SUPERCONDUCTING SHIELDS FOR EXCLUSION AND FIELD SHAPING*

S. J. St. Lorant Stanford Linear Accelerator Center, Stanford University Stanford, California 94305

Abstract

Superconducting shields provide a means of obtain­ as they undergo a superconducting transition, some ing arbitrarily reduced magnetic fields over considerable flux will invariably remain trapped in the metal. These volumesfor use in diverse applications. Various shield­ regions of flux remain pinned in the superconductor ing techniques for high and low flux densities are sur­ either by non-superconducting impurities or by specific veyed and some recent results discussed. In particular, states of the lattice of the superconductor. In a sense, the flux shielding properties of laminates of Nb3Sn have it is the perfect conductivity of the material which been studied in fields up to 2.5 T. Measurements are makes complete elimination of the trapped flux so dif­ presented on the shielding efficiency of the laminate as ficult. Of course, the number of pinning sites can be a function of the number of lamellae. the properties of reduced considerably by using very pure, strain-free overlap regions, and the methods of construction. The materials. but the shields then become very difficult to practical application of these devices is illustrated. make and rather delicate to handle.

Introduction The next step is to perform the shielding process in increments: effectively to reduce the external field The purpose of this report is twofold: The first is by some other means before the shield is made super­ to present a representative cross section of past work conducting, so that tbe Meissner effect will have to re­ on shielding of magnetic fields with superconductors for move a correspondingly reduced amount of flux from various applications. We find that this work falls natu­ the material. An interesting realization of this step­ rally into two groups, characterized by the magnitude wise process has been described by Vant-Hull and of the flux density to be excluded. Thus, on the one Mercereau. 2 Many superconductors, and in partic­ hand, we have shielding of apparatus against fields of ular, show a spectacular drop in the electrical resistiv­ the order of a microtesla, methods which usually make ity just before the metal becomes superconducting. The use of the Meissner effect and which in the ultimate low­ magnetic shield in the form of a tin cylinder is rotated field region exploit the quantization of flux. On the rapidly while being cooled through the transition region. other hand, we are concerned with superconducting An ambient will induce eddy currents in shields in which the screening currents are of a magni­ the metal which will tend to shield the region inside the tude characteristic of the high-field Type II alloy super­ cylinder from the non-axial components of the field. As conductors. It is this latter type of magnetic shielding, the cylinder is cooled through the transition region, an the study and application of which to practical problems almost perfect Meissner effect will be achieved because form the second part of this article. We have found, the effective resistivity of the metal decreases rapidly moreover, that it appears to be possible to combine which in turn enhances the flux exclusion both sets of properties of superconductors to obtain by several orders of magnitude before any region of the large regions of truly zero magnetic field. shield is able to trap flux. The authors were able to achieve an ultimate trapped field somewhat less than Review of Shielding Techniques 10-10 tesla.

Magnetic shielding with superconductors is, of Before leaving the subject of very low fields, we course, an old concept; however, the approach to shield­ mention one method which uses the phenomenon of flux ing tended to be largely empirical. The literature is quantization, whereby in principle the field can be re­ full of references to lead or tin foil screens surrounding duced to zero. Suppose a folded superconducting con­ particular experiments to reduce stray magnetic fields, tainer is cooled in a magnetic field; it will exclude some but little attention seems to have been devoted to a sys­ flux and it will trap the remainder. If the container is tematic examination of the conditions required to do so now unfolded, the flux density inside is reduced and efficiently. The Space Age changed this. Magnetom­ hence the field is lowered. Another folded container in­ eters for space application have to be tested and cali­ serted into the first and cooled slowly will repeat the brated in low ambient fields, typically of the order of flux expulsion process and, on being unfolded, similarly 10-8 tesla or less. Such flux densities are achievable reduce the field still further. Thus, given enough steps, with conventional shielded rooms but further reduction it is possible to reduce the field far enough until less in the field can only be effected by exploiting the intrin­ than half a quantized flux unit remains. If the conditions sic propertL s of superconductors. Thus, Hildebrandt1 are then right, a current is induced in the inner folded describes a fairly typical combination of a p.-metal container which exactly cancels this flux and produces shielded room and a dewar containing superconducting the lowest quantized state inside, i. e. 3 there remains lead shells which make use of the Meissner effect to zero flux inside the container. Brown has constructed achieve a final static magnetic field of less than 2 x 10-10 such a flux "vacuum pump" and has achieved remanent tesla. Unfortunately this technique is limited by the fact magnetic fields less than 10-10 tesla in two stages. that a perfect Meissner effect is extremely difficult to Variants of these and other exotic devices are described achieve in practice. Although most superconducting in some detail in a review by Hamilton. 4 metals will exclude a large fraction of the applied field The shielding of magnetic fields in excess of about *Work supported by the U. S. Atomic Energy Commission 250 millitesla required virtually that a new technology

