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Opportunities in High Magnetic Field Science (2005)

Chapter: 3 Technological Challenges and Opportunities for Developing Higher Fields

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Suggested Citation:"3 Technological Challenges and Opportunities for Developing Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
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3
Technological Challenges and Opportunities for Developing Higher Fields

The design and construction of magnets that operate at high field is an art form that requires the balancing of many conflicting requirements. This chapter starts with a discussion of the nature and magnitude of the challenges that must be met and concludes with a description of the opportunities that now exist for improving magnet performance. A common thread in this discussion is the distinction between structural problems that limit what can be done and intrinsic material properties that often have not yet been exceeded, suggesting that it should be possible to build magnets that operate at fields significantly higher than any available today.

WHAT IS THE CHALLENGE?

Four different types of electromagnets are used to generate high fields: (1) resistive DC magnets, (2) (resistive) pulsed-field magnets, (3) superconducting DC magnets, and (4) hybrid magnets, combining both resistive and superconducting elements. The maximum fields obtainable from magnets of all four types have increased significantly over the past several decades, but the increases have come in increments of 10-20 percent rather than in large jumps (such as by factors of 10 or more), and it is important to understand why this is likely to remain the case (Figure 3.1 shows how record field strengths have increased with time).

The reason advances in maximum field strength are hard to obtain is that, everything else being equal, both the energy stored in the field of a magnet and the

Suggested Citation:"3 Technological Challenges and Opportunities for Developing Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
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FIGURE 3.1 Available maximum field strength for nondestructive magnets at facilities around the world as a function of year. The maximum fields delivered by different types of magnets are indicated separately. Figure courtesy of G. Boebinger, National High Magnetic Field Laboratory.

stresses it experiences increase as the square of the field strength. Thus, the higher the field at which a magnet operates, the more complicated the stress and energy management techniques that must be used to ensure its physical integrity and the safety of those who work with it. All aspects of magnet design are affected, including the choice of materials for electrical conductors and structural components,

Suggested Citation:"3 Technological Challenges and Opportunities for Developing Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
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the electrical insulation systems employed, and so on. In the case of superconducting magnets, field stability and quench protection pose additional issues.

The magnitudes of the energies and stresses that must be confronted are best illustrated with some numerical examples. The energy stored in the field of the highest-field magnet for high-resolution solution NMR now operating, which is the 21.6-T superconducting NMR magnet at the National Institute for Materials Science (Japan), is 33 MJ. This is roughly the kinetic energy of a 40-metric-ton railroad locomotive moving at 150 kilometers per hour, and the magnet is designed so that it will dissipate all of that energy harmlessly in the event the windings of its magnet become nonsuperconducting for any reason. Magnetic stresses are easiest to work out for long solenoidal magnets, because the mechanical forces generated by the interaction of the current in the windings of a solenoid with its own magnetic field are similar to an internal hydrostatic pressure. If the central magnetic field is 6 T, the equivalent pressure is 140 atm, which is about the working pressure of the typical gas cylinder. If the central field is 10 T, the internal pressure is about 400 atm, which is approximately the yield strength of annealed copper at room temperature. A 1-GHz NMR magnet (23.5 T) must withstand the equivalent of an internal pressure of over 2,000 atm and thus must be built of materials having yield strengths significantly greater than that of copper. Similarly large stresses must be dealt with in nonsolenoidal high-field magnets.

The magnetic field of a high-field magnet is generated only by the current-carrying conductor in its coil, but the coil necessarily includes other components. For example, none of the materials used as conductors in high-field magnets today is strong enough to withstand the stresses generated, so all magnets include mechanical support systems, which contribute nothing to the field. Electrical insulation and a thermal management system are also required. The higher the field to be generated, the more non-current-carrying material a coil must include and the lower its bulk-average current density. However, the lower the bulk-average current density, the bigger (and more expensive) the magnet must be to deliver a specified field. Given these facts, it should not be surprising that the inner bores of magnets tend to fall as the maximum field at the windings increases (see Figure 3.2), which reduces the amount of field energy that must be managed. Further, as just explained, the overall winding pack current density also tends to fall as stored energy rises (see Figure 3.3). (Table 3.1 describes the magnets shown in Figures 3.2 and 3.3.) Other issues that designers must consider include joints for both resistive and superconducting magnets and AC loss, stability, and quench problems for superconducting magnets. Finally, it is generally the case that no single aspect of a high-field magnet design can be altered independent of the others, so even incremental improvements in magnet performance must be hard fought for.

Suggested Citation:"3 Technological Challenges and Opportunities for Developing Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
×

FIGURE 3.2 The stresses in solenoid scale with the square of the magnetic field, limiting the highest field magnets to small bore sizes and sample volumes. See Table 3.1 for legend. Figure courtesy of J.R. Miller, National High Magnetic Field Laboratory.

FIGURE 3.3 Overall winding pack current density decreases as the stored energy increases because increased amounts of structure, insulation, and thermal management are required to limit stresses and protect the magnet. See Table 3.1 for legend. Figure courtesy of J.R. Miller, National High Magnetic Field Laboratory.

Suggested Citation:"3 Technological Challenges and Opportunities for Developing Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
×

TABLE 3.1 Description and Location of Magnets in Figures 3.2 and 3.3.

Item

Location

Description/Intended Use

T1a

Japan

DC solenoid/conductor testing

T2a

Japan

DC solenoid/conductor testing

T4a

Japan

DC solenoid/conductor testing

T5a

Japan

DC split solenoid/conductor testing

T6a

Japan

DC solenoid/conductor testing

T11a

Japan

DC pancake module/superconducting motors and energy storage (SMES) development

T13a

Japan

DC solenoid/SMES development

T14a

Japan

Pulsed pancake module/SMES development

NRIM 40 T HOb

Tsukuba, Japan

DC solenoid/research facility

NHMFL 45 T HO

Tallahassee, Florida

DC solenoid/research facility

FENIXc

Livermore, California

DC split solenoid/conductor testing

ITER CS model coild

Japan Atomic Energy Research Institute

Pulsed solenoid/technology demonstration

NRIM 21 Te

Tsukuba, Japan

DC solenoid/research facility

NRIM 900 MHzf

Tsukuba, Japan

DC solenoid/research facility

NOTE: Includes a historical sample of foreign and domestic magnets. NRIM, National Research Institute for Metals; NHMFL, National High Magnetic Field Laboratory.

aO. Tsukamoto, S. Torii, T. Takao, N. Amemiya, S. Fukui, T. Hoshino, A. Ishiyama, A.Ninomiya, H. Yamaguchi, and T. Satow, Recent technical trends of superconducting magnets in Japan., IEEE Transactions on Applied Superconductivity 9(2), 547 (1999).

bH. Morita and S. Ito, Development of a 40 T Class Hybrid Magnet, High Magnetic Fields: Applications, Generation, and Materials, H. Schneider-Muntau, ed., Singapore, World Scientific, Singapore, 1997.

cD.S. Slack, R.E. Patrick, and J.R. Miller, FENIX: A test facility for ITER and other new superconducting magnets, IEEE Transactions on Magnetics 27(2), 1835 (1991).

dH. Tsuji, JAERI, Private communication, April 19, 2000.

eR. Hirose, T. Kamikado, O. Ozaki, M. Yoshikawa, T. Hase, M. Shimida, and Y. Kawate, 21.7 T superconducting magnet using (Nb,Ti)3Sn conductor with 14%-Sin Bronze, Proceedings of the 15th International Conference on Magnet Technology, Beijing, China, October 20-24, 1997.

fT. Kiyoshi, A. Sato, H. Wada, S. Hayashi, M. Shimada, and Y. Kawate, Development of 1 GHz superconducting NMR magnet at TML/NRIM, IEEE Transactions Applied Superconductivity 9(2), 559 (1999) and T. Kiyoshi, NRIM, private communication, March 6, 2000.

