on materials that transition quickly between a state of effectively lossless conduction and a state which dissipates significant power directly or significantly changes the magnetic coupling between other circuit elements. Most bulk material processes have been adjusted to maximize the value of J/sub C/ rather than to provide the well defined, sharp transition just mentioned. This study compares the dissipative properties of sintered YBCO with equiaxed, unoriented grains to that of two different types of melt-processed thick film material with plate-like grains in c-axis orientation. Dissipative properties are measured under current densities of 10 times J/sub C/ or more. Isothermal conditions are maintained through the use of submillisecond feedback-controlled current pulses. Significant differences are noted in the high-current flux flow properties, with the sintered samples developing Ohmic behavior and the samples with oriented microstructures developing voltages proportional to I/sup 2/. Sharp transitions at J/sub C/ and extreme dependence on applied magnetic field were noted in all cases.