227 of superconductors be developed. We find the first ref­ A shielding tube for that experiment was con­ erence outlining the possible use of the then newly dis­ structed and the e~eriment was successfully concluded covered superconducting compound of and tin5 earlier this year. 2 We understand at the time of for magnetic shielding in an early paper by Autler, 6 writing that the CERN flux shield, which uses niobium who, in the same article, made some rather remarkable titanium for the shielding elements, has also passed predictions concerning the use of this material. The its acceptance tests. macroscopic theory of of hard supercon­ ductors due to Bean, 7,8 and Anderson9 and the experi­ Cylindrical Flux Shields mental work of Kim et al. 10, 11 underscored the useful­ ness of Type II superconductors for such applications as In this description of the SLAC work on supercon­ magnetic field shielding and magnetic field stabilization. ducting flux excluders, we should like to briefly mention Thus Dunlap et al. 12 described the operation of a some of the steps in our development program and we simple tubular field stabilizer constructed very much should like to comment on certain gray areas which along the lines first suggested by Autler6 and elaborated would benefit from further study. on by Foner,13 while Hempstead et al. 14 investigated the inductive behavior of a closed system of supercon­ Our early work was based on the assumption that ductors. This study encompassed not only flux shield­ well-cooled, oriented, niobium-titanium foil is essential ing and flux trapping but also discussed some flux con­ to any large-scale construction, as at the time we could centration schemes to achieve various degrees of in­ not visualize a method of construction other than an as­ tensification in field strength, including . sembly of spirally-wound sheets. Experiments not di­ rectly concerned with magnetic shielding, however, very It may seem rather surprising that after such an soon convinced us that motion of the foils under mag­ auspicious a beginning of a new technology, applications netic forces, rather than inadequate cooling, prevented should lag behind. However, we must not forget that the us from reaching the theoretically desirable screening production of these high-field superconducting alloys in current density in the superconductor. Niobium­ a form useful for shielding purposes and in any ~uantity titanium is, in metallurgical terms, a singularly un­ was far from simple. Benaroya and Mogensen, 5 who cooperative alloy. This fact, more than anything else, undertook a study of the shielding properties of Nb3Sn convinced us that the solution to our shielding problem with a view of making superconducting plate shields for was very closely coupled to our choice of supercon­ a deflector system for a high-intensity variable-energy ductor. However, before giving up niobium-titanium cyclotron, give a good insight into the fabrication tech­ completely, we undertook a series of tests of flux niques of the day. It was not a process that every lab­ shields made for us by the Linde Division of Union Car­ oratory would undertake lightly. Incidentally, these bide Corporation, 17 with disappointing results. authors were among the first to study the shielding properties of a hard superconductor for practical rea­ We were thus effectively reduced to experimenta­ sons. They were followed by others: Minet et al. 16 tion with Nb3Sn tape. As a construction based on 5-cm developed a simple analog model for use in field shaping wide spirally-wrapped modules was quite obviously problems involving superconductors, Haid et al. 17 ex­ totally unsuitable for any kind of design involving long plored the feasibility of making superconducting flux shielded lengths, we proceeded to develop the concept shields by a process of plasma deposition of mixed nio­ of combining two long semi-cylindrical shells into an bium and tin powders. A little later, Kawabe et al. 18 equally long cylinder. studied the magnetic shielding by superconducting Nb­ 40 Zr-10 Ti hollow cylinders. We found in our tests that no matter how carefully the superconductor was interleaved with copper and About this time, an unexpected sector of the in­ processed into a shielding tube, the shielding behavior dustry began to show a keen interest in the shielding would tend to be erratic and the samples would often characteristics of superconductors: the manufacturers exhibit mechanical damage on completion of the meas­ of electron microscopes. As Echarri and Spadoni19 urements. Furthermore, as we mentioned before, commented in their review article, here at last was a while the quality of the cooling did not seem to affect possibility of constructing highly corrected electron the performance of the shield, the rate at which the ex­ lenses of short focal length. The work of Dietrich et a1.20 ternal field was applied did. We therefore coated our is a good example of the degree of shielding sophistica­ material with a variety of metallic bonding agents, in­ tion achievable. cluding Wood's alloy and various types of soft solder and soldered a number of tapes together in an effort to While electron microscopy provided the incentive eliminate all relative motion of the superconductors. for the study of small compact shielding systems, it Figure 1 is typical of the improvement that this treat­ was high-energy that demanded further develop­ ment produced. Figure 2 illustrates the characteristic ment. The idea of a field-free beam channel leading to shielding curve of a short sample tube after we had de­ a typical particle detector, be it a bubble chamber or a veloped the fabrication technique. 25 The smoothness spectrometer system, has always claimed more than of the penetration of the applied field into the shielding its share of the particle physicist's attention. Firth tube and the degree of reversibility attained proved to et al. 21 and Schmeissner and Haebe122 at CERN de­ be a very sensitive diagnostic of the mechanical condi­ scribed some preliminary work on field-free beam tion of the sample. For example, inadequate bonding paths using superconducting flux shields. This was fol­ or an excessive reheating cycle resulted in a discontin­ lowed almost simultaneously by a proposal for an ex­ uous flux penetration pattern. Into a "good" sample, periment at the Stanford Linear Accelerator Center, re­ the flux would penetrate smoothly and no flux jumps quiring a 4-m long and 1.3-cm average inside diameter would be observed. flux exclusion tube. 23

