IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 19, NO. 3, JUNE 2009 3577

Shielding of AC Magnetic Fields by Coils and Sheetsof Superconducting Tapes

Jozef Kvitkovic, John Voccio, and Sastry V. Pamidi

Abstract—Magnetic field shielding properties of coils and sheetsof silver sheathed ��� ��������������� tapes (1G super-conductor tapes) and ������ coated conductors (2G super-conductor tapes) are reported. Shielding properties were measuredat 77 K, with varying field amplitude and frequency of externalmagnetic field, using Hall probes and a phase sensitive technique.Shielding factor data on several coils and sheets are presented as afunction of external field amplitude and frequency. Magnetic fieldshielding of up to 50% was observed at field amplitudes of 10 mT.Differences in frequency dependence of shielding properties of testshields fabricated from 1G and 2G conductors are discussed.

Index Terms—AC magnetic field, BSCCO coils, Hall probe, mag-netic shielding, YBCO coils.

I. INTRODUCTION

S UCCESSFUL commercial production of high temperaturesuperconductors in the form of tapes opened new possibil-

ities to fabricate low cost magnetic shields. Some scientific ex-periments require shielding from background magnetic fields tominimize noise and interference. Equipment with strong mag-netic fields needs to be shielded to protect surroundings frommagnetic fields. Superconducting shields can be made smallerin size and lighter than shields made of ferromagnetic metalsbecause of their higher saturation induction in constant mag-netic fields and thinner than shields made of nonmagnetic highconductivity metals because of the stronger skin effect in lowfrequency alternating magnetic fields. Magnetic shielding prop-erties of BSCCO-2212 bulk cylinder were reported by Matsubaet al. [1]. The shielding factor of the cylinder did not exhibit anyfrequency dependence. Wang and Sayer investigated magneticshielding properties of packed powder and thin films of YBCOin frequency range of 10 Hz to 3 kHz [2]. They concluded thatthe effectiveness of the shields depended on material composi-tion, microstructure, sample shape and dimensions. Karthikeyanet al. reported propagation, penetration, and shielding of ACmagnetic field through plasma sprayed YBCO coatings [3]. Asmost of the recent developments in superconducting materialsare in commercializing the tape form, it would be beneficial to

Manuscript received August 21, 2008. First published June 30, 2009; currentversion published July 15, 2009.

J. Kvitkovic is with the Center for Advanced Power Systems, Florida StateUniversity, Tallahassee, FL 32310, USA. He is on leave from the Institute ofElectrical Engineering, Slovak Academy of Sciences, Bratislava, Slovak Re-public (e-mail: kvitkovic@caps.fsu.edu).

J. Voccio is with American Superconductor Corporation, Devens, MA 01434-4020 USA.

S. V. Pamidi are with the Center for Advanced Power Systems, Florida StateUniversity, Tallahassee, FL 32310 USA (e-mail: kvitkovic@caps.fsu.edu).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TASC.2009.2018856

understand the suitability of superconducting tapes for fabri-cating magnetic shields. Little information is available on the ef-ficacy of superconducting tapes in shielding AC magnetic fields.To be able to use commercial tape conductors for shielding ap-plications, it is necessary to understand their shielding proper-ties in various geometries.

This paper describes the results of the studies on shields fab-ricated from 1G and 2G tapes. The paper also describes the tech-nique used to measure very small variable frequency AC mag-netic fields at cryogenic temperatures using Hall probes and theexperimental facility used to measure magnetic fields inside anexperimental shield.

II. EXPERIMENTAL

Superconducting shields were fabricated using two differentsuperconducting tapes, namely brass-laminated, silver-sheathedBi-2223 (1G tape) and YBCO coated conductor (2G tape). Bothwere manufactured by American Superconductor Corporation[4]. The Bi-2223 tape has 85 untwisted filaments encased in asilver matrix. The outer dimensions of the insulated tape were4.3 mm 0.4 mm and its self-field critical current at 77 K with1 criterion was 115 A.

For YBCO tape the nickel-tungsten (Ni-W) substrate wasformed using the rolling-assisted biaxially-textured substrate(RABiTS) technology. The substrate was coated with thenecessary buffer layers. A thin layer of YBCO was depositedonto the buffer layers using the metal-oxide deposition (MOD)process. The initial experimental samples used in magneticshielding trials are 12 mm wide and had self-field criticalcurrent of 200 A/cm-width at 77 K.

Superconducting shields were fabricated in coil and sheetgeometries. Fig. 1 depicts photographs of some typical shieldsfabricated and characterized. Single layer coils of 1G tape werefabricated using 3.3 m of conductor. The solenoid coil waswound on a 50 mm fiberglass epoxy former with 21 turns. Thecoil length was 9 cm. The turn-to-turn gap was minimized bypacking the turns next to one another as closely as possible.After characterizing the shielding properties of the single layercoil, a second layer was wound by offsetting the layers by thehalf-width of the conductor such that the second layer coversthe turn-to-turn gaps of the first layer. Coils of 2G tape werefabricated in a similar fashion with 10 turns. The coil lengthwas 12 cm and we used 1.6 m of the tape. The coil ends werenot short-circuited.

