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Register a new userLarge Predicted Self-Field Critical Current Enhancements In Superconducting Strips Using Magnetic Screens
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A transport current distribution over a wide superconducting sheet is shown to strongly change in a presence of bulk magnetic screens of a soft magnet with a high permeability. Depending on the geometry, the effect may drastically suppress or protect the Meissner state of the sheet through the enhancement or suppression of the edge barrier critical current. The total transport current in the magnetically screened Meissner state is expected to compete with the critical current of the flux-filled sheet only for samples whose critical current is initially essentially controlled by the edge barrier effect.
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Recent theoretical and experimental research on low-bulk-pinning superconducting strips has revealed striking dome-like magnetic-field distributions due to geometrical edge barriers. The observed magnetic-flux profiles differ strongly from those in strips in which bulk pinning is dominant. In this paper we theoretically describe the current and field distributions of a superconducting strip under the combined influence of both a geometrical edge barrier and bulk pinning at the strips critical current Ic, where a longitudinal voltage first appears. We calculate Ic and find its dependence upon a perpendicular applied magnetic field Ha. The behavior is governed by a parameter p, defined as the ratio of the bulk-pinning critical current Ip to the geometrical-barrier critical current Is0. We find that when p > 2/pi and Ip is field-independent, Ic vs Ha exhibits a plateau for small Ha, followed by the dependence Ic-Ip ~ 1/Ha in higher magnetic fields.
The critical current of a thin superconducting strip of width $W$ much larger than the Ginzburg-Landau coherence length $xi$ but much smaller than the Pearl length $Lambda = 2 lambda^2/d$ is maximized when the strip is straight with defect-free edges. When a perpendicular magnetic field is applied to a long straight strip, the critical current initially decreases linearly with $H$ but then decreases more slowly with $H$ when vortices or antivortices are forced into the strip. However, in a superconducting strip containing sharp 90-degree or 180-degree turns, the zero-field critical current at H=0 is reduced because vortices or antivortices are preferentially nucleated at the inner corners of the turns, where current crowding occurs. Using both analytic London-model calculations and time-dependent Ginzburg-Landau simulations, we predict that in such asymmetric strips the resulting critical current can be {it increased} by applying a perpendicular magnetic field that induces a current-density contribution opposing the applied current density at the inner corners. This effect should apply to all turns that bend in the same direction.
Analytic expressions for alternating current (ac) loss in radially arranged superconducting strips are presented. We adopt the weight-function approach to obtain the field distributions in the critical state model, and we have developed an analytic method to calculate hysteretic ac loss in superconducting strips for small-current amplitude. We present the dependence of the ac loss in radial strips upon the configuration of the strips and upon the number of strips. The results show that behavior of the ac loss of radial strips carrying bidirectional currents differs significantly from that carrying unidirectional currents.
A simple analytical expression is presented for hysteretic ac loss $Q$ of a superconducting strip simultaneously exposed to an ac transport current $I_0cosomega t$ and a phase-different ac magnetic field $H_0cos(omega t+theta_0)$. On the basis of Beans critical state model, we calculate $Q$ for small current amplitude $I_0ll I_c$, for small magnetic field amplitude $H_0ll I_c/2pi a$, and for arbitrary phase difference $theta_0$, where $I_c$ is the critical current and $2a$ is the width of the strip. The resulting expression for $Q=Q(I_0,H_0,theta_0)$ is a simple biquadratic function of both $I_0$ and $H_0$, and $Q$ becomes maximum (minimum) when $theta_0=0$ or $pi$ ($theta_0=pi/2$).
We experimentally study effect of single circular hole on the critical current $I_c$ of narrow superconducting strip with width $W$ much smaller than Pearl penetration depth $Lambda$. We found nonmonotonous dependence of $I_c$ on the location of a hole across the strip and a weak dependence of $I_c$ on radius of hole has been found in case of hole with $xi ll R ll W$ ($xi$ is a superconducting coherence length) which is placed in the center of strip. The observed effects are caused by competition of two mechanisms of destruction of superconductivity - the entrance of vortex via edge of the strip and the nucleation of the vortex-antivortex pair near the hole. The mechanisms are clearly distinguishable by difference in dependence of $I_c$ on weak magnetic field.
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