Magnet technology has advanced, year-over-year, pulled by the desire of scientists for ever-higher magnetic fields. But progress has been paced by new developments in properties of materials needed to build these high-field magnets and by innovations in overcoming the technical challenges that arise from integrating the materials into a stable, safe, and economical magnet.
The technology challenges can thus be grouped into advancing specific aspects of magnet materials properties and by applying intensive and clever engineering design innovations and manufacturing processes. All magnets must simultaneously satisfy a number of often competing electrical, electromagnetic, structural, thermal, and economic constraints.
High-field magnets are generally categorized as resistive, superconducting, and hybrid (i.e., an outer superconducting magnet and an inner resistive magnet). They can be further divided into steady-state (continuous wave, or CW) and pulsed (or transient). For high field research magnets, the coils are mostly solenoids generating primarily axial magnetic fields along the bore centerline. Both resistive and superconducting magnets are usually operated CW, but the highest magnetic fields are generated from pulsed resistive, cryogenically precooled coils. Cryogenic precooling permits significantly higher and longer pulse lengths than can be achieved with identical magnets operated without precooling.
For other high-field applications such as high energy physics accelerators, magnetic confinement fusion, and medical applications, the most advanced systems require large-scale superconducting magnets. Superconducting magnets permit the generation of very large magnetic field volumes with minimal electrical power
input and can provide extremely stable temporal magnetic fields when the magnet leads are shorted through a superconducting switch. Superconducting magnets can either be operated CW (detector solenoids, toroidal field coils, magnetic resonance imaging, or MRI, magnets) or ramped (synchrotron dipoles and quadrupoles), or pulsed (ohmic heating coils, poloidal field coils).
The technology challenges are often different for each variety of magnet, but there is a basic subset of issues that all magnet designs must address to achieve higher fields.
Resistive electromagnets have been used industrially for well over a century and frequently have been designed for steady-state operations. For research applications, high-field (and high-stress), steady-state, water-cooled solenoid magnets constructed with Bitter plates were operating 50 years ago. They are the workhorse magnets of the National High Magnetic Field Laboratory (NHMFL) and other high-magnetic-field laboratories.
Resistive magnets can be simple, robust, and relatively inexpensive. For steady operation, it is only necessary to provide an electrical power source sufficient to overcome steady resistive losses in the magnets and a cooling system sufficient to remove the dissipated power from the magnets. In forced convection designs operating steadily at room temperature, pressurized coolant (frequently water) is pumped through passages in the magnet’s copper conductors, then through a heat exchanger to eject heat into the environment, and then returned to the pump. Steady cooling at high power densities requires that coolant flow paths in the conductor must be kept very short and coolant flow rates kept very high.
The key point allowing resistive magnets to be used for steady-state applications in large sizes is that the required electromagnet current density decreases with increases in the designed size of the electromagnet, as J ∝ B/R. Power density decreases even more, as ηJ2 ∝ η(B/R)2, so cooling large magnets becomes relatively easier. Furthermore, with the total conductor volume proportional to R3, it follows that total magnet power varies as Pmagnet ∝ B2 R.
For resistive magnets the conductor challenge is to balance high electrical conductivity with high mechanical strength since they are in opposition for practical materials (Figure 7.1). Higher-strength copper alloys or copper strengthened with nanoparticles or filaments have higher resistance than pure copper while offering higher yield stress and modulus. Thus, to achieve higher magnetic fields, the required electrical power and cooling power must also be substantially increased
FIGURE 7.1 Electrical conductivity relative to International Annealed Copper Standard versus tensile strength of copper alloys. SOURCE: Data from Diehl Metall.
to keep the working stresses within limits. The CW Bitter magnets at NHMFL are water cooled, while the pulsed-field magnets at Los Alamos National Laboratory (LANL) are precooled by liquid nitrogen and then operated adiabatically during the pulse. The pulse rate is limited by the time required to recool the magnet.
Superconducting magnets usually are more complicated to design and build than resistive magnets and also are more expensive. On the other hand, operating costs are lower because they require only a small amount of electrical power to keep them cold by their cryogenic refrigeration system. There are many technical challenges and considerations for high field superconducting magnets, including the following:
Critical current density is a function of field, temperature, strain, mechanical strength, and wire piece length. Cost and availability have become a major issue particularly when evaluating the trade-offs between high-temperature superconductors (HTS) and low-temperature superconductors (LTS) because the number of manufacturers is limited and the fabrication and processing methods are still under development.
The most commonly used LTS conductors are ductile alloys of NbTi (47 wt% Ti, Tc ~ 9 K) and the brittle intermetallic compound of Nb3Sn. Manufacture is quite complex since the conductors are most useful as multifilamentary composites requiring many assembly, processing, and control steps. This requires sophisticated
quality control (QC) and quality assurance (QA) programs that result in high costs. Owing to the highly brittle nature of Nb3Sn, the wires must be processed to small final diameters while the Nb and Sn elements are separate, and then the Nb3Sn compound is formed by a high-temperature reaction treatment, typically in the range of 650°C for as long as 100 hours. This process usually requires that the coil be wound from the unreacted wire while it is still ductile, then the entire coil undergoes reaction heat treatment so as not to mechanically strain the reacted wire. The reaction heat treatment poses additional problems for the electrical insulation, which often is applied to the wire before winding and thus must survive the reaction stage.
