Opportunities in High Magnetic Field Science (2005) / Chapter Skim
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3 Technological Challenges and Opportunities for Developing Higher Fields
3 Technological Challenges and Opportunities for Developing Higher Fields
Pages 69-102
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pulsed-field magnets, (3) superconducting DC magnets, and (4)
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Boebinger, National High Magnetic Field Laboratory. stresses it experiences increase as the square of the field strength.
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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.
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Figure courtesy of J.R. Miller, National High Magnetic Field Laboratory.
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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.
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Image courtesy of National High Magnetic Field Laboratory.
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. 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.
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FIGURE 3.5 The field strength and pulse duration times of representative nondestructive pulsed magnets from facilities around the world (see Appendix B)
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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.
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However, in general, experiments at the limits of sen sitivity and high resolution can only be performed using steady or slowly varying magnetic fields. As developments in technology make measurement instrumenta tion smaller and faster, though, pulsed fields will continue to offer a valuable alternative.
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CH A L L E N G E S A N DO P P O R T U N I T I E S F O RD E V E L O P I N G H I G H E R F I E L D S 79 FIGURE 3.7 The fatigue stresses in high-field pulsed magnets often lead to dramatic failures. Images courtesy of National High Magnetic Field Laboratory.
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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 experi ment 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.
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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.
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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.
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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)
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The per formance 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, rela tively little is known about how practical conductors made of high-temperature superconductors or MgB2 can be fully optimized, as is discussed below.
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. 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.
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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
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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.
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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)
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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.
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90 O P P O R T U N I T I E S I N H I G H M A G N E T I CF I E L DS C I E N C E MgB2 MgB2 FIGURE 3.10 Upper critical fields (Hc2) for Nb47wt%Ti, Nb3Sn, and MgB2 and irreversibility fields (Hirr)
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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.
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92 O P P O R T U N I T I E S I NH I G HM A G N E T I CF I E L DS C I E N C E 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)
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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.
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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.
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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.
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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 avail able commercially. As noted earlier, a 920-MHz NMR magnet (21.6 T)
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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.
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Increases in magnet size reduce performance because the farther con ductors 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)
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CH A L L E N G E S A N DO P P O R T U N I T I E S F O RD E V E L O P I N G H I G H E RF I E L D S 99 Superconducting Resistive Outsert Magnet Insert Magnet Outsert Magnet Cryostat Helium Return and Bus Duct FIGURE 3.12 View of the NHMFL 45-T hybrid magnet. Images courtesy of National High Magnetic Field Laboratory.
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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.
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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.
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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.
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