Liquid helium-free superconducting magnets and their applications

ICEC 15 Proceedings

Liquid Helium-Free Superconducting Magnets and Their Applications

K. Watanabe, S. Awaji, T. Fukase, Y. Yamada**, J. Sakuraba*, F. Hata*, C. K. Chong*, T. Hasebe* and M. Ishihara* Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-77, Japan *Sumitomo Heavy Industries, Ltd., 63-30 Yuhigaoka, Hiratsuka 254, Japan A new superconducting magnet (4.6 T, 38 mm room temperature bore) which consists of a low-T~ Nb3Sn superconducting magnet and high-To Bi2Sr2Ca2Cu3Olo current leads has been working in vacuum for about 7400 hours without trouble. An experiment at a field of 3.7 T, which is generated by an operating current of 370 A, has been continuously performed for 1200 hours. The high-T~ current leads have experienced 13 thermal cycles from room temperature to 10 K. It is found that the high-T~ current leads can hold excellent superconducting properties for a long enough time to be practically used. As a next step, an 11 T-52 mm room temperature bore, a 6 T-220 mm room temperature bore and a 5 T- ~ 50 mm × 10 mm room temperature gap liquid helium-free superconducting magnet are now under construction.

INTRODUCTION Liquid helium has successfully enabled us to realize a high field superconducting magnet, while it is holding back high field applications in a broad range of chemical, biological and medical investigations, for instance. This is because liquid helium is troublesome to handle and is expensive to use. In addition, a large cryostat with a complicated structure is required to house liquid helium. To eliminate the use of liquid helium for superconducting magnets, cryocooler-cooled superconducting magnet systems have been designed.[1-3] However, difficulties of large heat loads due to copper current leads were encountered for the usage of a cryocooler with small refrigeration capacity. Since the liquid helium-free magnet is operated in vacuum, it is impossible to adopt the conventional helium gas flow current leads. The key of the design for a cryocooler-cooled superconducting magnet lies in how heat loads of the current leads at the second stage can be reduced within the refrigeration capacity. Recently discovered high-T~ superconductors are expected for current leads with low thermal conductivity. We first demonstrated the compact liquid helium-free superconducting magnet system using Bi2Sr2Ca2Cu3010 (Bi(2223)) current leads.[4] This paper describes the performance test of Bi(2223) current leads combined with a practical superconducting magnet. Moreover, a high field, a large bore and a split-type liquid helium-free superconducting magnet under construction are presented.

PERFORMANCE OF Bi(2223) CURRENT LEADS Tubular Bi(2223) current leads with size of 23 mm in outer diameter, 20 mm in inner diameter and 140 mm in length were made by a powder-sintering process using a cold isostatic pressing method. Plasmasprayed Ag was coated onto both sides as a current terminal of 20 mm in length. These Bi(2223) current leads with a cross section of N1 cm2 exhibit the transport critical current of more than 1000 A at 77.3 K and zero external magnetic field. Figure 1 shows the Jo-vs-B characteristics at 77.3 K and 4.2 K in fields perpendicular to the transport current direction for Bi(2223) leads. Although a large history effect of J~ t present address : Faculty of Engineering, Tokai University, 1117 Kitakaname, Hiratsuka 259-12, Japan Cryogenics 1994Vol 34 ICEC Supplement 639

ICEC 15 Proceedings

B (T) 2

4

6

8

IO

12

14

Table 1. Performance of Bi(2223) Current Leads 10 4

E u

<

holding time in vacuum

14440 hours

cooling time in vacuum

7390 hours

continuous holding time at 370 A operation

1200 hours

continuous holding time at 400 A operation

96 hours

thermal cycles from room temperature to 10 K

13 times

T =77.3 K Bi(2223) current lead I0

i 0

2(30

L

i

i

400

600

800

B

i

I0(0)

i

1200

i

1400

(Gauss)