228 70 tion is a regenerative process, we would expect a dis­ (0) turbance to propagate through the tube and tend to drive it into the normal state. Our experimental evi­ 60 dence contradicts this: No matter how large the dis­ turbance, no matter how small the amount of copper 50 "stabilizer, " as long as the mechanical integrity of the tube remained unimpaired, the shield continued to ex­ clude the same amount of flux. As Fig. 3 shows, the > 40 E measurements made under very different conditions W are reproducible to a remarkable degree, even to the l? 20 w f-

0.65 1.00 0 1.32 1.60 1.88 Figure 5 indicates the scale of the final apparatus: EXTERNAL MAGNETIC FIELD, T The pole pieces of the magnet are 1. 5 m in diameter and the shielding tube and its associated cryostat is Fig. 2. Flux penetration into a bonded shielding tube. over 7 m long. The actual construction of the shield is illustrated in Fig. 6: The superconducting semi­ cylindrical shells are soldered to the copper-plated The results of the measurements are summarized stainless steel support tube, and the entire bonded as­ in Fig. 3. It is a plot of the external field at which flux sembly is further clamped with a number of brass and penetration begins, as a function of the number of layers stainless steel clamps, two of which are shown. of superconductor in the same tube. The points reflect the measurements of numerous shielding tubes having The flux shield behaved as predicted by the test various lengths, diameters, and differing in the amount ~f program. The system has now been thermally-cycled of superconducting ribbon used. 26 The regularity between ambient and liquid helium temperatures some this graph is striking, particularly when one takes mto sixteen times, and has remained cold for periods of up account that a considerable fraction of the superconduc­ to two weeks. No deterioration in performance has tor is always in the metastable critical state. This state been observed. On several occasions, partial flux as we know from other evidence, is unstable, and small penetration took place as a resuIt of malfunctioning of disturbances in the applied field or in the temperature the magnet and its associated power supplies. These will tend to produce local flux instabilities which may events had no detectable effect on the shielding per­ drive that region into the normal state. As flux penetra- formance. We have concluded that the field penetration

