Superconducting Cylinder Research Articles

Evaluation of the magnetic field within a BPSCCO cylinder superimposed by multi-layered soft-iron cylinders by use of an HTS DC-SQUID

The evaluation of the maximum shielded magnetic flux density B/sub s/ of a high-critical temperature superconducting (HTS) cylinder acting as a magnetic shielding vessel, is required to improve the value of B/sub s/ for practical use. The authors have improved the value of B/sub s/ by the superposition of multi-layered soft-iron cylinders over a Bi-Pb-Sr-Ca-Cu-O cylinder which is termed the superimposed cylinder. Little is known, however, of the characteristics and evaluation procedures for the magnetic flux density B/sub ia/ within the innermost of the superimposed cylinders when the value of the external magnetic flux density B/sub ex/ is less than that of B/sub s/. The present research has evaluated the shielding effect for the superimposed cylinder with the use of an HTS DC-SQUID magnetometer and spectrum analyzer. The present paper examines the magnetic step response of B/sub in/ to the applied B/sub ex/, the characteristics of B/sub in/ when exposed to a B/sub ex/ having a value less than that of B/sub s/, and the magnetic noise power spectra within the superimposed cylinder. In addition, a discussion is conducted on the relationship between the number of layers of the multi-layered soft-iron cylinder and the value of B/sub in/.

Superconducting magnetic shields for SQUID applications

We investigate the shielding of superconducting and μ-metal forms in axial and transverse directed background magnetic noise fields. Analytical expressions are obtained for the improvement in signal-to-noise ratio obtained by placing a superconducting disk in the presence of a dipole source and a uniform noise field. Axial and transverse shielding factors are then compared for identical superconducting and μ-metal cylinders. The signal-to-noise ratio is found to be infinite at certain points inside a superconducting cylinder as well as a superconducting cylinder with a central partition. Shielding factors obtained here are relevant to SQUID measurements of small dipole source fields in the presence of large background noise fields such as those encountered in biomagnetism and nondestructive evaluation.

Magnetic characteristics of a high-T/sub c/ superconducting cylinder for magnetic shielding type superconducting fault current limiter

The superconducting fault current limiter (SCFCL) is expected to be the first application of high-T/sub C/ superconductors (HTSs) in power systems. To develop a magnetic shielding type superconducting fault current limiter, we have carried out some fundamental experiments concerning the magnetic shielding characteristics of an HTS bulk cylinder. In this paper, the experimental results of magnetic flux penetration into a Bi(2223) cylinder are shown. AC magnetic field is applied to the sample by a primary winding (copper coil) excited by AC triangular- and sinusoidal-waveform currents in the frequency range of 0.1 Hz to 100 Hz. We also developed a finite element method (FEM) computer program for evaluation of the dynamic electromagnetic behavior of the HTS cylinder in a time-varying external magnetic field. The results of computer simulations considering the voltage-current (E-J) characteristic are compared with experiments.

A possible device for selective detection of magnetic field sources using superconducting and ferromagnetic cylinders

The authors have studied a system, consisting of superconducting and ferromagnetic cylinders, and a magnetic field sensor for selectively detecting a magnetic dipole parallel to the extension of the axis of the cylinders. The superconducting cylinder has an open slit in its side so that the magnetic field from the dipole can be measured selectively according to its diamagnetic characteristics. The magnetic fields detected by the sensor are estimated with the aid of magnetic field analysis. It may be possible to use the system for positioning of one or two parallel dipoles. The system will be useful for nondestructive testing of metallic materials.

Thermoelectric effect in the hollow superconducting cylinder of Pb and Pb-In alloy

The technique for measuring of the thermoelectric effect in superconductors near the critical temperatureTC has been elaborated. The measured magnetic flux was induced by temperature gradient (ΔT∼1K) between joints. The experimental study of the thermoelectric effect has been carried out in the hollow bimetallic superconducting cylinder consisting of the Pb and Pb-In alloy. The observed temperature dependence of the thermoexcited magnetic flux has been well approximated by the logarithmical function. The magnitude order of the thermopower estimated from the experimental data coincides with the known low temperature thermopowers of metals.

Thermoelectrical effect in the hollow superconducting cylinder

The technique of determination the thermoelectrical effect in superconductings samples near the critical temperature Tc has been elaborated. In this method the thermoexcited magnetic flux is measured in a closed contour composed of two superconductors and with substantial temperature gradient (ΔT≈1 K) between joints. The experimental study of the thermoelectrical effect has been carried out in the hollow bimetallic superconducting cylinder of Pb and Pb-In alloy. The observed temperature dependence of the thermoexcited magnetic flux has been well approximated by the logarithmical function.