SOURCE: J.R. Miller, NHMFL.

RESISTIVE DC MAGNETS

Resistive DC electromagnets have been in use since the first half of the 19th century, and they can be as simple as a solenoid made of insulated copper wire. In addition to the force and energy problems described above, high-field resistive magnets present two unique challenges. First, they consume electric power in large quantities and hence need large DC power supplies. Second, most of the power they consume is converted into heat; thermal destruction is a serious problem.

Suggested Citation:"3 Technological Challenges and Opportunities for Developing Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
×

Present Status

The most powerful resistive DC magnets operating today are magnets of a modified Bitter design that generate fields up to 33 T (see Figure 3.4). The field strengths of these magnets are limited primarily by power availability and by cooling issues, but also to some degree by the mechanical strength of materials in the assembly. For economic reasons, high-field DC magnets are found only at large

FIGURE 3.4 Typical plate from a (resistive) Bitter magnet. The small slots are for the circulation of cooling water. The ring of large holes is for axial prestress and accommodates structural tie rods. Image courtesy of National High Magnetic Field Laboratory.

Suggested Citation:"3 Technological Challenges and Opportunities for Developing Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
×

facilities that have the financial means to build and operate the associated power supplies and cooling systems, as well as meet the other operating and maintenance costs of these magnets, which are substantial.

To control energy costs, resistive DC magnets are energized only when experiments are conducted with them and thus are routinely cycled between their energized and unenergized states. This cycling leads to fatigue, limiting their operating lifetimes, which are typically 1,000 to 2,000 hours. It is reasonable to anticipate that resistive DC magnets that produce fields significantly higher than 33 T will eventually be built. Before this happens, however, new materials will have to be developed that combine high mechanical strength and low resistivity, and new structural designs may be required.

Outlook for Resistive DC Magnets

As described above, the difficulty of supplying sufficient power and cooling to a resistive electromagnet severely limits the development of higher-field resistive DC magnets. The physical strengths of the materials used within the magnet are also near their limits. Nevertheless, resistive DC magnets will continue to be important because record steady-state fields can be achieved by pairing resistive magnets with superconducting magnets. The Tallahassee site of the NHMFL is where most of the work on resistive magents is now being done in the United States. The objective of this work is to improve the field strength of the resistive outsert of NHMFL’s 45-T hybrid magnet (see the section “Pulsed Magnets”).

PULSED MAGNETS

Pulsed magnets have long played an important role in high-field science, primarily because they can generate much stronger magnetic fields than DC magnets. However, they do so for only short periods of time (10 to 100 ms), and they tend to have very small bores (10-20 mm). Pulsed magnets are resistive magnets that take advantage of the capacity of materials and power systems to withstand extreme conditions transiently. Figure 3.5 is a plot of the peak magnetic field produced by the pulsed-field magnets now operating around the world versus the duration of the field pulse generated (see Appendix B for more information). By providing access to fields far higher than those accessible using steady-state magnets, pulsed magnets offer opportunities for new discoveries that can take research in new directions; detailed follow-up with steady-state fields is often required, however.

The 60-T long-pulse magnet at NHMFL at Los Alamos National Laboratory (LANL) is a good example of a device of this type (Figure 3.6 shows its (massive) power supply). It produced field pulses having a strength of 60 T for 100 ms in a

Suggested Citation:"3 Technological Challenges and Opportunities for Developing Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
×

FIGURE 3.5 The field strength and pulse duration times of representative nondestructive pulsed magnets from facilities around the world (see Appendix B).

32-mm bore before failing prematurely in 2000. Pulsed magnets are often pre-cooled to cryogenic temperatures using liquid nitrogen and then energized with a pulse of electric current obtained from a large capacitor bank. Pulse length is limited by the heat capacity of the magnet and by the adiabatic temperature rise of its windings.

The generation of high pulsed fields requires advanced materials for conductors and careful attention to stress management. The ideal conductor would combine high mechanical strength with high electrical conductivity and high heat capacity, properties that are unhappily in conflict, so compromises must be made. Whatever the conductor used, magnets are usually built up in nested layers, each designed to withstand its own radial force by hoop tension and each often reinforced structurally by shells, bands, or fibers of nonconducting or low conductivity material. Design parameters for each layer, such as conductor size, material, current density, and reinforcement, are all varied to optimize magnet performance.

Pulsed magnets that operate in the 50- to 60-T range are relatively common, but only a few facilities have, or plan to build, magnets that produce fields that are

Suggested Citation:"3 Technological Challenges and Opportunities for Developing Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
×

much higher, because 60 T is the field strength at which the mechanical strength of the materials used becomes limiting. Nevertheless, pulsed magnets have been built that achieve 70 T, but their pulse lifetimes are extremely short. As this report is being written, two institutions (NHMFL at Los Alamos and the High Field Laboratory at Dresden, Germany) are developing 100-T, multishot pulsed magnets. One approach is to build a magnet consisting of a large outer coil that produces a long pulse with a peak field about 55 T and a small inner coil that generates a much shorter 45-T pulse in synchrony.

Fields substantially in excess of 100 T can be produced by pulsed methods. They are obtained by discharging large amounts of electric current through single-turn coils. The resulting magnetic field pulse ends when the coil vaporizes. Because the plasma so created is driven outwards by magnetic forces, the experimental sample may survive.

FIGURE 3.6 The 1,430-MVA electric generator used by the NHMFL-LANL for pulsing very high magnetic field resistive magnets. Photo by K.N. Roark, Los Alamos National Laboratory.

Suggested Citation:"3 Technological Challenges and Opportunities for Developing Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
×

Not surprisingly, multishot pulsed magnets suffer from cycling-induced mechanical and electrical fatigue. To alleviate these problems, some designs allow for limited plastic deformation of the windings. Electrical insulating materials must withstand cycles of high voltages and mechanical stress. Discontinuities in windings, structures, and insulation systems create mechanical and electrical stress concentrations where failure can occur. They are usually found at transition regions in windings, joints between subcoils, and at coil terminals. Given the magnitude of the energy stored in these magnets when they are fully energized, their end-of-life failure modes are often sudden and highly destructive (see Figure 3.7). Protection of users and surrounding equipment is an important consideration for this class of magnets.

Advantages and Disadvantages

Pulsed magnets are in widespread use today, especially outside the United States. The rapid development of instrumentation in recent years, notably in high-speed electronics and optical detector arrays, has greatly increased the range of work that can be undertaken in pulsed fields. Some topics in semiconductor physics are particularly well suited to such measurements since, due to the low concentrations of free carriers, there are no particularly severe problems associated with sample heating. Additionally, advances in the speed and miniaturization of electronics have increased the range of experiments that can be usefully performed using pulsed-field magnets. However, in general, experiments at the limits of sensitivity and high resolution can only be performed using steady or slowly varying magnetic fields. As developments in technology make measurement instrumentation smaller and faster, though, pulsed fields will continue to offer a valuable alternative. In fact, 16 of the 28 foreign high-field facilities surveyed in Table B.1 in Appendix B provide only pulsed-field magnets for their users, and only 2 of them employ DC magnets exclusively. There are many reasons for this bias.

Advantages

Pulsed-field magnets are cheaper to build and operate than high-field DC-magnets. For fields in the 30-T range, where there is overlap with what can be obtained from resistive DC magnets, pulsed magnets are generally much cheaper to construct and operate largely because they require much less infrastructure (power supplies and cooling facilities).1 For facilities that operate many pulsed

1  

The experience of C.C. Agosta and his students at Clark University provides an example, albeit extreme. They recently built a pulsed magnet that delivers a peak field of 50 T with a rise time of 12 ms. The complete system, including the power supply, cost less than $150,000.

Suggested Citation:"3 Technological Challenges and Opportunities for Developing Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
×

FIGURE 3.7 The fatigue stresses in high-field pulsed magnets often lead to dramatic failures. Images courtesy of National High Magnetic Field Laboratory.