229 TRANSVERSE EXTERNAL MAGNETIC FIELD I I I I LOCKSCREW [ ~~~S~Y~::E :@@I(@WP?W«IIlIIllllZZZlZZWlVJ/i1 OCTAGONAL ~ ~ ~ ~ ~ • HALL PLATE POSITIONS TO STAINLESS STEEL MEASURE INTERNAL FIELD SPACER WWMIWIWWlWm»mzmmmr::aAYERS BRASS SPLIT RING

I I I OF Nb Sn TAPE SUPERCONDUCTOR ASSEMBLY -1.0 -0.5 0 0.5 1.0 em 3 LINE OF CONTACT BETWEEN THE EXPERIMENTAL ARRANGEMENT TWO SEMI-CYLINDRICAL SHELLS SUPERCONDUCTOR LAMINATIONS 0.2 I I EFFECTIVE LAYERS OF TAPE COPPER PLATED STAINLESS STEEL SUPPORT TUBE 0.1 - POSITION A· 45 -

o Fig. 6. Detail of the construction of the flux exclusion :J::::::::=::::::POS~'"'' tube. , ," 1 was the result of abrupt motion of the shielding tube within the cryostat in spite of the massive supports, I- 0.2 POSITION C rather than being due to changes in the external flux. c:i ...J W c:: 0.1 On one occasion, we have used the incident elec­ u tron beam to map the residual magnetic field inside the ~ tube at various external flux densities. ~ 0 40.3 ::;'"' POSITION D· 15 As Fig. 7 shows, at 1. 5 T, the field penetration, ...J of the order of 10-2 T, is sufficient to deflect the beam ~ 0.2 almost totally into the walls of the shielding tube. Here

0.3 RESIDUAL MAGNETIC FIELD DUE TO PERSISTENT CURRENTS Because the shielding tube does not qUite clear the fringe field of the magnet, the beam arrives at the en­ 0.2 trance beam window bent slightly down. We have cal­ culated that the two transverse (to the beam direction) 0.1 components of the magnetic field are 6.4 and 10.4 mT, respectively, when the external impressed field is 1 T. o o 20 40 60 80 100 MAGNET CURRENT, A L-_--'- ---'------' -'-- ---' o 0.65 1.00 1.32 1.60 1.88 EXTERNAL MAGNETIC FIELD, T HELIUM GAS OUTLET '-.

Fig. 4. Flux shielding characteristics of overlap regions. HELIUM SUPPLY VESSEL

HELIUM TANK MAGNET BEAM ENTRY WINDOW CABLE '1/' CABLE VAPOR COOLED SUPPORT 7/ //'1/ / // '" RADIATION SHIELD l f' , I BEAM.Q~

TARGET '''" "" .,NDDW I

DRIFT SECTION ------

}rr,

Fif{. 5. General layout of cryostat with flux exclusion tube.

2'3!J 7 Further Work EXTERNAL MAGNETIC FIELD: 0 T The flux penetration into the tube is a very intrigu­ 6 ing phenomenon: We have analyzed the field distribu­ tions in and around the tube and we have found that the 5 major contributions to the trapped field arise from the multipole distributions in the superconductor induced by 4 the external field. 27 We have further concluded that when the slit separating the two half-shells exceeds the penetration depth of the superconductor, the field gen­ 3 erated by the screening currents is negligible and the external field penetrates the slit. On the other hand, 2 when the shell separation is less than the penetration depth, the external field is prevented from entering into the interior of the tube. Thus, to eliminate the internal field entirely, even in the presence of the unavoidable misalignments, we could add inner shells whose slits o were rotated 90° with respect to the outside slits. This EXTERNAL MAGNETIC FIELD: 1 T would certainly reduce the field leakage but it would not completely eliminate the inductive effects of the screen­ ----...... ing currents. As the remanent internal fields can be ",/ .... " / " reduced to 200 mT or less, this suggests making use of the Meissner effect as described earlier. Work is /I '"\ now in progress to study these alternatives; in partic­ I \ ular, we are interested in mapping the internal fields l \ to determine the order of multipolarity and to examine \ J the process of flux penetration. ~ I \~ / References ... ---/'" 1. A. F. Hildebrandt, Rev. Phys. Appliquee 5, 49 (1970). -

2. L. L. Vant-Hull and J. E. Mercereau, Rev. Sci. Instr. 34, 1238 (1963). EXTERNAL MAGNETIC FIELD: 1.5 T 7 3. R. E. Brown, Rev. Sci. Instr. 39, 549 (1968).

