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Opportunities in High Magnetic Field Science 1 Introduction THE IMPORTANCE OF MAGNETISM IN THE MODERN WORLD Because humans do not sense magnetic fields, it took a long time for the importance of magnetism in the natural world to be appreciated. The first magnetic device to come into wide use was the magnetic compass. It is believed to have been invented in China around 200 B.C., but it was not fully understood until the 19th century, when systematic investigation of magnetic phenomena began. In 1819, H.C. Oersted discovered that electric currents engender magnetic fields, and a few years later, M. Faraday discovered how to use magnetism to interconvert mechanical and electrical energy—that is, how to build electric motors and dynamos. These advances and others related to magnetism had a profound impact on society. Magnetic devices are now so deeply integrated into everyday life that the average citizen takes most of them for granted. It is assumed that when a light switch is turned on, a generator at some unknown location will deliver electric power to the appropriate lightbulb. It is assumed that when the ignition switch of an automobile is turned, an electric motor will start the car. It is assumed that when the appropriate sequence of key strokes is made on a computer keyboard, information will be recorded faithfully on a (magnetic) disc. About the only time people are likely to be aware of having encountered magnet technology is when they go to the hospital for a magnetic resonance imaging (MRI) scan. Even so, they are unlikely to realize that the drug used to treat the disease diagnosed by MRI is itself the product of research that relied heavily on magnet technology.
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Opportunities in High Magnetic Field Science Magnetic devices are even more important in the scientific world than they are in everyday life. Instruments that take advantage of magnetic phenomena or that use magnets to produce electromagnetic radiation are used in many fields, the most spectacular being the accelerators employed in high-energy physics to study the structure of subatomic particles. High-field magnets are used to control particle trajectories in accelerators, and over the years, the interest of the high-energy physics community in increasing the energies at which accelerators operate has been a powerful driver of magnet technology. THE SIGNIFICANCE OF HIGH MAGNETIC FIELD RESEARCH Most of the magnetic devices important to the public do not generate high fields, MRI being the exception. Thus it is reasonable to ask why the nation should support high magnetic field science and technology. Before this question can be answered, some background information must be supplied. The committee starts by reminding the reader that there are two kinds of magnets: permanent magnets and electromagnets. Permanent magnets are made of substances like iron (Fe), cobalt (Co), and nickel (Ni), the atoms of which have large magnetic moments. When those substances are in their unmagnetized states, the magnetic moments of their atoms are randomly oriented. Magnetization is achieved by making the magnetic moments of their constituent atoms point in the same direction, which can be done by exposing the substances to an external magnetic field. What distinguishes a magnetizable substance from another substance that contains atoms with magnetic moments is that once the magnetic moments of its atoms have become aligned, they remain that way. Thus, a piece of iron that has been exposed briefly to a magnetic field emerges with a net magnetic moment that persists; it has become a permanent magnet. A compass needle is a permanent magnet, and permanent magnets can produce fields up to about 2 T. Permanent magnets have many practical uses, and the search for magnetizable materials with improved properties is ongoing; its goals include increasing the efficiency of electrical motors and generators. Electromagnets can be made of any material that conducts electricity, regardless of the magnetic properties of its atoms, and they produce magnetic fields via the Oersted effect whenever an electric current flows through them. Electromagnets are commonly made from coils of an electrical conductor. Since the field contributed by each turn in a coil adds to that of its neighbors, and the field per turn increases with electric current, the more turns in the coil and the greater the current put through it, the stronger the magnetic field that results. All high-field magnets—that is, magnets that generate fields substantially greater than 2 T (the limit of permanent magnetization for iron)—are electromagnets.
