Review of stability in high temperature superconductors with emphasis on flux jumping

We have experimentally studied the magnetic shielding properties of a cylindrical shell of BiPbSrCaCuO subjected to low frequency AC axial magnetic fields. The magnetic response has been investigated as a function of the dimensions of the tube, the magnitude of the applied field and the frequency. These results are explained quantitatively by employing the method of Brandt (1998 Phys. Rev. B 58 6506) with a Jc(B) law appropriate for a polycrystalline material. Specifically, we observe that the applied field can sweep into the central region either through the thickness of the shield or through the opening ends, the latter mechanism being suppressed for long tubes. For the first time, we systematically detail the spatial variation of the shielding factor (the ratio of the applied field over the internal magnetic field) along the axis of a high-temperature superconducting tube. The shielding factor is shown to be constant in a region around the centre of the tube, and to decrease as an exponential in the vicinity of the ends. This spatial dependence comes from the competition between two mechanisms of field penetration. The frequency dependence of the shielding factor is also discussed and shown to follow a power law arising from the finite creep exponent n.

Magnetic shielding properties of high-temperature superconducting tubes subjected to axial fields

The onset of magnetic flux penetration through the wall of a tube of sintered YBa2Cu3O7 has been measured in the temperature range of 4 to 90 K. These measurements have been used to calculate the temperature dependence of the intergranular critical transport current employing three critical-state models. Good agreement was obtained between this calculated critical current and direct critical current measurements on rod-shaped samples of the same material. The critical-state magnetic field profiles were used to explain the observed voltage waveforms induced in a pick-up coil inside the bore of tubular samples when excited by an AC field. Best agreement was obtained using a model in which the critical current (jc) has a magnetic field dependence given by j c = j 0 H 2 0 (H 2 +H 2 0 ) where j0 and H0 depend only on temperature.

Flux pinning in tubes of a ceramic superconductor

© 2002-2011 IEEE. YBCO tubes of ∼10 mm diameter closed at one extremity were engineered by a buffer-aided top seeded melt growth fabrication process. These tubes can act as efficient 'dc' magnetic shields and are observed to reduce axial flux densities of 1.5 T by a factor of 100 at 20 K. Such performances are comparable in magnitude to the record threshold inductions reported for bulk MgB2 and Bi-2212 materials at lower temperatures. Magnetic shielding measurements for open and closed tubes at 77 K also show that the presence of the cap improves substantially the shielding performance at the closed extremity since it reduces the penetration through the open end. This fabrication technique is extremely promising for shielding 'dc' stray fields generated by high temperature superconductor magnets operated in a temperature range obtained by cryocoolers, liquid hydrogen (20 K), or liquid neon (27 K).

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Magnetic Shielding above 1 T at 20 K with Bulk, Large Grain YBCO Tubes Made by Buffer-Aided Top Seeded Melt Growth

The shielding properties of high-temperature superconductors in DC and AC magnetic fields are discussed using Bean's critical-state model and models, which account for the field dependence of the critical current density. Experimental work on the shielding properties of superconducting YBa 2 Cu 3 O 7−δ and conductive (aluminum and cooper) screens is presented. The maximum value of the shielded field for YBa 2 Cu 3 O 7−δ /Ag screens was measured at 3 mT. Shielding by superconducting screens was found independent of frequency over the range 80 Hz−10 5 Hz. The limited shielding capability of ceramic superconductors results from the magnetic field dependence and low values of the intergrain current density in bulk materials. There is a practical limit to the external field that can be shielded by a superconductor, characteristic of the material and size independent. Increasing of the shield thickness will not increase the maximum value of the field which can be shielded.

Shielding effects in ceramic superconductors

High‐Tc superconducting tubes have been developed for magnetic shielding of SQUIDs at 77 K. The characteristics of such tubes show that adequate shielding exists for magnetic fields up to a critical field determined by the current density of the tube. Relaxation phenomena with two different time dependencies are observed when the external magnetic field enters the tubes.

S. Kulkarni

Papers

Magnetic shielding and relaxation characteristics of superconducting YBa2Cu3O7 tubes