Appliquee~, 6 4. W. O. Hamilton, Rev. Sci. 41 (1970). 5. J. E. Kunzler, E. Buehler, F. S. L. Hsu, and 5 J. H. Wernick, Phys. Rev. Letters~, 89 (1961).

4 6. S. H. Autler, Proc. Int. Cont. on High Magnetic Fields, MIT, 1961 (MIT Press and John Wiley & Sons, Inc., New York) p. 324. 3 7. C. P. Bean, Phys. Rev. Letters~, 250(1962). 2 8. C. P. Bean and M. V. Doyle, J. Appl. Phys. 33, 3334 (1962).

9. P. W. Anderson, Phys. Rev. Letters~, 309 (1962). o 012345678 10. Y. B. Kim, C. F. Hempstead, and A. R. Strnad, 1905.2 BENDING MAGNET, SOUTH (millivolts) 2179Cl Phys. Rev. Letters~, 306 (1962).

11. Y. B. Kim, C. F. Hempstead, and A. R. Strnad, Fig. 7. Deflection of the beam by the remanent mag­ Phys. Rev. 129, 528 (1963). netic field in the shielding tube . Solid line: Effective beam profile at exit of 12. R. D. Dunlap, C. F. Hempstead, andY. B. Kim, shielding tube. J. Appl. Phys. 34, 3147 (1963). Chain line: Effective beam profile at entrance 13. S. Foner, Rev. Sci. Instr. 34, 293 (1963). to shielding tube. pashed line: Defines effective beam aperture 14. C. F. Hempstead, Y. B. Kim, and A. R. Strnad, limits determined by counting rate. J. Appl. Phys. 34, 3226 (1963).

231 15. R. Benaroya and H. P. Mogensen, J. Appl. Phys. 21. M. Firth, L. Krempasky, and F. Schmeisser, 37, 2162 (1966). CERN TC-LTL Report (1969).

16. R. Minet, H. A. Combet, J. Y. LeTraon, and 22. F. Schmeissner and E. U. Haebel, Int. Com. on J. Schmouker, IEEE Trans. Magnetics MAG-3, Instrumentation for High Energy Physics, Dubna, 271 (1967). 1970, USSR, p. 161.

17. D. A. Haid, C. A. Houck, and C. R. McKinsey, 23. B. Dieterle, W. Lakin, F. Martin, E. Petraske, Tech. Report AFAPL-TR-67-155 (1968), Air M. L. Perl, J. Tenenbaum, and W. T. Toner, Force Aero Propulsion Laboratory, Wright­ SLAC Proposal No. 65 (1970). Patterson Air Force Base, Ohio. 24. F. Martin, S. J. St. Lorant, and W. T. Toner, 18. U. Kawabe, T. Doi, and M. Kudo, J. Appl. Phys. SLAC-PUB-I040 (1972). To be published in Nucl. 40, 10 (1969). Instr. and Methods.

19. A. Echarri and M. Spadoni, Cryogenics 11, 274 25. A. C. Newton, F. Martin, S. J. St. Lorant, and (1971). W. T. Toner, SLAC-PUB-1102 (1972). Submitted to Rev. Sci. Instr. 20. 1. Dietrich, R. G. Maier, R. Weyl, and H. Zerbst, Third Int. Cryogenic Engineering Com., Berlin, 26. Nb3Sn ribbon, supplied in 5-cm width by Kawecki 1970 (Heywood-Temple Industrial Publications Ltd. , Berylco Industries, P.O. Box 1462, Reading, Pa., London), p. 422. U. S. distributors for Thomson-CSF , 51 Boulevard de la Republique, 78, Chatou, France.

27. F. Martin and S. J. St. Lorant, SLAC-PUB-1104 (1972). to be published in J. Appl. Phys.

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