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Magnetic shielding, superconductivity

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Magnetic Shielding Characteristics of Hybrid High Temperature Superconductor/Ferromagnetic Material Multilayer Shields

Magnetic shielding characteristics of hybrid shields consisting of second generation high temperature superconducting tapes (2GHTS) and ferromagnetic materials are presented. Shields with multiple layers with several combinations of superconducting and ferromagnetic materials showed interesting patterns. Shielding factors as high as 95 % are observed for 3-layer hybrid shielding structures. The data presented offers new ways to achieve the required shielding factors in a given application with minimal use of expensive superconductor.

Superconductors (S) and magnetic materials (M) have many useful and interesting properties. One of the important properties is their ability to shield magnetic fields. A variety of shapes have been used to achieve the best magnetic shielding solution. Studies have been conducted to ascertain the relative merits of each type of material for a given magnetic shielding need. When trying to achieve the highest possible magnetic shielding factor (SF) in economical fashion, both classes of materials were used individually and in combination (S, M, SM, and SMS). Hybrid configurations were explored particularly to reduce the amount of expensive superconductor tape used.

Results of the magnetic shield made by the superposition of four layers of a soft iron over the BPSCCO cylinder were reported [1]. Research has examined a new analysis method for the maximum shielded magnetic flux density for a multilayered cylinder. It was found that the magnetic field values at the center of four layered superimposed cylinders were almost the same as the value at the center of the single BPSCCO cylinder without an external magnetic field. The maximum shielded magnetic flux density of 61 mT is found to be about 4 times larger than that of a single BPSCCO cylinder.

Shielding properties of a hybrid MgB2/Fe structure consisting of two coaxial cups, one made of MgB2 and the second from Fe, subjected to an external magnetic field parallel to their axis were studied in [2]. The height and radius of each cup were chosen of similar dimensions. Measurements were performed at 20 and 30 K – typical temperatures for MgB2 devices. Magnetic field shielding of hybrid system was compared with individual components. The shielding factor was measured as a function of the sensor position along the cup axis and the external magnetic field. In the hybrid configuration, the shielding factor at higher magnetic fields was more than three times higher than that measured with the superconducting cup alone at temperature 20 K [2].

Previously developed open-type cylindrical magnetic shields suffer from magnetic field which penetrates into the magnetic shield from its open ends. Therefore, axial length of the shield must be more than two times longer than its diameter [3]. An enhanced type of open-ended cylindrical magnetic shield using high-Tc superconducting rings and flexible ferromagnetic sheets was developed. It uses superconducting rings to reduce the magnetic field that penetrates into the magnetic shield from its open ends. The developed cylinder [3] is an optimum shield for magneto-cardiograph because of its open, small, and lightweight structure.

The study of magnetic shielding has been done with both materials separately in the past, but in new experiments the authors use both materials at the same time to shield as much magnetic field as possible. Obtaining a high value of shielding factor is one of the most important steps for application of very sensitive detectors. As the sensitive detectors use SQUIDs, the magnetic field value at the detector location must be below 10-7 T [4]. To further decrease the field around the sensor, a sheet of niobium was bent into the shape of a cube without one side. The superconductor could form an ideal shield because of the perfect diamagnetism caused by Meissner effect. If the field exceeds the niobium critical field, magnetic flux will penetrate into the superconductor. Measurements indicate that the design based on test shield should satisfy the requirements as the external magnetic field was lower than 60 mT which is the critical field of niobium [4].

S and M materials can be used for magnetic shields while standing alone and in hybrid structures. Also, placing the magnetic material layer in close vicinity of the superconductor to screen the magnetic field may result in a higher critical current [5]. During the past decade use of S and M materials in the hybrid structures have also brought promising results in building large magnetic shields. Our previous shielding studies dealt with the superconductor material only [6]-[10].

In this paper we explored the magnetic shielding efficiency of the different heterostructures using combination of S and M materials. We studied cylindrical and sheet shapes to identify the hybrid material configuration that would offer the highest magnetic field shielding. To have a better understanding on how the materials interact with each other, we measured the shielding characteristics starting from one layer of S or M material and added additional layers to observe the influence of each additional layer on shielding efficacy. The maximum number of layers that were used to create the magnetic shield was three in many combinations. For studies on 2-layer shields, we measured four combinations and for 3-layer structures we measured eight different combinations.