A magnetic shielding type superconducting fault current limiter using a Bi2212 thick film cylinder

The application of a superconductor to a fault current limiter in a power system has been researched. The authors developed a magnetic shielding type superconducting fault current limiter using a Bi2212 thick film cylinder. This limiter consists of a primary winding on a superconducting cylinder for generating an AC magnetic field, the cylinder for shielding the field, an iron core for magnetic members and a control ring for controlling fault currents. When a fault occurs, the superconductive cylinder quenches and a magnetic circuit is formed inside the cylinder. As a result, the limiter has a high impedance to limit the fault current. The present study shows the limiting and recovery characteristics of the fault current limiter. >

Magnetic self-field entry into a current-carrying type-II superconductor. II. Helical vortices in a longitudinal magnetic field.

The previous consideration of the irreversible magnetic self-field entry into a current-carrying type-II superconductor (SC) [Phys. Rev. B 49, 6950 (1994)] is extended to the case of an applied longitudinal magnetic field. The structure of the helical Abrikosov vortex is found exactly in the framework of the London approximation. The field-dependent critical current of a spontaneous entry of the right-handed flux helix against the surface Bean-Livingston barrier into the SC cylinder of arbitrary radius R is calculated, as well as the critical current of the flux-line instability with respect to the left-handed helical expansion. On the basis of these two mechanisms, a diagram of the resistive state in current-field coordinates is evaluated. The direction of the optimal helix for which the surface barrier is the least and which, hence, is the first to enter the sample turns out to coincide with that of the full magnetic field at the surface only in the case of a thick cylinder with R\ensuremath{\gg}\ensuremath{\lambda}, where \ensuremath{\lambda} is the magnetic penetration depth. For the latter case, the generalized Silsbee's rule of a breakdown of nondissipative state is formulated.

Helical magnetic vortex in a type-II superconductor: An exact solution of an edge barrier problem

An exact solution of problem of magnetic helical vortex entry into a long superconducting (SC) cylinder through a surface Bean-Livingston barrier is found in London approximation. Gibbs energy of system is calculated when transport current and longitudinal magnetic field are applied. A critical current of spontaneous vortex penetration into SC is found and compared to the critical current of left-handed helical instability.

Magnetic self-field entry into a current-carrying type-II superconductor.

A problem of the irreversible entry of the Abrikosov vortex ring, induced by the self-field of a transport current, into a long type-II superconductor cylinder of arbitrary radius R is solved exactly in the London approximation. The magnetic field and current distribution for the toroidal vortex inside the cylinder are evaluated. The Gibbs free energy of the system is calculated when the transport current is applied. The critical current ${\mathit{j}}_{\mathit{c}}$ of a spontaneous vortex penetration into the superconductor through a surface Bean-Livingston barrier is found to be independent of the radius of the cylinder and close to the depairing current. However, the dependence of the width of edge barrier on the transport current in a thin cylinder is found to differ qualitatively from that in a thick cylinder. As a result of this, in the first case there is no characteristic current but ${\mathit{j}}_{\mathit{c}}$, while in the second case a characteristic current ${\mathit{j}}_{\mathit{c}1}$\ensuremath{\simeq}${\mathit{j}}_{\mathit{c}}$/\ensuremath{\kappa} arises (\ensuremath{\kappa} the Ginzburg-Landau parameter) at which the barrier width drops down to values of the order of the magnetic-field-penetration depth \ensuremath{\lambda}, which allows for vortex entry on the surface defects of the size of \ensuremath{\lambda}. The latter result is discussed in reference to the high-critical-current observations on the microbridges of high-temperature superconductors.

Improvement of magnetic shielding effects: the superposition of a double-magnetic cylinder over a high-T/sub c/ superconducting cylinder

The authors obtained improved magnetic shielding by superimposing a double-magnetic cylinder over a YBCO (Y-Ba-Cu-O) cylinder. The value of the magnetic shielded field for the superimposed cylinder was 47 times larger than that of the YBCO cylinder. The magnetic shielded field obtained when superimposing a soft iron single-cylinder over a YBCO cylinder was analyzed using the Bean, Wills, and Mager models, with an important criterion being derived for designing an effective shield of the chamber. >

Discontinuity of Magnetic Flux Trapped in Multiply Connected Superconductor

A theoretical basis is discussed for the experimentally observed discontinuity of magnetic flux trapped in multiply connected superconductors. The discontinuity is related to the modification of gauge field with Goldston boson in quantum-condensed fluid. Several experimental observations 1 ),2) reveal that there exists some discrepancy between the total trapped magnetic flux f/)!n existing in a multiply connected supercon­ ductor and total flux f/)ex observed outside the superconductor. Lischke 1 ) measured the temperature dependence of the total flux trapped in a superconductor cylinder with use of Aharanov-Bohm effect,3) The superconductor cylinder is with inside radius p and thickness d as seen in the insert of Fig_ 1. The observed temperature dependence of f/)ex is shown in Fig_ 1 by solid line. When p'2>d and J..>d, a calcula­ tion 4 ) shows that the total trapped flux f/)in existing in the cylinder is