Superconducting shields with sheet geometry were fabricatedby gluing sections of superconductor tapes on fiberglass platewith dimensions 60 mm 100 mm, as shown in Fig. 2. A pair ofidentical sheets arranged parallel to each other with a 20 mm gap

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3578 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 19, NO. 3, JUNE 2009

Fig. 1. Photographs of shield coils fabricated using 1G (a) and 2G (b) super-conductor tapes.

Fig. 2. Photograph of two sheets of 2G conductor used as shields.

constituted the shields with sheet geometry. Additional layerswere added after characterization of the single-layer shields. Aswith the coils, the second layer was offset by a half-width ofthe tape to cover inter-turn gaps. The separation between thetwo sheets was varied from 20 mm to 30 mm to understand theeffect of separation distance on shielding properties.

External AC magnetic fields with variable amplitude and fre-quency were produced using special double helix dipole magnetwhich has two sets of tilted copper coils, each consisting of threelayers. The tilted coils are connected in series in such way as tocancel the axial field component and produce only transversefield which is perpendicular to the magnet axis. The magnetcoils are cooled with liquid nitrogen. The magnet produces uni-form transverse magnetic field with less than 1% variation ina cylindrical region with 75 mm in diameter and 100 mm inlength. Details of the magnet design are described elsewhere[6].

AC magnetic fields were measured with AC Hall probe mag-netometry. The AC Hall probe magnetometer consists of a Hallprobe (HHP-MP) prepared at the Institute of Electrical Engi-neering, Slovak Academy of Sciences, Bratislava, Slovakia, aDC current source (Lake Shore, C120) and a lock-in amplifier

Fig. 3. Block diagram of the instrumentation used in measurements ofshielding properties. B with arrow represents the direction of the externalmagnetic field.

(Stanford Research Systems, SR 830). Fig. 3 depicts a schematicof the measurement instrumentation. The Hall probes are suit-able for cryogenic temperatures and are known to provide highlylinear output when used in DC magnetic fields. The Hall probehas an active area of and a sensitivity of 73.2mV/T at a nominal control current of 10 mA. The details of theHall probes used are described elsewhere [4].

The Hall probes have been extensively used for DC applica-tions. No data was available, however, on their suitability formeasuring variable frequency AC fields. The Hall probes werechosen for their very small active area. Several attempts to usethe Hall probes using DC excitation current and measurementof AC voltage resulted in non-linear and frequency-dependentresponse. Large inductive signals mask the actual response ofthe Hall probe. Phase sensitive and frequency-locked measure-ments produced satisfactory results, but a small non-linear com-ponent was present superimposed on an otherwise linear signal.The non-linear component of the signal could, however, be elim-inated by averaging the signals under normal and reverse con-trol current. The measurements using the modified protocol pro-duced highly linear and reproducible results. The response ofthe Hall probes with the modified method has no discernablefrequency dependence.

The shielding properties of test shields were characterized at77 K by placing them in the uniform magnetic field region ofthe double helix magnet with the magnetic field perpendicularto the axis of the shields. The radial component of the magneticfield inside the various shields (coils and sheets) immersed inliquid nitrogen was measured as a function of field amplitudeand frequency. After each measurement the shields were heatedto the room temperature slowly ( 1 K/min) in order to preventthermal shock to the superconducting tape.

III. RESULTS AND DISCUSSION

The AC magnetic field inside the magnet with and withoutthe test shield was measured using Hall probes by followingthe phase-locked measurement protocol described in the experi-mental section. Shielding properties of the test shields fabricated

KVITKOVIC et al.: SHIELDING OF AC MAGNETIC FIELDS BY COILS & SHEETS OF SUPERCONDUCTING TAPES 3579

Fig. 4. Shielding factor versus peak external magnetic field for 1-layer coilfabricated from 1G superconductor.

from 1G and 2G superconductor were characterized at 77 K inpeak fields up to 50 mT in the frequency range of 10–100 Hz.The shielding efficacy as denoted by percentage shielding is de-fined as the percentage of external field that is shielded.

, where isexternal magnetic field and is magnetic field in the centerof the shield. All measurements reported here are for the orien-tation with the magnetic field perpendicular to the axis of thecoils and to the broad surface of the sheets.

Fig. 4 depicts the variation of shielding factor as a functionof peak field for frequencies ranging from 10 Hz to 100 Hz ob-served for the 1-layer coil made of 1G superconductor. As seenin the figure, the shielding factor drops initially with field andlevels off for all frequencies. It can also be seen that the shieldingfactor increases with increasing frequency. This frequency de-pendence of shielding factor is in contrast to that observed for2G coils as explained later.