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Opportunities in High Magnetic Field Science Scientists have been building electromagnets that deliver fields of ever-increasing strength since the 19th century. Two issues have had to be confronted at every step of the way. First, the field of an energized electromagnet exerts forces on its own structure that increase as the square of the field strength and that will destroy it if not contained. Second, if the electrical conductor of which the magnet is made is resistive, as was always the case before 1960 or so, heating may also cause it to fail. Thus, the construction of magnets that operate at high fields is, and has always been, an engineering challenge. The justification for building ever more powerful magnets remains today what it was at the outset—namely, the scientific and, ultimately, the social benefits of research done using more powerful magnets. Over the years, advances in magnet technology have paid huge dividends, not only to those interested in the science made accessible by increases in field strength but also to the much larger community able to work at lower fields. The technological advances that have made it possible to build magnets that push the field strength envelope have often made it easier and cheaper to build magnets that produce less extreme fields. There is every reason to believe that advances in magnet technology will be as rewarding in the future as they were in the past. Research on the magnetic properties of materials, particularly those that are superconducting (e.g., high-Tc and high-field superconductors, advanced sensors), and on magnet design and construction can have substantial economic payoffs. Electric motors and generators will operate with improved efficiency. More efficient ways will be found to transmit electric power over long distances. Better information storage devices will be made. On the scientific side, improvements in magnet technology will lead to better instrumentation for studying the structure and properties of materials of all kinds, at all length scales. The benefits to the public of most of the scientific advances will be less direct but no less real. It is appropriate, therefore, that the nation support high magnetic field research and that the state of high magnetic field science and technology in the United States be assessed periodically.1 THE TASK OF THE COMMITTEE In the summer of 2003, the National Research Council established the Committee on Opportunities in High Magnetic Field Science in response to an informal request from the National Science Foundation (NSF). The committee was charged with (1) assessing the current state and future prospects of high-field magnetic 1 See, for example, National Research Council, High-Magnetic-Field Research and Facilities, Washington, D.C., National Academy Press, 1979; and National Science Foundation, Final Report of NSF Panel on Large Magnetic Fields, Arlington, Va., National Science Foundation, 1988.
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Opportunities in High Magnetic Field Science science and technology in the United States, (2) assessing the current status of U.S. high-field efforts in the international context, (3) identifying particularly promising multidisciplinary areas for research and development, and (4) reviewing major initiatives in the construction of new high-field magnets and setting priorities for the coming decade. In its deliberations, the committee took as its purview both the disciplines relevant to the generation of high magnetic fields and those that would benefit if higher fields could be generated. DEFINITION OF HIGH MAGNETIC FIELD The field a magnet produces is “high” if it tests the limits of the mechanical and/or electromagnetic properties of the materials of which the magnet is made. In many instances, the quantity that determines whether a magnet is high field is the amount of energy stored in its field, which is proportional to the integral of the square of its field strength over the volume affected. Thus, a magnet having a maximum field strength around 8 T and a bore large enough to accommodate a human being is as much a high-field magnet as the much smaller bore magnet in a nuclear magnetic resonance (NMR) spectrometer operating at 20 T. Some magnets operate in a pulsed mode, which alleviates some of the constraints that limit the fields achievable by DC magnets. “High field” in the pulsed mode is a function of pulse duration and might be considered as starting around 60 T. (See Box 1.1 for a list of some other magnetic field strengths.) While this definition of high field will not help the reader decide whether a magnet of one type operating at field x is a higher field magnet than a magnet of anther type operating at field y, it does make clear why it is difficult to increase the maximum field strength delivered by magnets of any given type. The materials in a high-field magnet of any given type are, by definition, close to failure. One of the objectives of this report is to identify those areas of magnet design and technology where future developments are likely to enable raising the field strengths delivered by high-field magnets, rendering today’s high-field magnets “conventional.” These developments should make it cheaper to build magnets like the best we have today, promoting their wider distribution. HIGH-FIELD MAGNETS As already noted, the construction of high-field magnets has always posed engineering challenges. Solenoids generating fields of about 2 T were built in the 19th century using resistive conductors, and even at fields that low, both the mechanical strength of the materials used and heating were issues. In the 1930s, W.F. Giauque and F. Bitter built water-cooled magnets of novel design from