Hybrid magnetic shields were characterized in external DC and AC magnetic fields at 77 K in two geometrical forms: as two parallel sheets and multilayer coils.

The support structure for two parallel shields was made of two 60 mm x 100 mm flat G10 plates. The superconducting tape of 45 mm x 100 mm dimensions was placed on each G10 plate, Fig. 1(a). Shields with single layer were characterized before placing the second layer. Position of Hall probe and magnetic shields on sample holder is shown in Fig. 1.

In the coil geometry YBCO tape of 45 mm width was wound on G10 coil former of 50 mm diameter and 10 cm length. There were 6 turns on coil former, and 1.1 m of tape was used. Neighboring turns were not isolated and overlapped by 10 mm. YBCO tape turns were fixed on coil former by a Kapton adhesive tape, Fig. 1(b).

As a source of the external magnetic field we used a helical copper magnet immersed in liquid nitrogen contained in a stainless steel cryostat. The hybrid magnetic shields were placed in a uniform AC or DC magnetic field with the field orientation perpendicular to the broad surface of the sheets and perpendicular to the axis of the coils. The copper magnet was supplied by a current from a Techron triple unit power rack, which was controlled by a DC or sinusoidal generator. The current supplied to the magnet was measured using an AC/DC Hall probe current transducer. From the magnet field constant of the magnet known from calculation and calibration, and measured current through the magnet we evaluated the external magnetic field values. In AC measurements a series resonance circuit with a capacitor bank was used to compensate the inductance of the magnet in each frequency.

A calibrated cryogenic Hall probe was used for internal magnetic field measurements in the shields. The Hall probe was positioned in the vertical and horizontal center of shields and they together were placed in the center of the magnet to be in the homogeneous magnetic field region. The Hall probe was supplied by a 10 mA control current from a Lakeshore DC constant current source. The Hall probe was calibrated AC and DC external magnetic fields without any magnetic shield present. The Hall probe sensitivities evaluated for each frequency and DC magnetic field were used to calculate internal magnetic field.

The superconducting material used was the second generation high temperature superconductor (2GHTS) in the form of 45 mm wide coated conductor manufactured by American Superconductor Corporation [11]. The 2GHTS conductors were prepared on Rolling Assisted Bi-axially Textured NiW Substrates (RABiTS) with several buffer layers. Superconductor layer was deposited on the substrate material by metal oxide deposition (MOD) technology. 2GHTS wire top was passivated by 1 µm silver layer. The YBCO tape has a total critical current of ~ 1.3 kA at 77 K in self magnetic field by using the 1 µV/cm Ic criterion.

The ferromagnetic material used was AD-MU-80 (18% Fe, 75% Ni, 2% Cr, 5% Cu) from Ad-Vance Magnetics Inc. [12] with initial permeability 75 000 at 4 mT and permeability at 20 mT of 100 000 and saturation magnetic field of 0.8 T. The magnetic tape has 0.16 mm thickness and 45 mm width.

The interlayer gap in the multilayer shields was kept as small as possible. The shields were held in place by cryogenic Kapton adhesive tape to the G10 fiberglass support sheet.

In AC Hall probe voltage measurements a signal from magnet current was fed via preamplifier to the reference input of the Lock-in amplifier. Before each measurement, the frequency was set on signal generator. The Lock-in phase was locked to the phase of the Hall probe signal before the measurement. For each measurement, the current through the magnet and the Hall probe voltage were recorded. For DC Hall probe voltage measurements we used Keithley 2001 multimeter.

Shielding factors of the hybrid shields were measured as a function of magnetic field magnitude (all reported AC magnetic field values are RMS values), frequency, and the number of layers made of S and M materials in various combinations. We are presenting only some combinations with higher shielding factor in order to keep figures clear. In case of the coil shield the external magnetic field was perpendicular to the axis of the cylindrical shield and in case of the sheet shield it was perpendicular to the wide face of the sheet. All measurements were conducted at temperature 77 K in liquid nitrogen bath.

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