Fig. 5 shows the comparison of variation of shielding factoras a function of peak field for 1-layer and 2-layer sheets of 1Gsuperconductor. Addition of second layer enhances shieldingproperties as seen from larger values of shielding factor for2-layer sheet for all frequencies. As discussed in the experi-mental section, the second layer not only adds additional su-perconducting layer but also covers the interturn gaps. The ob-served increase can be attributed to the absence of through gapsin 2-layer sheet because the addition of a third layer failed toenhance shielding properties beyond what was observed in the2-layer sheet. The general behavior of shielding factor versuspeak field plots of 1-layer and 2-layer sheets, however, is sim-ilar. Further, the general shape of shielding plots of sheets issimilar to that of 1-layer coil shown in Fig. 4.

The variation of shielding factor as a function of peak fieldfor 1-layer and 2-layer coils of 2G superconductor is presentedin Fig. 6. As observed in 1G sheets, addition of second layer en-hances shielding properties of 2G coil. One of the distinguishingfeatures of 2G coils is that absence of frequency dependence ofshielding factors. The curves of all frequencies between 10 Hzand 100 Hz sit one on another. The second feature of 2G coils isthe sharp drop of shielding factor with increasing field. As seenin Fig. 6, shielding almost vanishes at peak field of 50 mT.

Fig. 5. Shielding factor versus peak external magnetic field for 1-layer and2-layer sheets fabricated from 1G superconductor.

Fig. 6. Shielding factor versus peak external magnetic field for 1-layer and2-layer coils fabricated from 2G superconductor.

Fig. 7 depicts the variation of shielding factor as a function ofpeak field for 1-layer and 2-layer sheets of 2G superconductor.As can be seen from a comparison of Figs. 6 and 7, the behaviorof 2G coils and sheets is similar both in the absence of signifi-cant frequency dependence and sharp drop of shielding proper-ties with increasing peak field values.

Fig. 8 shows the effect of separation distance (G) betweenthe parallel two layer plates of a sheet type 2G shields. As ex-pected, for a given pair of parallel sheets, shielding efficacy de-creases with increasing separation. As seen in Fig. 8, as the sep-aration between 2-layer sheets increase from 20 mm to 30 mm,shielding properties diminished significantly.

As seen from the data presented in Figs. 4–8, both coil andsheet form shields fabricated from commercial 1G and 2Gsuperconducting tape exhibit significant shielding properties.Shielding factors of up to 50% were observed at lower mag-netic fields. Surprisingly, shielding factors observed for 2Gconductor drop faster than for 1G conductor. It is not clearif the difference is due to the difference in the optimization

3580 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 19, NO. 3, JUNE 2009

Fig. 7. Shielding factor versus peak external magnetic field for 1-layer and2-layer sheets fabricated from 2G superconductor, gap was 30 mm.

Fig. 8. Dependence of shielding properties on the separation gap (G) betweenthe pair of parallel two layer plates in 2G superconductor sheet geometry.

of processing conditions or the difference is inherent in thematerials. Matsuba et al. [1] did not observe any frequency de-pendence of shielding properties in their study on bulk Bi-2212.It appears that coils of 2G behave similar to bulk material inshielding magnetic fields. Significant difference observed inthe relative frequency dependence of 1G shields is perhapsdue to the filament structure of 1G conductor. The filamentsget electromagnetically coupled as the frequency increases andthe increased coupling leads to enhanced shielding at higher

frequencies. It is also important to note the presence of largevolume fraction of silver in 1G conductor. It is difficult toassess the role of silver and substrate from the results presentedhere. Further studies are necessary to understand the detailedproperties of superconducting shields fabricated using com-mercial superconducting tapes. With expected improvementsin superconducting properties of 2G conductor and availabilityof long lengths will enable fabrication of superconductingmagnetic shields for various applications.

IV. CONCLUSIONS

The test shields were fabricated in coil and sheet geometriesusing commercial 1G and 2G superconducting tapes. Hallprobes have been successfully applied to measure AC magneticfields by establishing a phase-sensitive technique. Shieldingfactors of up to 50% have been demonstrated at AC fields of

10 mT with frequencies in the 10–100 Hz range. Some inter-esting differences in frequency dependence of shielding factorsbetween 1G and 2G conductors were observed. Further studiesare necessary to understand the role of filament structure andhighly conductive normal metal in superconducting tapes onshielding properties. Such studies will be continued with 1Gand 2G conductors with improved superconducting propertiesand wider tapes. The results presented in this paper point tothe potential possibility of fabricating magnetic shields usingcommercial 1G and 2G superconducting tapes.

ACKNOWLEDGMENT

The authors thank Steve Ranner and Danny Crook for cryo-genic design and experimental assistance.

REFERENCES

[1] H. Matsuba, A. Yahara, and D. Irisawa, “Magnetic shielding prop-erties of HTc superconductor,” Supercond. Sci. Technol., vol. 5, pp.S432–S439, 1992.

[2] J. Wang and M. Sayer, “Low frequency magnetic shielding ofYBaCuO:Ag high temperature superconductors,” Cryogenics, vol. 33,no. 12, pp. 1164–1169, 1993.

[3] J. Karthikeyan, A. S. Palthankar, P. Chaddah, and K. P. Sreekumar,“Study of the propagation of an AC magnetic field through supercon-ducting thick films,” Supercond. Sci. Technol., vol. 4, pp. 250–256,1991.

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