Observation of Decrease of Supercurrent in the Gapless Region

The dependence of superfluid current density $J$ on superfluid velocity $v$ was measured in a hollow super-conducting cylinder of indium placed in an axial low-frequency magnetic field. In order to enter the $v$ region for which a decrease of $J$ as a function of $v$ is predicted, the interior of the cylinder was filled with a superconducting rod. The measured quantities were the flux ${\ensuremath{\varphi}}_{J}$ arising from $J$ and the magnitude ${H}_{e}$ of the low-frequency magnetic field which is related to $v$. The experimental results are graphs showing ${\ensuremath{\varphi}}_{J}$ as a function of ${H}_{e}$. At temperatures close to the transition temperature of the film, the ${\ensuremath{\varphi}}_{J}\ensuremath{-}{H}_{e}$ graphs were linear for small ${H}_{e}$, whereas for larger ${H}_{e}$, a maximum in ${\ensuremath{\varphi}}_{J}$ followed by a decrease was clearly observed. The temperature dependence and the maximum value of ${\ensuremath{\varphi}}_{J}$ were found---within experimental accuracy---to be in agreement with the predictions of the Ginzburg-Landau theory.

Transition of a homogeneous type II superconductor cylinder under the influence of axial current

We have calculated the magnetic field distribution and the resistance of a homogeneous type II superconductor cylinder at transition under the influence of axial current in zero external magnetic field. In this calculation we have expressed the magnetic field dependence on flow resistivityϱ f by two or more potential functions and have shown that the accuracy of this approximation is sufficient. For some values of the lower critical field and various temperatures, the magnetic field distribution and the resistance of the cylinder are calculated as a function of the applied electrical field and/or the applied current. The temperature dependence of the electric field at which, the normal state appears on the surface of the cylinder is also given.

Low-Frequency Permeabilities of a Superconductor due to Surface Currents

Superconductors which can carry critical persistent currents around the circumference of a specimen may dissipate energy when an alternating magnetic field ${h}_{0}cos\ensuremath{\omega}t$ is superimposed on a magnetic field ${\mathrm{H}}_{0}$ which is swept at a constant rate $\frac{d{H}_{0}}{\mathrm{dt}}$. We have calculated, for a long macroscopic superconducting cylinder, the real and imaginary parts of the permeabilities due to the surface currents alone for the first three Fourier components of the internal field. The results are complicated and depend on two parameters, namely, $\frac{(\frac{d{H}_{0}}{\mathrm{dt}})}{\ensuremath{\omega}{h}_{0}}$ and $\frac{{h}_{c}({H}_{0})}{{h}_{0}}$. The function ${h}_{c}({H}_{0})$ is a measure of the current-carrying capacity of the surface of the superconductor.

Properties of a Thin Hollow Superconducting Cylinder

The properties of a thin, superconducting cylindrical tube are considered by means of the Ginsburg-Landau theory. The equations have been solved under the condition that the magnitude of the energy gap (order parameter) is independent of position but not independent of magnetic field ${H}_{0}$ or of fluxoid quantum number $n$. This latter dependence on ${H}_{0}$ and $n$, which is usually ignored, is found to lead to modification of the expressions for the various superconducting properties. In the limit $\frac{\mathrm{rd}}{{\ensuremath{\lambda}}^{2}}\ensuremath{\le}1$, where $r$ is the radius, $d$ the thickness, and $\ensuremath{\lambda}$ the penetration depth, the energy gap goes smoothly to zero with magnetic field for a fixed $n$, defining a second-order phase transition. The magnetization, the current density, and the free energy also go smoothly to zero, with the curves for different $n$ being displaced by $n$ times the field corresponding to unit quantum. For this case the transition temperature is periodic with magnetic field with period corresponding to unit quantum, which agree with the experiments of Little and Parks and the calculation of Tinkham. In the limit $\mathrm{rd}\ensuremath{\ge}{\ensuremath{\lambda}}^{2}$, the energy gap drops discontinuously to zero at critical conditions, giving a first-order phase transition, with a corresponding influence on the other properties of the superconductor; also the possibility of a thermodynamically metastable region with respect to the normal state arises. In order to obtain agreement with the measurements of critical persistent currents and the measurements of flux quantization, it must be assumed that $n$ is a good quantum number under any conditions as long as the system remains superconducting. This leads to metastable states which for the $\mathrm{rd}\ensuremath{\ge}{\ensuremath{\lambda}}^{2}$ case includes states which are thermodynamically metastable as well.