Figure 1 J¢ properties in fields at 4.2 K and 77.3 K for the Bi(2223) current lead. Magnetic fields are applied perpendicular to the transport current. in increasing and decreasing fields appears at 4.2 K, which indicates weak links at grain boundaries for Bi(2223) leads[5], the Jc properties are one order of magnitude higher than those at 77.3 K in low fields. Since the c-axis direction of the Bi(2223) lead with a preferred orientation is perpendicular to the tubular surface, Bi(2223) leads have a weak point for the critical current density at 77.3 K for B_I_I. The Bi(2223) leads indicate a current capacity of 500 A at 77.3 K and 200 G for B-L I for magnet design. In the case of oxygen-free high conductivity copper (OFHC) leads combined with a cryocooler, the current level was limited to less than about 40 A[1], because of the small refrigeration capacity. The thermal conductivity integrated from 77.3 .K to 10 K is estimated to be about 600 W/cm for OFHC. If one selects the operating current of 400 A, a large heat load of 9.5 W due to the thermal conductivity is generated for a pair of copper current leads with optimum size of 5.5 mm in diameter and 300 mm in length, and further Jule heating of copper is added. On the other hand, the heat load of a pair of Bi(2223) leads is expected to be 0.2 W using the thermal conductivity value of about 1 W/cm. An attempt was made to demonstrate a compact superconducting magnet system using large capacity of 500 A current leads. Then, an epoch-making cryocooler-cooled 10 K-4.6 T superconducting magnet with no use of liquid helium, consisting of a low-T~ Nb3Sn superconducting magnet and high-To Bi(2223) current leads, was realized. [4] Table 1 lists the high performance of Bi(2223) leads. The system has been working for about 7400 hours up to now, and has experienced numerous quenches (40 times) for a test. In order to investigate the relation between the amount of the induced martensitic phase and the holding time in fields for stainless steel 304L[6], the experiment was continuously performed for 1200 hours at 3.7 T which is generated by an operating current of 370 A. No variation of the contact resistance for the Bi(2223) leads was observed. It is found from such performance tests that the Bi(2223) current leads with no reinforcement work excellently in vacuum for a long time.

LIQUID HELIUM-FREE SUPERCONDUCTING MAGNETS I I T High Field Superconducting Masnet The critical currents in fields up to 23 T for Nb tube processed (Nb, Ti)3Sn wires which have Wire characlcristics of Cu/non Cu ratio of 0.89, filament number of 54, filament diameter of 88/~ m and wire d iamclcr of 0.9 mm were measured. The prediction of the field dependence of I c at various temperatures i~ ~h(~wn in Fig. 2, using the Kramer-type scaling law of the form Ic=A(Bc2)XSB-°5(1-B/Bc2)z55, where A is ~ t illi,g parameter and B~2 the upper critical field. [7] The critical current of 200 A is obtainable at l 1 T f;40

(:ry(~{l~;rli(:s 1994 Vo134 ICEC Supplement

ICEC 15 Proceedings

800

' '

/

J

~ ~ ~ IK [ ' ~ I .

'

I

I

I

'

I

'

I

'

I

_

Nb3Sn:Ti 600

cryocooler

,,o0o,,oK,

i

0K~ I

s-.,0oo,,on

i

=4g ,-?~ 400

/GM

'

ro,

(2223)

112~-~~

200 0

,

0

,

~

UI"

I ~ ,

5

10

Bff)

15

20 r'

' d

Figure 2 Ic-vs-B properties at various temperatures Figure 3 Liquid helium-free 11 T high field for Nb tube processed multifilamentary (Nb, Ti)3Sn superconducting magnet system with the compact superconducting wires. cryostat of outer diameter of 450 mm, room temperature bore of 52 mm and cryostat height of 900 mm. and 8 K. We designed the operating current of 180 A at coil temperature of 7.5 K for the maximum field of 11.3 T. Figure 3 shows an outline of the liquid helium-free 11 T superconducting magnet. Heat loads are estimated to be about 0.30 W for a second stage with Bi(2223) leads and about 30 W for a first stage with copper leads of the cryocooler. An experimental room temperature bore of 52 mm is available. This liquid helium-free high field magnet will be superior to traditional NbTi superconducting magnets immersed in liquid helium. 6 T Large Bore Superconducting Magnet The high field superconducting magnet with a large experimental room temperature bore is of importance for a number of practical applications. A great interest is focused on basic research into chemical or biological effects in high field, which are being carried out using a 220 mm room temperature bore superconducting magnet with use of liquid helium at High Field Laboratory for Superconducting Materials, Tohoku University. As an outstanding example, a morphological study on electroless deposition of a silver metal-leaf in high field recently attracted strong attention.[8] The drastic change in the growth morphology from a random direction at B=0 to a spiral direction at B=4 T was observed. This fact indicates that the growth pattern is strongly affected by applying field. Important discoveries of a ube J