Suggested Citation:"3 Technological Challenges and Opportunities for Developing Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
×

magnets, a further economic advantage arises from the fact that a single power supply can often service several magnets in parallel.

Improvements in instrumentation have significantly increased the effectiveness with which pulsed fields can be used. Because of progress in electronics, instrumentation available commercially today can acquire and store in memory far more data within the span of a single magnetic field pulse than was previously possible. Significant numbers of observations can now be made and recorded within a few milliseconds, and this capability has unquestionably improved the scientific utility of pulsed magnets. In some cases, the dependence of some property of a sample on magnetic field strength, from zero field to the maximum field of a pulse, can be determined today in a single shot. Electrical transport and optical studies are easily done using pulsed-field magnets, and recently an NMR experiment was reported that used a pulsed field of 33 T with a rise time of 10 ms.2

Pulsed-field magnets can generate much higher fields than DC magnets. At the present time, the only magnets available that can deliver fields greater than ~45 T are pulsed magnets, and most of the science done with these devices is done using magnets that deliver relatively long pulses. The MegaGauss Laboratory at the Institute for Solid State Physics at Kashiwa, Japan, is an impressive exception in this regard. It offers users routine access to destructive, short-pulse magnets that deliver field strengths up to 622 T. There is important science to be done at these very high fields. For example, there is reason to believe that superconducting DC magnets incorporating high-temperature superconducting (HTS) wire will deliver fields higher than those built using ordinary, low-temperature superconducting wire. Before any of these HTS materials can be used this way, their critical fields and currents (Hc and Jc) will have to be determined experimentally at all temperatures below their critical temperatures (Tc). Pulsed field magnets will have to be used for much of this research because many of the materials under consideration—for example, (1) oxide HTS materials, such as YBCO, and (2) the more recently discovered MgB2—have critical fields far higher than those generated by any DC magnet. In general, the access to higher fields offered by pulsed magnets can lead to the discovery of new phenomena.

Disadvantages

All the above notwithstanding, pulsed magnets are not the solution to all problems. Pulsed-field magnets cannot be used to measure field-dependent

2  

J. Haase, D. Eckert, H. Siegel, H. Eschrig, K.H. Muller, and F. Steglich, Concepts in Magnetic Resonance Part B, 19B(1), 9-13 (2003).

Suggested Citation:"3 Technological Challenges and Opportunities for Developing Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
×

properties of materials that do not equilibrate in times less than the pulse width. Examples of the kinds of measurements for which pulsed magnets are not optimal are measurements of specific heats and thermal conductivities, although new techniques have recently been developed for use in 60-T long-pulse magnets. Quantum Hall effect measurements have also proved to be very difficult in pulsed fields, and DC magnets are essential for most NMR and MRI applications.

Pulsed-field magnets are awkward to use for measurements that require signal averaging. The repetition rate of pulsed-field magnets is very low, often less 0.1 min–1. Thus, if signal averaging is required for some measurement, data accumulation on pulsed magnets is likely to be very time consuming. For all such measurements, DC magnets will be preferred, provided they can deliver the field strength required.

The Potential for Expanding the Use of Pulsed Magnets

Discussions with NHMFL users, who mainly come from the materials science community, revealed their concern about access to the DC magnets at NHMFL, which reflects both the high user demand and the restricted operating schedule of the laboratory. The way DC magnets are used at NHMFL ought to be analyzed to find out if a significant fraction of the experiments could have been carried out using pulsed magnets. If that fraction is large, an obvious way to alleviate the current bottleneck would be to offer users improved access to appropriately equipped pulsed magnets. A robust pulsed magnet with a bore of 25 mm, a peak field of 45 T (equal to today’s highest DC field), a pulse rise time of 15 ms, and a fall time of 100 ms would allow exploring the highest magnetic fields in ways that could take research in new directions. Additionally, advances in high-speed electronics, instrumentation, and miniaturization also offer the potential to allow greater experimental access to higher fields. Such a pulsed magnet could probably be built for considerably less than $500,000; the alternative—namely, the construction of a new 45-T DC hybrid magnet—would be vastly more expensive. Another approach to improving user access to high fields would be to increase the operating hours for the existing DC magnets.

Outlook for Pulsed Magnets

Despite their limitations, pulsed magnets produce the highest magnetic fields and are an important part of the nation’s high-field portfolio. The scientific motivation for carrying out experiments at the highest available fields is compelling, and the possibility of observing qualitatively new science in this regime is very real. A 100-T, long-pulse magnet would have a good chance of achieving this goal. By

Suggested Citation:"3 Technological Challenges and Opportunities for Developing Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
×

“long pulse” is meant a field pulse that is reasonably stable for at least 100 ms. A highly focused materials development program, coupled with an intensive program of high-field magnet design, analysis, and engineering, will be required to make a substantial leap forward in the magnetic fields produced by pulsed methods. A large investment in the power supply and conditioning equipment will also be needed since power demand increases as the square of the magnetic field. In this connection it should be pointed out that it is no accident that LANL is the home of the nation’s foremost pulsed magnet program. The LANL program (see Figure 3.6) takes advantage of some remarkable energy generation and storage equipment that became available at Los Alamos only because of the cancellation of an unrelated fusion experiment. A decision to expand the capabilities of the Los Alamos power system would have to be based on a careful evaluation of existing opportunities as well as consideration of greenfield ventures.

SUPERCONDUCTING MAGNETS

The first superconducting magnets were built about 40 years ago, and great progress has been made since then. The many important successes achieved in the past 2 years make one optimistic about the prospects for constructing superconducting magnets in the next decade that are much more powerful than any in existence today.

High on the list of relevant successes is the host of advances in high-field applications of Nb3Sn. For example, the model central solenoid coil for the International Thermonuclear Experimental Reactor (ITER) fusion experiment, which is by far the largest fast-ramping superconducting magnet ever made, has achieved 13 T with an 8-s ramp-up to full current. It has a 46,000-A, force flow, fully force-supported conductor made of 720 superconducting Nb3Sn and 320 copper strands. When the international decision is made to proceed with ITER construction, the annual production of Nb3Sn wire will have to increase severalfold, a development that will probably reduce the cost of this critical material for all users. In addition, with the support of the particle accelerator community, Nb3Sn conductors were developed recently that operate at a current density of 3 × 105 A/cm2 at 12 T, which is more than double what was possible 5 years earlier. A dipole magnet has been constructed from this material at Lawrence Berkeley National Laboratory (LBNL) that operates at 16 T, a field scarcely thought possible a few years ago. The Large Hadron Collider (LHC), which is scheduled to come online in 2007, uses Nb-Ti magnets that operate at about 9 T in superfluid helium. The possibility that the center of mass collision energy of the LHC might be doubled by replacing those magnets with Nb3Sn magnets of the LBNL type has stimulated a new European program in Nb3Sn conductor technology. Finally, the latest generation of high-

Suggested Citation:"3 Technological Challenges and Opportunities for Developing Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
×

field NMR magnets operate in a persistent mode at 21.4 T (900 MHz) at about 1.8 K. Better Nb3Sn conductors that incorporate insights gained from the high-energy physics (HEP) and fusion programs should make it possible to build 1-GHz NMR systems.

An advance of a different sort was made recently by an Oxford Instruments/NHMFL team, which built a 25-T magnet containing both Nb3Sn, and Bi-2212 coils. Bi-2212 is a high-temperature superconductor, and 25 T is a new record for a superconducting magnet. The design used should make it possible to build magnets that operate at 30 T at least. The HTS technology used in this magnet is being fostered by the Department of Energy’s commitment in 2003-2004 to a major expansion of its industrial superconducting partnership initiative. This initiative should lead to the construction of three underground power cables based on Bi-2223 that have power-handling capabilities exceeding 100 MVA with voltages up to 138 kV. At the same time, a 100-MVA superconducting generator that uses Bi-2223 is being built by GE. Long-term hopes for the widespread application of superconductors in utility applications depend on development of a new generation of low-cost HTS superconductors using novel designs that layer YBa2Cu3O7-x thinly on a metal alloy substrate, yielding dramatic improvements in critical current density. The first industrial production of lengths of this material using scalable, continuous processes took place in 2003.