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Opportunities in High Magnetic Field Science BOX 1.1 SOME MAGNETIC FIELD STRENGTHS In outer space the magnetic flux density is between 10−10 T and 10−8 T. Earth’s magnetic field at latitude 50° is 2 × 10−5 T and at the equator (latitude of 0°), 3.1 × 10−5 T. The magnetic field of a horseshoe magnet is ~0.1 T. The magnets in clinical medical MRI spectrometers operate around 4 T; the current highest field is 9.4 T. In a sunspot, the field is ~10 T. Strongest continuous magnetic field yet produced in a laboratory: About 20 T with a single superconducting magnet, 25 T with a superconducting magnet that has a high-temperature superconducting magnet insert (2003), 35 T with a resistive magnet (2002), and 45.2 T with a hybrid magnet (2003). Strongest (pulsed) magnetic field yet obtained nondestructively in the laboratory: 80 T (1997) for ~10 ms. Strongest (pulsed) magnetic field ever achieved (with explosives) in the laboratory (Sarov, Russia), 2,800 T. The field on a neutron star is 106 T to 108 T. Maximum theoretical field strength for a neutron star, and therefore for any known phenomenon, is 1013 T. resistive conductors that produced steady-state fields of about 10 T, a big advance. Bitter magnets are still used today. Modern versions produce fields of 30-35 T. While they are relatively inexpensive to build (tens of thousands of dollars), they are costly to operate because of the power they consume and the cooling they require. At fields in this range the energies stored in a magnet of useful size are so large that mechanical failure can have dangerous consequences. In 1911, H.K. Onnes discovered that many metals become superconducting at temperatures close to 0 K. For magnet designers, superconductivity looked like a godsend. The flow of current through the coils of a superconducting electromagnet generates no heat because there is no resistance, so no cooling is required beyond that needed to maintain its coils in the superconducting state. In addition, once energized, superconducting magnets consume no power and do not have to be connected permanently to a power supply. However, it was soon discovered that no matter how cold they are, the metals in which superconductivity was first demonstrated become resistive when exposed to magnetic fields much lower than those generated by the resistive electromagnets of the day. The quenching of the superconducting state by external magnetic fields occurs in all superconducting materials, not just the metals studied by Onnes. What varies from one super-
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Opportunities in High Magnetic Field Science conductor to the next is the field strength at which quenching occurs—i.e., the critical field—and consequently the highest field the material can deliver when it is formed into a magnet. Only in 1961 were materials discovered that remain superconducting in fields high enough to be interesting to magnet designers, and the use of these materials for magnet fabrication has exploded since then. The number of superconducting magnets operating in instruments in laboratories and hospitals around the world is hard to estimate, but the committee was told by an industry representative that every year manufacturers sell about 2,000 MRI instruments and roughly 500 NMR spectrometers. These instruments contain superconducting magnets collectively worth billions of dollars. Other arenas in which superconducting magnets are used on a large scale are high-energy physics and fusion research. The demand for superconducting wire suitable for high-performance magnets increased enormously in response to the construction of the Large Hadron Collider (LHC) at CERN and will increase even more as construction of the International Thermonuclear Experimental Reactor (ITER) gets under way. As is explained in the body of this report, the construction of magnets from superconducting wire is a complex art. The performance of all such magnets is limited by the properties of the superconductors from which they are made, especially their critical fields. Mechanical strength and fabricability are also vital issues. These challenges notwithstanding, the maximum strengths of the fields produced by superconducting magnets have gradually increased to about 25 T. Hybrid magnets, which consist of a resistive solenoid inside a superconducting solenoid, can deliver substantially higher DC magnetic fields (about 45 T), but of course they continuously consume power and generate heat in their normal conducting sections. In 1986, materials were discovered that superconduct at temperatures up to 130 K, much higher than the highest temperature achieved by previously known superconducting materials (about 23 K). These high-temperature superconductors are ceramic copper oxides, which suffer from intrinsically weak links at internal grain boundaries, making the fabrication of magnets from them extremely difficult. They are very interesting to magnet designers, however, because their critical fields are far higher than those of any of the superconductors now routinely used for magnet fabrication. The technical challenges they pose are being overcome, so the field strengths that can be obtained from superconducting magnets are likely to increase significantly in the next few years. Resistive magnets can generate fields with strengths greater than about 45 T, but only for short times. The NHMFL has magnets that generate fields of about 60 T for tenths of a second, 65 T for hundredths of a second, or about 200 T for milliseconds. If partial or total instrument destruction can be tolerated, fields well
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Opportunities in High Magnetic Field Science above 300 T can be generated for microseconds. As the duration of the field pulse a magnet delivers declines, however, so too does its utility as a tool for scientific research. Consequently, the committee took the view that both the technologies and the science associated with fields of very short duration (less than a few milliseconds) lie outside the scope of its inquiry.2 This report has been written for readers who have a technical background and at least some familiarity with high magnetic field science. The committee’s decision to write at this level was made following discussions with the NSF. The body of this report begins with an overview of the science that is being done using high-field magnets and the science opportunities and challenges that might open up if higher-field magnets were developed. It closes with a discussion of magnet technology that explains why the fields generated by today’s most powerful DC magnets are less than two orders of magnitude stronger than those available to scientists in the 19th century, and points out the opportunities that now exist for developing more powerful magnets. This report includes several appendixes the readers may find useful, such as descriptions of selected high-field facilities around the globe, tutorials on advanced topics, and a glossary of common terms. 2 Most pulsed magnet techniques offering high fields of duration less than 1 ms are of the destructive variety, where the magnet is designed to explode with each use. For instance, single-turn coil magnet devices driven by a capacitor can generate fields of 100-250 T for 4-8 µs.
Representative terms from entire chapter: