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Performance evaluation of high temperature superconducting coil Mitsugi Yamaguchi a, * , Satoshi Fukui a , Itsuya Muta b , Taketsune Nakamura b a Graduate School of Science and Technology, Niigata University, Ikarashi 8050, Niigata 950-2181, Japan b Graduate School of Engineering, Kyoto University, Yoshida Honmachi, Sakyo, Kyoto 606-8501, Japan Abstract The current–voltage characteristics of Ag-sheathed Bi-2223 tape for high-temperature superconducting coils are measured as a function of the magnetic field and its flux angle to the tape at liquid nitrogen temperature. The ap- proximate characteristic equations on critical currents and n values are obtained from this result. Then, the electric field distribution inside the coil can be calculated with the approximate characteristic equations. The current–voltage characteristics of high-temperature superconducting coil are analyzed and compared with the test result of conduction- cooled coil, producing relatively good agreements. The coil performances can be made clearer with the assistance of characteristic equations at the coil design stage and, further, the optimum coil design will be made possible. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Critical current; n Value; Flux density; Flux angle; Ag-sheathed Bi-2223 tape 1. Introduction The performance of BSCCO Ag-sheathed tape was improved, and high temperature supercon- ductivity (HTS) coils arrived at the level of a practical application. If the critical current of the HTS tape is made larger, it would be possible to generate a high magnetic field with HTS coils and they would enable us applications for MRI, NMR, energy storage device, magnetic separator, etc. However, the critical current density is small, in particular in magnetic fields and in addition, a BSCCO Ag-sheathed tape has an anisotropy against the magnetic field direction. It is required to estimate a flux flow loss and a critical current I c of the coil at the design stage in advance to make the critical current of HTS coil larger, namely to increase the current carrying capacity. The critical current I c of the tape is the function of the flux angle, temperature and magnetic field. For this reason the current carrying characteristics of HTS coil cannot be evaluated easily. In view of these technical backgrounds, I c and n value of the tape are first measured as a function of the flux angle h, the temperature T of the super- conductor and the flux density B. Based on these basic data, a way of evaluating the current carry- ing characteristics of HTS coil is pursued. The approximate equations of I c and n value were derived for the BSCCO Ag-sheathed tape Physica C 372–376 (2002) 1406–1409 www.elsevier.com/locate/physc * Corresponding author. Tel./fax: +81-25-262-7369. E-mail address: [email protected] (M. Yamaguchi). 0921-4534/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII:S0921-4534(02)01040-7

Performance evaluation of high temperature superconducting coil

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Page 1: Performance evaluation of high temperature superconducting coil

Performance evaluation of high temperaturesuperconducting coil

Mitsugi Yamaguchi a,*, Satoshi Fukui a, Itsuya Muta b, Taketsune Nakamura b

a Graduate School of Science and Technology, Niigata University, Ikarashi 8050, Niigata 950-2181, Japanb Graduate School of Engineering, Kyoto University, Yoshida Honmachi, Sakyo, Kyoto 606-8501, Japan

Abstract

The current–voltage characteristics of Ag-sheathed Bi-2223 tape for high-temperature superconducting coils are

measured as a function of the magnetic field and its flux angle to the tape at liquid nitrogen temperature. The ap-

proximate characteristic equations on critical currents and n values are obtained from this result. Then, the electric field

distribution inside the coil can be calculated with the approximate characteristic equations. The current–voltage

characteristics of high-temperature superconducting coil are analyzed and compared with the test result of conduction-

cooled coil, producing relatively good agreements. The coil performances can be made clearer with the assistance of

characteristic equations at the coil design stage and, further, the optimum coil design will be made possible.

� 2002 Elsevier Science B.V. All rights reserved.

Keywords: Critical current; n Value; Flux density; Flux angle; Ag-sheathed Bi-2223 tape

1. Introduction

The performance of BSCCO Ag-sheathed tapewas improved, and high temperature supercon-ductivity (HTS) coils arrived at the level of apractical application. If the critical current of theHTS tape is made larger, it would be possible togenerate a high magnetic field with HTS coils andthey would enable us applications for MRI, NMR,energy storage device, magnetic separator, etc.

However, the critical current density is small,in particular in magnetic fields and in addition,a BSCCO Ag-sheathed tape has an anisotropy

against the magnetic field direction. It is requiredto estimate a flux flow loss and a critical current Icof the coil at the design stage in advance to makethe critical current of HTS coil larger, namely toincrease the current carrying capacity. The criticalcurrent Ic of the tape is the function of the fluxangle, temperature and magnetic field. For thisreason the current carrying characteristics of HTScoil cannot be evaluated easily.

In view of these technical backgrounds, Ic and nvalue of the tape are first measured as a function ofthe flux angle h, the temperature T of the super-conductor and the flux density B. Based on thesebasic data, a way of evaluating the current carry-ing characteristics of HTS coil is pursued.

The approximate equations of Ic and n valuewere derived for the BSCCO Ag-sheathed tape

Physica C 372–376 (2002) 1406–1409

www.elsevier.com/locate/physc

*Corresponding author. Tel./fax: +81-25-262-7369.

E-mail address: [email protected] (M. Yamaguchi).

0921-4534/02/$ - see front matter � 2002 Elsevier Science B.V. All rights reserved.

PII: S0921 -4534 (02 )01040 -7

Page 2: Performance evaluation of high temperature superconducting coil

from those measured data. Then, the current car-rying characteristics of HTS coil were analyzedby using the above approximate equation. More-over, the current–voltage characteristics of HTScoil were measured and compared with the anal-ysis.

2. Evaluation of the Bi-2223/Ag tape

The current–voltage characteristics of the Bi-2223/Ag tape used for HTS coil were measured toobtain the basic data concerning the critical cur-rent Ic and n value. Table 1 shows the dimension ofthe Bi-2223/Ag tape used for the measurement.

The current–voltage relation of the tape wasmeasured against the flux angle at liquid nitro-gen temperature. The measured parameters ofthe magnetic field are 0.5, 1.0 and 2.0 T and theflux angle of 0–70�. A critical current Ic and ann value were specified by the measured current–voltage characteristics of the Bi-2223/Ag tape,where Ic was defined by 0.5 lV/cm. The critical cur-rent characteristics of Bi-2223/Ag tape are shownin Fig. 1.

3. Approximate equations of critical current and nvalue

The relation between the current and the volt-age of high temperature superconductors is ex-pressed by the next equation.

V ¼ V0IIc

� �n

ð1Þ

In the above equation, a critical current Ic and an nvalue are given by the function of the flux angle h,the cooling temperature T and the flux density B.The approximate equations of Ic and n value areobtained as a function of B and h from the tapetest result, where the temperature T is 77 K. Thenext equations were obtained

IcðB; hÞ ¼ 21:2 exp f � f ðhÞ � Bg ð2Þ

f ðhÞ ¼ ð2:02� 10�8Þh5 � ð2:95� 10�6Þh4

þ ð1:04� 10�4Þh3 þ ð1:09� 10�3Þh2

þ 0:03h þ 1:70 ð3Þ

nðB; hÞ ¼ 16:8 exp f � gðhÞBg ð4Þ

gðhÞ ¼ ð�1:81� 10�9Þh5 þ ð5:10� 10�7Þh4

� ð4:87� 10�5Þh3 þ ð1:29� 10�3Þh2

þ 0:06h þ 0:97 ð5Þ

The current carrying characteristics of super-conductors inside the HTS coil can be analyzed byusing these approximate equations.

4. Evaluation of HTS coil performance

The current–voltage characteristics of HTS coilwere measured. Table 2 lists the dimensions of themeasured HTS coil. The conduction cooling with arefrigerator was employed in this coil. The coilterminal voltage, which supplied a current to theHTS coil, was measured at the temperature of 77K. The measured critical current and n value ofHTS coil are 6.7 A and 9.6 respectively.

The distribution of the flux angle and the fluxdensity based on the coil critical current of 6.7 Aobtained from the test result was analyzed. Fig. 2Fig. 1. Critical current characteristics of Bi-2223/Ag tape.

Table 1

Specification of Bi-2223/Ag tape

Width (mm) 3.20

Thickness (mm) 0.21

Number of filament 55

Silver ratio 2.8

M. Yamaguchi et al. / Physica C 372–376 (2002) 1406–1409 1407

Page 3: Performance evaluation of high temperature superconducting coil

illustrates how fluxes intersect the superconduc-tors. The maximum radial field of 0.16 T appearsat the middle of the coil edge and the maximumaxial field of 0.45 T occurs at the center in the coilbore.

The distribution of the critical current Ic insidethe coil was analyzed by using the approximateequations as depicted in Fig. 3. The maximumcritical current is 20.4 A, and the maximum n valueis 16.4 at the particular location of the coil, how-ever they are 9.85 A and 10.9 respectively at thepoint of the maximum field.

The current–voltage characteristics of HTScoil was analyzed by using Eqs. (1)–(5) describedabove. Fig. 4 shows analyzed results of the cur-rent–voltage characteristics of the entire HTS coil

Table 2

Specification of HTS coil

Number of turns 7365

Inner diameter (mm) 50

Outer diameter (mm) 160

Height (mm) 100

Current (A) 6.70

Maximum field (T) 0.45

Number of double pancakes 15

Inductance (H) 2.86

Fig. 2. Field angle distribution inside the HTS coil.

Fig. 3. Critical current distribution inside the HTS coil.

Fig. 4. Current–voltage characteristics of HTS coil.

Fig. 5. Electric field distribution inside the HTS coil.

1408 M. Yamaguchi et al. / Physica C 372–376 (2002) 1406–1409

Page 4: Performance evaluation of high temperature superconducting coil

together with the test result. The difference of thecritical current Ic between the measurement andthe analysis is 0.45 A as to the whole HTS coil,and the analyzed n value is more than doublethe measured one. The cause of these differences isconsidered because of that approximate equationsof the critical current and n value do not count theperformance degradation by winding the Bi-2223/Ag tape into the coil and/or non-uniformity ofproperties of BSCCO Ag-sheathed tape with thelength. Fig. 5 indicates the electric field distribu-tion inside the coil. The maximum electric fieldof 4.8 lV/cm appears at the middle of the coiledge.

5. Conclusion

The current carrying characteristics of HTS coilcan be evaluated by using the approximate equa-tions of the critical current Ic and n value derivedfrom test results of 2223/Ag tape. The effectivity ofthis analytical approach for the characteristic es-timation of HTS coil was verified by the compar-ison with experiment. The performance of HTScoil can be estimated at the design stage with thisevaluation method. Hereafter, the relation of theloss of the HTS coil at the steady state and thecooling capability of the refrigerator will be ex-amined.

M. Yamaguchi et al. / Physica C 372–376 (2002) 1406–1409 1409