Finally we note the demonstration in 2003 of critical field behavior in a new superconductor, MgB2, that has upper critical fields exceeding those of Nb3Sn at all fields and temperatures. MgB2 suffers neither from the grain boundary, weak-link problems of most HTS materials, which dictate that they be made in strongly textured, often tapelike geometries, nor from the large flux creep that makes most HTS materials unsuitable for operation in the persistent mode, which applications like solution NMR require. Many groups have already prepared prototype wires of this substance. MgB2 offers some exciting near-term opportunities.

Superconductors Used for Magnet Construction

Several thousand materials are known to be superconducting under appropriate conditions, but virtually all superconducting magnets have been made from just three of them: (1) the body-centered, cubic solid solution alloy Nb-Ti (47 wt% Ti), with a Tc of 9 K, (2) the cubic A15-structure, intermetallic compound Nb3Sn, with a Tc of 18 K, and (3) the orthorhombic trilayer cuprate (Bi,Pb)2Sr2Ca2Cu3O10-x (Bi-2223), with a Tc of 110 K. Occasional use is made of Nb3Al (Tc ≈ 17 K; see subsection “Materials Occasionally Used” for more discussion) and the bilayer cuprate version of Bi-2223, Bi2Sr2CaCu2O8-x (Bi-2212) (Tc ≈ 90 K), and both are available commercially on special order. Significant effort has been mounted

Suggested Citation:"3 Technological Challenges and Opportunities for Developing Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
×

worldwide since the mid-1990s to make conductors of the less anisotropic cuprate YBa2Cu3O7-δ (YBCO), with a Tc of 92 K, in the form of a quasi single-crystal, multilayer thin film known as a coated conductor. The hexagonal AlB2-structure compound MgB2, with a Tc of 39 K, discovered to be superconducting only in early 2001, is now attracting serious attention as a competitor for both the lower-Tc Nb-based conductors and the higher-Tc Bi and Y cuprate conductors.

As will become clear below, the performance of electrical conductors made from superconducting materials is highly sensitive to fabrication details. The performance of kilometer lengths of conductor may fall far below expectations based on measurements made on laboratory-size samples unless scrupulous attention is paid to every detail of the manufacturing process. One of the reasons for optimism about the future of superconducting magnet technology is that at this point, relatively little is known about how practical conductors made of high-temperature superconductors or MgB2 can be fully optimized, as is discussed below. Substantial improvements in performance are likely to occur as the community works its way along the learning curve.

Economic Considerations

From the point of view of magnet construction, the critical temperature of a superconducting material is less important than the feasibility of fabricating multikilometer lengths of conductor, at reasonable cost, in a form suitable for magnet construction and with critical current densities (Jc) of more than 105 A/cm2 over a wide range of fields and temperatures. The cost-performance metric is dollars per kiloampere-meter of conductor. The Jc for all such substances is strongly field- and temperature-dependent because its magnitude is determined by the volumetric pinning interaction force Fp (Jc × B, where B is the magnetic field), which results from the interaction of quantized superconducting-state flux vortices and the nanostructure of a superconductor. In principle, a homogeneous superconductor with uniform microstructure does not support long-range, bulk super-currents, but if it is treated to give it defects where superconductivity is locally depressed or destroyed at scales on the order of the superconducting coherence length ξ, Jc can reach 106 to 107 A/cm2. Because ξ lies within the range 0.5 to 5 nm for all useful conductors, even atomic-scale defects can produce the pinning interactions required. Superconducting conductors useful for magnet construction are thus fabricated with very high defect densities (>1016 cm−2) and defect dimensions of ξ or less, which makes practical superconductors genuine nanostructured materials.

Suggested Citation:"3 Technological Challenges and Opportunities for Developing Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
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Additional Conductor Requirements: Hirr

In addition to needing high Jc and reasonable costs of fabrication, the superconductors used in high-field magnets must have a high irreversibility field, Hirr, since Jc becomes zero at Hirr. The definition of Hirr is fundamentally different for anisotropic high-temperature superconductors and isotropic, low-Tc superconductors (LTS). The classical LTS materials, Nb-Ti and Nb3Sn, are both cubic and isotropic, and Hirr is defined by the phase transition between the superconducting and the normal states of the material, which occurs at the upper critical field, Hc2. Smearing of Hc2 due to the inhomogeneities that must be introduced to give the materials a high Jc makes Hirr ~0.9 Hc2. The large structural and electronic anisotropy of the HTS materials leads to Hc2 anisotropies on the order of 100 for Bi-2212 and Bi-2223, Hc2 and Jc being much smaller for fields perpendicular to the CuO2 planes than for fields parallel to them. Large anisotropies are a direct consequence of the electronic structure of the cuprates, whose parent state is insulating. Superconductivity is strongest within the CuO2 planes and weakest in the charge reservoir layers that dope holes into the CuO2 layers and make them metallic. The charge reservoir layer in the Bi compounds is a double Bi-O layer, which is itself only poorly conducting. These cuprates can therefore be regarded as multilayers that are composed of dirty, normal metal layers (the Bi-O and adjacent Sr-O layers) alternating with superconducting block layers of CuO2 and their separating Ca-O layer, which together form a repeating superconducting/normal/superconducting stack. In the lower Hc2 orientation of fields, which is perpendicular to the planes, vortices within the superconducting state are greatly weakened whenever they pass through Bi-O layers. This weakening, coupled to strong thermal activation at higher temperatures, results in the destruction of bulk currents at fields of Hirr much lower than Hc2.

The reason there is so much interest in YBa2Cu3O7-δ today is that its charge reservoir layer is metallic, making the anisotropy of its Hc2 much smaller than for any other HTS cuprate, which in turn strongly raises Hirr. The highest Tc cuprate is HgBa2Ca2Cu3O10-x, with a Tc of 132 K, but it is even more anisotropic than the Bi compounds just mentioned and for this and other reasons is not being seriously pursued for conductor applications. The accessible superconducting state space of finite Jc for actual or potential conductor materials is indicated in Figure 3.8. Interestingly, MgB2 appears to exhibit BCS-related superconductivity rather than HTS-related behavior despite its relatively high Tc, just below 40 K. Even though MgB2 is variably anisotropic, depending on its alloying state, its Hirr is about 0.9 Hc2.

A useful superconducting conductor must also be electromagnetically stable. Because superconductors with high Jc can shield strong fields, they are thermodynamically unstable when carrying bulk transport currents and thus capable of

Suggested Citation:"3 Technological Challenges and Opportunities for Developing Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
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FIGURE 3.8 The field dependence of critical current densities for candidate conductor materials suitable for high-field magnet construction. The data shown were taken using conductors fabricated from the materials indicated. These data are obtained from the University of Wisconsin’s Applied Superconductivity Center; the center maintains a database of critical current density measurements and provides free online access at http://www.asc.wisc.edu/plot/plot.htm. Figure courtesy of Peter J. Lee, University of Wisconsin at Madison.

Suggested Citation:"3 Technological Challenges and Opportunities for Developing Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
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flux-jumping to the more stable state of lower Jc. To prevent this, they must be subdivided so that the stored energy of the shielded field, which is proportional to Jcd, where d is the transverse dimension of the superconductor, is small enough not to quench the superconducting state. A high-conductivity matrix of Cu in Nb-base superconductors or Ag in Bi-base conductors provides the stability required both during superconducting operation and during the occasional destructions of the superconducting state that occur in a magnet quench. Thus, a useful superconducting conductor is inherently a composite of superconductor and normal metal in which superconducting filaments are embedded in sufficient normal metal to ensure electromagnetically stable operation. Various ways in which this requirement can be met are shown in Figure 3.9.

FIGURE 3.9 Representative multifilamentary conductors made from (a) Nb47wt%Ti, (b) Nb3Sn, (c) Bi-2212, (d) Bi-2223, and (e) MgB2. The matrices for the conductors are high-purity copper for (a), (b), and the outer sheath of (e) and pure silver for (c) and (d). The filaments of MgB2 are surrounded by 316 stainless steel in (e). Conductors were manufactured by Oxford Instruments Superconducting Technology (a) and (c), ShapeMetal Innovation (b), American Superconductor Corporation (d), and Hitachi Cable in collaboration with the National Institute for Materials Science (e).

Suggested Citation:"3 Technological Challenges and Opportunities for Developing Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
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Critical Current Densities

A final important parameter for characterizing the performance of superconducting materials in high-field magnets is the critical current density Jc, the density of current flow that extinguishes superconductivity in a particular sample of a superconductor. Although the critical current density of any sample is limited by the intrinsic properties of the superconducting material it contains, it is highly dependent on extrinsic factors, such as the microstructure of the sample being considered and the way the superconducting material it contains was processed. In the limit of zero applied field and zero temperature, all superconductors useful for magnets have Jc values exceeding 106 A/cm2. For magnets with meter-size bores (e.g., fusion or particle-detector magnets), superconducting wires that have Jc values lying at 104 A/cm2 or more suffice, but for small magnets (e.g., solenoids or beam-steering magnets), where apertures are a few centimeters or so, Jc values of 105 A/cm2 or more are required. The magnitude of Jc as a function of the magnetic field HJc(H)—is equal to the summed interaction of individual vortex pinnings with the defect sites. Broadly speaking, one can distinguish two situations. In one situation, there is full summation of all interactions over all H-T space. This is the case for Nb-Ti, where flux pinning is well understood and the fabrication process is designed to optimize Jc(H). More generally, however, the density of pinning interactions is less than optimum, and the summation of pinning forces is subject to the collective properties of the vortex lattice. A compendium of Jc(H) data, provided in Figure 3.8, shows that field dependence is generally greater in the collective case. High relative values of Jc(H) are more easily designed and obtained with Nb-Ti. The main-ring dipole magnets of the LHC at CERN are good examples of magnets operating in this regime. The recent Nb3Sn magnets used in 900-MHz NMR spectrometers are exceptional in operating up to about 75 percent of Hc2. An advance beyond 1 GHz is unlikely with Nb3Sn.

Existing Conductor Materials

Nb-Ti

Nb-Ti is a strong, tough, ductile, well-understood material with a Tc of 9 K, an Hirr(4.2 K) of about 10.5 T, and an Hc2(4.2 K) of about 12 T.3 It is the workhorse of the superconducting industry, being used for all MRI and accelerator magnets. It is universally used for fields up to about 8 T, at which point Nb3Sn becomes

3  

Throughout this report, the notation Nb-Ti denotes the alloy, which can have a range of elemental combinations, normally niobium alloyed with about 47 wt% titanium.

Suggested Citation:"3 Technological Challenges and Opportunities for Developing Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
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more attractive. If Nb-Ti magnets are cooled to between 1.8 and 2.0 K using superfluid helium, they can operate at fields as high as 12 T. The advantages of Nb-Ti are several. It is well understood how to fabricate wires of Nb-Ti that have optimized nanostructures and high Jc. The alloy is strong and tough, making Ni-Ti conductors robust in fabrication and application. Its cost-performance ratio is low, about $1 per kiloampere-meter (/kA-m) at 5 T and 4.2 K. It can be made in many designs, with filament sizes ranging from about 1 to 100 mm surrounded by matrices of pure Cu or, occasionally, pure Al. If necessary, alloyed Cu can be used to enhance transverse resistivity so that the filaments are less easily coupled by high magnet charge rates or AC use. A typical accelerator conductor containing a few thousand 8-µm diameter Nb47Ti filaments embedded in a Cu matrix is shown in Figure 3.9a. MRI magnet conductors are generally simpler because filament diameters are larger, about 50 µm.

Nb3Sn

Nb3Sn is an intermetallic A15 compound that exists over a range of compositions: Nb combined with anywhere from 18 to 25 percent by (atomic) weight Sn. In 1961 its use for high-field superconducting magnets was discovered, and it remains one of the two most important materials for such magnets. A great deal of effort was devoted to its development as a conductor for magnets in the 1960s and 1970s and, again, in the last 3 or 4 years, when great strides were made, as discussed earlier. This developmental work, which has been driven by the needs of three communities—NMR, fusion, and, more recently, high-energy physics—has greatly improved our understanding of Nb3Sn. Because Nb3Sn is now very well understood, it is easier than ever to make from it conductors with enhanced Jc and Hc2, even though its Hc2 at liquid-helium temperatures is about 28 T (see Figure 3.10). It is the superconductor of choice for magnets producing fields from 10 T to more than 20 T. It is conceivable that it will be used to build NMR magnets that operate at 23.4 T (1 GHz) but unlikely that it will go much beyond that because of the intrinsic Hc2 limitation of the material itself.

Nb3Sn wire can be manufactured in many different forms to suit design requirements, one of which is shown in Figure 3.9b. However, once formed, Nb3Sn is notoriously brittle. More than that, the critical current densities of Nb3Sn conductors are a strong function of applied longitudinal strain, an effect that increases with the magnetic field strength. To control these problems, Nb3Sn coils are usually manufactured by a process called insulate-wind-react. The conductor is produced in a form in which its Nb and Sn components are adjacent but separate. Electrical insulation is wrapped around the conductor while it is in this ductile state and the conductor is wound into the coil form required. The completed coil is then treated

Suggested Citation:"3 Technological Challenges and Opportunities for Developing Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
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FIGURE 3.10 Upper critical fields (Hc2) for Nb47wt%Ti, Nb3Sn, and MgB2 and irreversibility fields (Hirr) for Bi-2223 and YBCO. Note that the two fields are very close for Nb47wt%Ti, Nb3Sn, and MgB2 (Hirr is 85-90 percent of Hc2) but far apart for all cuprate superconductors. For MgB2, Bi-2223, and YBCO, the values plotted are the lower values appropriate for fields perpendicular to the strongest superconducting planes, the B planes for MgB2 and the CuO2 planes for the cuprates. Values plotted are the highest found for each compound.

at a high temperature, which causes its Nb and Sn components to mix, forming Nb3Sn. Extreme care must be taken with the coil once it has reached this stage because the wire in it is no longer ductile. Large thermal stresses may develop when such a magnet is cooled to cryogenic temperatures, and when the coil is energized, Lorentz forces create additional stresses. All of these effects must be appropriately anticipated if a useful magnet is to result.

Suggested Citation:"3 Technological Challenges and Opportunities for Developing Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
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Considerable effort has been expended to find optimum conditions for the heat treatment of Nb3Sn conductors. Treatment at 650-700°C is compatible with the limitations imposed by using glass-fiber insulation, so Nb3Sn treated like this is suitable for the construction of small-bore magnets for NMR and laboratory use. This rather low temperature has the additional advantage of producing a fine-grained (~100-nm) Nb3Sn phase that provides grain boundary vortex pinning, which maintains a higher Jc. However, at temperatures this low, the Sn content of the A15 phase does not equilibrate, and gradients of Sn content occur across filament layers, which are a few microns thick; these differences lead to gradients in the superconducting properties, complicating the optimization of wire properties.