;se I

/liquid helium

.t: E N

Figure 4 Liquid helium-free 6 T large bore superconducting magnet system is compared with a conventional superconducting magnet system using liquid helium. Cryogenics 1994Vol 34 ICEC Supplement 641

tCEC 15 Proceedings

chemical and a biological effect in high fields are expected in near future. In order to offer an easyoperation of such a large bore superconducting magnet, a 6 T- 220 mm room temperature bore superconducting magnet without use of cryogenic fluids is constructed. The coil temperature of 6 K is selected using a 4 K cryocooler with hybrid regenerators of Er3Ni and ErNio.gCoo.1. A liquid helium-free superconducting magnet which is wound with NbTi wires and impregnated with epoxy is realized. The cryostat housing the liquid helium-free superconducting magnet is extremely compact and simple as shown in Fig. 4. The cryostat with size of 220 mm in room temperature bore, about 800 mm in outer diameter and 750 mm in height is specified in comparison with the complicated cryostat using liquid helium with size of the same bore of 220 mm, 1170 mm in outer diameter and about 1650 mm in height. d410

5 T Split-Type Superconducting Magnet Recently, X-ray diffraction experiments have been performed under high temperature or high pressure, but high field research in X-ray diffraction has not yet been realized. So far it has been impossible to design a superconducting magnet accompanied with liquid helium on the goniometer. We are now constructing a liquid helium-free 5 T Helmholtz-type superconducting magnet as shown in Fig. 5, which can be combined with an X-ray diffraction apparatus and provides a very powerful tool to investigate the magnetic phase transitions. The 5 T- 9650 mm × 10 mm room temperature gap superconducting magnet working in vacuum and made as compact as possible is realized. This split magnet employs Nb tube processed (Nb, Ti)3Sn wires mentioned above, and will generate a maximum field of 6.7 T at the Windings. The operating current of 190 A and coil temperature of 9 K was selected. It will be very easy to extend the application of the liquid helium-free split-type superconducting magnet to the neutron diffraction and scattering experiments.

temperature (¢ 50ram)

temperature (IOmm) Ib~Sn coil

stage ~.223) nt lead tim stage

ryocoole r

Figure5 Liquid helium-free 5 T split-type superconducting magnet system with room temperature gap of 10 mm and bore of 50 mm.

ACKNOWLEDGMENTS This work is supported by Toray Science Foundation, and Grant-in-Aids from Scientific Research from the Ministry of Education, Science and Culture, Japan. We are grateful to Professor T. Masumoto of IMR, Tohoku University for his encouragement of this project.

REFERENCES Hoenig, M. O., IEEE Trans. Magn. (1983) MAG-19 880 van der Laan, M. T. G., Tax, R. B., ten Kate, H. H. J. and van de Klundert, L. J.M., Adv. Cryo_. Eng. (1992) 37 1517 Masuyama, S., Yamamoto, H. and Mastubara, Y., IEEE Trans. AppI. Supercond. (1993) 3 262 Watanabe, K., Yamada, Y., Sakuraba, J., Hata, F., Chang, C. K., Hasebe, T. and Ishihara, M., J. AppL Phys. (1993) 32 L488 Watanabe, K., Noto, K., Morita, H., Fujimori, H., Mizuno, K., Aomine, T., Ni, B., Matsushita, T., Yamafuji, K. and Muto, Y., Cryogenics (1989) 29 263 Shibata, K., Kurita, Y., Shimonosono, T., Murakami, Y., Awaji, S. and Watanabe, K., Proc. of International Cryogenic Materials Conferfnce, Albuquerque, 1993 Watanabe, K., Noto, K. and Muto, Y., IEEE Trans. Ma~n. (1991) 27 1759 Magi, I., Okubo, S. and Nakagawa, Y., J. Phys. Sac. Jpn. (1991) 60 3200

642 Cryogenics 1994 Vol 34 ICEC Supplement