Bi-2223

Bi-2223 conductors of the sort shown in Figure 3.9d are the first and so far the only fully commercial HTS conductor material. The highly aspected shape (~20:1) of the conductor follows that of the filaments of which it is composed and of the crystal structure of (Bi,Pb)2Sr2Ca2Cu3O10-x itself. One consequence of the parent insulating state of HTS materials is that grain boundaries tend to be weak-linked unless there is a very low level of misorientation. Making the conductor as a highly aspected tape favors a high degree of texture. However, many obstacles to current flow still remain in such conductors, even when they are textured. Supercurrent must thus percolate through a network of such obstacles, reducing the expected current density values by a factor of 5 or so. As Figure 3.8 shows, Jc values for Bi-2223 do not yet compete with those of Nb-based conductors, even at 4.2 K, owing entirely to the reduction of useful cross section produced by barriers to current flow, the most important of which are more angular grain boundaries.

HTS conductor development has largely been motivated by the desire to replace Cu and Fe in utility applications. Since such devices work either in self-fields of about 0.1 T for power cables or 1 to 2 T for transformers, motors, and generators, Bi-2223 can be used at quite high temperatures, though as Figure 3.10 shows, Hirr falls rapidly with field, precluding multitesla applications above 30 K or so. Even though the Tc is 110 K, it can only be used in weak fields at 77 K (boiling liquid nitrogen).

Bi-2223 is the workhorse of virtually all HTS utility applications (there are a few uses for Bi-2212), but it suffers from having to compete in cost with Cu and Fe. It is estimated that Bi-2223 conductors would have to cost ~$10/kA-m at 77 K in order to compete with Cu and Fe, but because of its still relatively low Jc and the high (about 60 percent) Ag content of wires made of it, it costs 10 times as much.

While Bi-2223 is the best HTS conductor available today, it is unclear whether it will remain so. Bi-2212 has better connectivity than Bi-2223 for reasons that are

Suggested Citation:"3 Technological Challenges and Opportunities for Developing Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
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not well understood and thus may emerge as a better low-temperature, high-field conductor than Bi-2223. In the long term, both substances may be replaced by biaxially textured YBCO, which has a much more favorable Hirr(T) curve (see Figure 3.10), lower anisotropy, and higher Jc (see Figure 3.8) and does not require Ag coating to be an effective conductor.

Materials Occasionally Used

Many other materials have been found that will superconduct under certain conditions and that might be used for magnet construction—for example, ternary alloys such as Nb-Ti-Ta, which is being studied as a possibility for accelerator magnets. Rather than discussing them all here, we will mention only one, the binary compound Nb3Al.

Nb3Al has a number of potential advantages over Nb3Sn, not least that the stoichiometric compound has a slightly higher Tc and an Hc2 that can reach 40 T. Its greatest shortcoming is that it does not easily form when it contains 25 wt% Al, such that when quenched from the high temperatures at which the stoichiometric composition is stable, it forms a disordered structure in which both Tc and Hc2 are lower. An important advantage of Nb3Al is that the strain sensitivity of its Jc, Tc, and Hc2 parameters is much less than that of Nb3Sn.

There are two principal approaches to making Nb3Al conductors. One uses jelly rolls of Nb and Al that are extruded and drawn into wire before being reacted at 700 to 900°C to form the A15 phase. The conductor that results has a Tc of about 16 K and an Hc2 of 20 to 24 T, which are both significantly lower than for competing Nb3Sn conductors. Because the properties of Nb3Al are little affected by strain, conductors made of it have been used in prototype coils for large fusion magnets. The second approach, which was pioneered in Japan, is to rapidly heat Nb-Al precursors to about 2000°C and then quench them in a liquid Ga bath. This maintains a ductile Nb-Al solid solution phase that can be then selectively crystallized into an off-stoichiometric but still high Jc and high Hc2 phase at lower temperatures. Some examples of the high Jc values that can be obtained by such a process are shown in Figure 3.8. It is, in fact, possible to find a range of magnetic field above ~20 T where Nb3Al is superior to Nb3Sn, and Japanese researchers are pursuing the use of such conductors in high-field NMR magnets. A variety of problems remain to be resolved. Among them are the viability and costs of such a complex process and finding a means of applying sufficient stabilizing normal metal—e.g., Cu or Au—to the conductor.

Suggested Citation:"3 Technological Challenges and Opportunities for Developing Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
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Emerging Superconducting Materials

While the normal and superconducting states of HTS materials remain incompletely understood, work on their practical application continues to advance. In fact, in many applications, HTS materials exhibit surprisingly conventional behavior. The property of HTS materials that has presented the greatest barrier to their practical use is the intrinsic weakness of the links at grain boundaries. This weakness makes the supercurrent density at grain boundaries less than within grains, reducing the overall supercurrent in conductors made of polycrystals. This problem can be alleviated by using epitaxial films in which grains are aligned in the plane of the substrate. Indeed, HTS epitaxial films are used in the wireless communications filter systems deployed today, with a few thousand such systems having been sold. For applications requiring a conductor (e.g., magnets, power transmission, energy storage, motors, and generators), tapes and wires of Bi-based HTS conductors are used, and the performance of these conductors has steadily improved. More recently, attention has focused on the development of tapes that consist of a thick film of YBCO deposited on ribbon substrates in which the grains have been aligned by various means so that grain boundary misorientation angles are small. Finally, discoveries about the superconducting properties of MgB2 have catapulted it to the fore. These useful developments notwithstanding, the industrial application of HTS materials is hampered by other physical issues, one being the need for expensive cooling systems to maintain them in the superconducting state and another being their brittleness.

Bi-2212

Bi2Sr2CaCu2O8-x is a two-CuO2-layer version of Bi-2223. It shares many of the features of the latter compound but has enough unique advantages so that it is still produced commercially in small quantities. One of its advantages is that it can be made as a round wire with high Jc. It appears that considerable self-alignment of Bi-2212 grains occurs during the partial Bi-powder precursor melt step in the manufacturing sequence, which is required to obtain a high Jc. Indeed, the Jc values of Bi-2212 round wires considerably exceed those of Bi-2223 wires. As Figure 3.8 shows, the field dependence of Jc for this material is very weak up to 30 T at 4.2 K. Indeed the very high field limits of this material, which are only poorly understood, are probably well above 50 T at 4 K. This is a direct consequence of the excellent connectivity of Bi-2212 conductors. A typical round wire conductor is shown in Figure 3.9c.

Some manufacturing problems remain to be worked out. For example, the temperature window for optimum processing is very tight, perhaps as little as 5°C

Suggested Citation:"3 Technological Challenges and Opportunities for Developing Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
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at 890°C. This makes the wind and react process that has proven so successful for Nb3Sn magnets more difficult to execute for Bi-2212. Also, like all conductor materials except Nb-Ti, Bi-2212 is brittle and, even when sheathed in Ag or Ag alloy, not very strong.

The 25-T superconducting magnet made by an Oxford/NHMFL team in 2003, which was mentioned earlier, demonstrates the application of Bi-2212 to small-bore solenoids. This technology is likely to make it possible to construct superconducting magnets that operate at 30 T. However, the large electronic anisotropy and strong flux creep of Bi-2212 make it unlikely to be a useful, persistent-mode material and may limit its use to driven magnets or to solid-state NMR. Builders of HEP magnets are also interested in Bi-2212 because the round shape of the wire makes it possible to cable 20-30 such wires into multikiloampere conductors suitable for accelerator dipole or quadrupole magnets.

YBCO-Coated Conductors

One key incentive for the study of YBCO-coated conductors is their exceptionally high Jc value, as shown in Figure 3.8. These values reflect the fact that biaxially textured conductors can be made of YBCO, in which there is almost no grain-boundary obstruction of current. The inherently low electronic anisotropy of YBCO (from 5 to 7) produces excellent flux pinning, giving it strong advantages in superconductor current density over any other HTS compound, including the Bi-based conductors. Prototypes of tape conductors made of this material are now in advanced development by several companies worldwide, suggesting that effective commercialization is imminent (see Figure 3.11).

Complicating any evaluation of the prospects for YBCO-coated conductors is the fact that coated conductors are fundamentally different from any other conductors used for magnet construction (compare Figures 3.8 and 3.10). In contrast to all earlier conductors, which are made by conventional metalworking processes such as extrusion and wire drawing, a coated conductor is made by sequential deposition of a multilayer oxide buffer, which is interposed between the Ni-alloy substrate and the YBCO. Purpose-built production lines of a new kind will therefore be needed. An additional issue with coated conductors is that only about 1 percent of the cross section of such wires is superconducting. Thus, overall conductor current densities are heavily diluted compared with competing conductors, where the superconductor fraction is typically 25 to 50 percent. However, the development of coated conductors is proceeding rapidly, such that prototype lengths have overall current densities that are competitive with the Bi-2223 conductors that they are designed to replace, even with such small fill factors. It is clearly encouraging that a very high current density, very high Hc2 conductor is possible

Suggested Citation:"3 Technological Challenges and Opportunities for Developing Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
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FIGURE 3.11 Illustrative longitudinal cross-sectional architecture of a 165-µm thick, neutral-axis, YBCO-coated conductor showing the copper stabilizer, the solder joining the Cu to the YBCO, and the Ni5W-textured template (a nickel-tungsten alloy with tungsten at 0.5 percent). The actual YBCO superconductor layer is about 1 µm thick and thus represents less than 1 percent of the cross section, in comparison with competing Nb-base, Bi-base, and MgB2 conductors, in which 25 to 50 percent of the cross section is superconductor. Image courtesy of American Superconductor Corporation.

with YBCO. As lengths longer than the present 10- to 100-m prototype lengths appear, it will become possible to make test coils that can be used to assess the true promise of YBCO-coated conductors for high-field magnets.

Magnesium Diboride

The most recent entry into this field is MgB2, an apparently simple binary compound that was discovered to be superconducting only in 2001. Its potential application space is indicated in Figure 3.10, which shows the transition temperature of MgB2 to be about twice that of Nb3Sn (35-39 K vs. 16-18 K). It can be alloyed so as to have a critical field superior to that of Nb3Sn at all temperatures. The superconducting characteristics of MgB2 are interesting not only because of its relatively high transition temperature but also because it has more than one superconducting energy gap, a state of affairs anticipated in BCS theory but never before

Suggested Citation:"3 Technological Challenges and Opportunities for Developing Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
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seen experimentally.4 MgB2 is a metal with a layered structure in which the boron atoms form hexagonal layers and the magnesium atoms are located between the boron layers, above and below the centers of the hexagons. The unusual properties of the material are thought to result from strong interactions between phonons and electron orbitals in the boron layer. But unlike previously known good superconductors, MgB2 does not have a high charge-carrier density, only simple s- and p-shell electrons are involved in its superconductivity, and it has a noncubic crystal structure. Other features make MgB2 even more remarkable: Only a few of the atomic-vibration modes are involved in its ability to superconduct, and only about half of its charge carriers are strongly affected during superconductivity.

The remarkable properties of MgB2 open a new window in superconductivity for fundamental as well as applied research. Prospects for the applications of MgB2 are enhanced by the inherently low cost of Mg and B. Conductors can be made by placing either B + Mg or MgB2 powders into metal tubes that are then drawn out into wires such as that shown in Figure 3.9e using conventional metal fabrication technology. Wires up to 1 km long have already been made by two or three companies in Japan, and this success has stimulated the development of lower-field, higher-temperature magnets for MRI. Furthermore, the density of MgB2 is comparable to that of aluminum, which may lead to new applications where weight is an important consideration. Present conductors do not yet have the capability indicated by the thin films whose Hc2 data are shown in Figure 3.10, and the current densities achieved are not yet fully competitive with those of Nb-based conductors. Nevertheless it is clear that with proper alloying and development, MgB2 could outperform any Nb-based conductor. For these reasons, the HEP magnet community has started to explore options for using MgB2 in the next generation of accelerator magnets. Equally exciting, the critical temperature of MgB2, 39 K, would allow electronic circuits based on this material to operate at 20-25 K, achievable by a compact cryocooler, which gives this material a significant advantage over low-temperature superconductors. Compared with the high-temperature superconductors, MgB2 is simpler, cheaper, and more stable over time.

Superconducting Magnet Design

Superconducting magnet technology has advanced to the point where 20-T laboratory magnets are not uncommon, and 900-MHz NMR magnets are available commercially. As noted earlier, a 920-MHz NMR magnet (21.6 T) having a 54-mm bore has been put into operation at the National Institute for Materials

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H.J. Choi, D. Roundy, H. Sun, M. Cohen, and S.G. Louie, The origin of the anomalous superconducting properties of MgB2, Nature 418, 758-760 (2002).

Suggested Citation:"3 Technological Challenges and Opportunities for Developing Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
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Science (NIMS), Tsukuba (Japan), and a 900-MHz, 63-mm bore magnet manufactured by Oxford Instruments is operating at Pacific Northwest National Laboratories (PNNL). The NHMFL in Tallahassee recently brought to full field its 104-mm bore, 21.15-T magnet, which is the largest bore 21.15-T magnet with persistent field in the world. The next big step is clearly going to be the development of 1-GHz (23.5 T) NMR magnets, but their size, complexity, and cost will be daunting.

High-field superconducting magnets present many of the same engineering problems as resistive and pulsed magnets—namely, very high stored energy and stresses—but they also have problems all their own. The superconductivity of superconducting materials is limited by the strength of the magnetic field they experience in a manner that depends on their operating temperatures, and all the superconducting materials now in use, except Nb-Ti, are mechanically weak, or intolerant of strain. In addition, today’s superconducting magnets require liquid-helium cooling systems that must be integrated into the coil without compromising its electrical or structural integrity. Even more challenging is the requirement that the energy in the field of an energized superconducting magnet be safely and passively dissipated in the event the coil quenches (i.e., ceases to be superconducting). The energy, which can amount to many tens of megajoules, must be dissipated within the magnet or to external energy-dump circuits. Failure during quench can occur due to overvoltage because of a too-rapid collapse of the magnetic field or, at the opposite extreme, because the magnetic energy dissipates in the generally small normal-state regions of the coil. To design superconducting magnets to withstand these perils is a complex but essential undertaking.

Some of the same engineering solutions to the structural problems of resistive magnets have been adopted for superconducting magnets. For example, magnets are often divided into radially independent subcoils to reduce the transmission of radial forces and to limit hoop stress. This strategy can also result in a more efficient use of materials because the coils can be “graded.” More expensive superconducting materials such as Nb3Sn can be used in the inner coils, while less expensive, more ductile conductors like Nb-Ti can be used in the outer coils. In addition, conductor size and number of turns can be varied in each subcoil to optimize the winding current density, and mechanical reinforcing structures can be incorporated within each subcoil as needed. These techniques are generally used for high-field NMR magnets because of their persistent-mode operation. Magnets intended to produce more transient fields often use internal heaters or external shunt resistors.

Mechanical strain is an extremely important issue for superconductors, as already noted, and electrothermal stability is another critical factor in the design and operation of superconducting magnets. All the solid components of a superconducting coil have extremely small heat capacities at 4 K and below, which

Suggested Citation:"3 Technological Challenges and Opportunities for Developing Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
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means that even a slight disturbance, such as the microheating generated by a local stick-slip friction event, can cause a small portion of the conductor to go normal, which generates resistive heating and may cause the entire coil to quench. This behavior requires a mechanical design different from that for a pulsed magnet, where some plastic deformation of the conductor and coil can be allowed.

Outlook for Superconducting Magnets

Experience shows that Nb3Sn reaches its practical limit of usable current density in the 21- to 23-T range, a limit that many current superconducting magnets have now reached. Nb3Sn technology might be further developed to the point that 1-GHz NMR magnets can be made using it, but it is unlikely that it will go much higher than that because of the intrinsic limitation on Hc2. A new superconducting material will probably be needed. An A15 compound such as Nb3Al might be suitable; this material is being developed at a very modest pace and has yet to be produced in large quantities. However, it is widely believed that some form of one of the newer HTS superconductors or MgB2 will emerge as the conductor of choice. Although these materials are widely recognized for their high critical temperatures, the more relevant measure of their usefulness is their very high upper critical fields. Like Nb3Sn, however, these materials are all weak structurally. Stress limitation will therefore be an engineering issue, although the strain tolerance of these materials is high enough to permit winding at reasonable bending diameters.

While the availability of new and improved superconducting materials offers hope that superconducting magnets can be built that deliver fields significantly greater than those available today, the challenges still to be met should not be underestimated. The intrinsic coupling of high magnetic field and high stored energy, leading to high stresses, will force magnet sizes to grow unless the yield strength and modulus of elasticity of materials used for their construction can be increased. Increases in magnet size reduce performance because the farther conductors are from the center of a magnet’s bore, the less they contribute to the magnetic field in the bore.

HYBRID MAGNETS

The highest DC magnetic fields available today are produced by hybrid magnets, which consist of an inner, water-cooled, DC resistive magnet surrounded by a superconducting outer magnet (see Figure 3.12). The 45-T (32-mm bore) magnet at the NHMFL, which is the most powerful DC magnet in the world, obtains 31 T from its resistive magnet and 14 T from its superconducting magnet. The outer diameter of the resistive magnet of course determines the bore size of the super-

Suggested Citation:"3 Technological Challenges and Opportunities for Developing Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
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FIGURE 3.12 View of the NHMFL 45-T hybrid magnet. Images courtesy of National High Magnetic Field Laboratory.

Suggested Citation:"3 Technological Challenges and Opportunities for Developing Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
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conducting magnet. Although the 14-T field produced by the superconducting coil of this magnet is significantly less than the peak field of the NIMS 920-MHz magnet mentioned earlier, the energy stored in its field is three times greater (115 MJ) because its bore is so much larger. The apparatus to support and sustain a hybrid magnet is also much larger; the Florida 45-T hybrid is more than 10 m tall.

Design Challenges

Unusual engineering is needed to solve the stress and cooling problems of a hybrid magnet because there is so much energy stored in its field. For these magnets, field homogeneity requirements are usually relatively modest. The superconductor in the 45-T hybrid is a cable-in-conduit conductor that consists of a multistrand, multistage superconducting cable encapsulated in a structural steel conduit (see Figure 3.13). The conductor is insulated and then wound into the coil form required. If Nb3Sn is the superconductor used, as is the case for two of the three coils in the superconducting portion of the 45-T magnet at Tallahasee, the reaction heat treatment must be performed after the windings are formed. Cooling is accomplished by forcing supercritical helium (or stagnant superfluid helium-2) through the interstices between cables in the conduit, which account for approximately 35 percent of its total volume. The advantages of this design include (1) very high electrothermal stability because of the high heat capacity of liquid helium and large heat transfer surface area to helium in the conduit, (2) integration of structural steel throughout the winding pack, and (3) high voltage integrity of the coil because of the complete coverage of conductor with insulation, which is achieved without compromising the cooling of the magnet.

The use of a multilevel cable allows for conductor currents of up to several kiloamperes and results in a lower terminal voltage when energy is dumped to an external room-temperature resistor if the magnet quenches or is deenergized for other reasons. However, high conductor current also requires the use of vapor-cooled current leads, which increases either helium consumption and/or refrigeration requirements. The NHMFL magnet also takes advantage of conductor grading to increase efficiency by using conductors of three different sizes for the three subcoils.

One of the main problems of hybrid magnets is protecting their superconducting components from the consequences of sudden resistive magnet shutdowns caused by failure of magnet integrity or of the power supply. The resulting rapid decrease in the resistive magnet field can produce AC losses in the superconducting windings large enough to make them quench. In addition, the large mutual coupling between the magnetic fields of the two coils generates a large Lorentz force, the sudden removal of which may cause mechanical displacement of the superconducting magnet, which could have dangerous mechanical or electrical consequences. Such

Suggested Citation:"3 Technological Challenges and Opportunities for Developing Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
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FIGURE 3.13 Cross section of one of the cable-in-conduit conductors used in the 45-T hybrid magnet at NHMFL. The stainless steel conduit provides structural support and the 35 percent void around the cable provides a helium cooling area. The overall winding pack current density is reduced by these elements because they cannot carry current to contribute to the magnetic field generated. Image courtesy of National High Magnetic Field Laboratory.

unplanned and disastrous quenches have occurred in the past, and newer designs take this possibility explicitly into account.

Outlook for Hybrid Magnets

NHMFL-Tallahassee is proposing the construction of a series-connected hybrid magnet, the resistive and superconducting components of which will be operated electrically in series from the same power supply. The high inductance of the superconducting magnet should reduce the current ripple in the resistive magnet, thereby improving the temporal stability of the magnetic field. A second important advantage of this design is that no overcurrent or unbalanced forces

Suggested Citation:"3 Technological Challenges and Opportunities for Developing Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
×

occur during a quench. In addition, the resistive coil will be designed so that it can be shimmed to higher field homogeneity than normal for a resistive magnet. The series-connected hybrid magnet is designed to produce 35 T (40-mm bore) with an inhomogeneity of less than 1 ppm in a 10-mm-diameter spherical volume. Hybrid magnets such as this one are intended to reduce the power consumed to produce the fields they generate rather than maximizing field strength.

COORDINATION OF MAGNET DEVELOPMENT

One striking characteristic of all of the sciences that use high magnetic fields is how constrained they all are by a single body of technological information—namely, magnet technology. This shared interest notwithstanding, each constituency has historically tended to develop the magnets it needs without much reference to the others. The reasons are several and obvious. The different communities have different missions and need magnets that differ correspondingly. In addition, they are supported by different funding agencies, each with its own perspective.

Recently the HEP and fusion communities have begun to more closely coordinate their development of Nb3Sn wire, an important first step in a healthy direction. Given the intense interest of all magnet user communities in magnet technology and engineering, it would seem to make sense for them to mount a coordinated effort to advance magnet technology. One component of such a coordinated effort might be the construction of high-field instrumentation at NHMFL specifically for the engineering research necessary for the production of high-performance magnets. A second component would certainly be the development of new materials for magnet construction, and a third would be an exploration of other constraints on magnet design and performance. The final ingredient would be a framework to coordinate and integrate communication among the different communities. This framework might start with something as simple as topical conferences and could extend to a management structure for operating a joint program.

It is clear from recent experience that magnets producing fields significantly higher than those available today are going to be either impossible or prohibitively expensive if current technology is used. Rather than supporting an all-out, brute-force effort to prevail using barely adequate technologies, it makes sense to find new approaches that will make it easier (and cheaper) to build the magnets needed. Essential to this enterprise will be the development of both resistive and superconducting materials with improved electrical, magnetic, and mechanical properties. This initiative could be a collaborative enterprise involving both publicly supported researchers and commercial enterprises, but however it is structured, it will certainly require substantial public support.

Suggested Citation:"3 Technological Challenges and Opportunities for Developing Higher Fields." National Research Council. 2005. Opportunities in High Magnetic Field Science. Washington, DC: The National Academies Press. doi: 10.17226/11211.
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High-field magnets—those that operate at the limits of the mechanical and/or electromagnetic properties of their structural materials—are used as research tools in a variety of scientific disciplines. The study of high magnetic fields themselves is also important in many areas such as astrophysics. Because of their importance in scientific research and the possibility of new breakthroughs, the National Science Foundation asked the National Research Council to assess the current state of and future prospects for high-field science and technology in the United States. This report presents the results of that assessment. It focuses on scientific and technological challenges and opportunities, and not on specific program activities. The report provides findings and recommendations about important research directions, the relative strength of U.S. efforts compared to other countries, and ways in which the program can operate more effectively.

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