For high-field applications of interest, NbTi is used for the lower field portion of the coil windings, typically up to 9 T, with Nb3Sn used in the inner, high-field layers with peak fields now up to ~23.5 T for the highest field nuclear magnetic resonance (NMR) magnets in operation at a proton frequency of 1.0 GHz.
Operation of a superconducting magnet in a cryogenic environment requires the use of a sophisticated cryostat and refrigeration system. The majority of magnets operate at or near 4.2 K, which is the boiling point of liquid nitrogen at 1 atm. The highest field NMR magnets are subcooled to ~2 K to achieve a higher critical current density, but this requires an even more complex cryogenic system, because helium becomes a superfluid below 2.2 K at atmospheric pressure.
To achieve magnetic fields higher than 24 T from an all-superconducting magnet, it will be necessary to utilize a HTS conductor for the portion of the coil operating in higher magnetic fields. Figure 7.2 shows a variety of the best-performing LTS and HTS superconductors that are presently in commercial production.
As noted earlier, NbTi and Nb3Sn wire production are highly mature and are available in long piece lengths suitable for contemporary magnets. The HTS conductors, on the other hand, still have a long way to go before realizing their full potential. The best performance highlighted in Figure 7.2 may not be available now with the properties stable over piece lengths required to build a magnet. Superconducting tapes produced by powder-in-tube methods such as (Bi,Pb)2Sr2Ca2Cu3O10-x (Bi-2223) (Tc ≈ 110 K) have seen the widest application so far. The highly anisotropic critical current density has, however, limited its use in high-field applications. More attractive for high-field magnet applications is Bi2Sr2CaCu2O8-x (Bi-2212) (Tc ≈ 90 K), which has high critical current density and low anisotropy at high magnetic fields and can be fabricated as a round wire, making coil winding much easier. Bi-2212 is presently made only in limited quantity for research purposes and is primarily considered for high-field use by the U.S. high-energy physics community.
The HTS material with greatest potential high-field application is the coated conductor YBa2Cu3O7-δ (YBCO), with a Tc ≈ 92 K. These tapes use a strong metallic substrate such as Hastelloy or a Ni-W alloy, providing very high tensile strength.
FIGURE 7.2 The critical current density versus magnetic field at 4.2 K for the best-performing LTS and HTS conductors presently in commercial production or development. Critical current densities for YBCO tape with B parallel to the plane of the tape (not shown in figure) are higher than 1,000 A/mm2 for all fields plotted on the graph (up to 31 T). SOURCE: Courtesy of David Larbalestier, Florida State University/NHMFL.
Presently these conductors can be made only as thin flat tapes. They exhibit a strong anisotropic critical current density in the presence of transverse magnetic fields. Doping the compound with zirconium and substituting gadolinium for yttrium have resulted in reduction and modification of this effect (Selvamanickam et al., 2012). This is very important for magnet operation since the performance of coils using YBCO has often been limited by the transverse magnetic field generated near ends of the coil rather than by the highest magnetic field, which is located at the coil inner radius, axial centerline, and parallel to the c-axis.
The 2008 discovery of superconductivity in fluorine-doped LaOFeAs was highly significant because, as it turned out, the quaternary compound was only the tip of the iceberg for a new class of iron-based superconductors that include a number of families of binaries, ternaries, and the other more structurally complex
pnictide and chalcogenide compounds that soon followed. In the classification by structure, the six families that have been discovered to date are known as 11, 111, 1111, 122, 32522, and 42622 compounds [1-13] (Selvamanickam et al., 2012; Sefat and Singh, 2011; Kumar et al., 2009; Yuan et al., 2009; Jaroszynski et al., 2008; Putti et al., 2010; Ozaki et al., 2012a, 2012b, 2011; Weiss et al., 2012; Gao et al., 2011; Wang et al., 2010; Qi et al., 2010) (see Figure 7.3).
One of the main features of the iron-based superconductors that have generated much interest is the unconventional multiband superconductivity originating from d-orbitals in the layers of paramagnetic iron ions, which would normally be antithetical to superconductivity by pair-breaking in the traditional mechanism of s-wave Cooper pairing. The relationship of the quintessential magnetic ion to superconductivity in these materials was surprising and motivated a focus on understanding the mechanisms of superconductivity and magnetic ordering in
FIGURE 7.3 Structural variation of the six families of iron-based superconductors. SOURCE: A. Sefat and D. Singh, 2011, Chemistry and electronic structure of iron-based superconductors, MRS Bulletin 36(8):615, reproduced with permission.
addition to the fundamental interplay between these two phenomena. The observation of antiferromagnetic ordering in the form of stripes versus the checkerboard pattern observed with the cuprates prompted further comparisons of similarities and differences between the iron-based superconductors and the cuprates. Magnetic correlations appear strongly in these families of materials so for this reason the iron-based superconductors are ideal for the study of the fundamental relationship between magnetism and superconductivity. Since some aspects of high-temperature superconductivity are still under debate a quarter of a century after the discovery of the cuprates, the iron-based superconductors offer another opportunity for the development of a fundamental understanding of the mechanism of high-temperature superconductivity.
The availability of high magnetic fields, particularly the 45 T hybrid dc field magnet in addition to the pulsed-field magnets at LANL, played a critical role in the exploration of many of the interesting characteristics exhibited by these superconductors. Experimental evidence of multiband superconductivity also quickly revealed that the higher-critical-temperature members of this new class of unconventional superconductors exhibited very high upper critical fields comparable to the cuprates. During this flurry of discoveries, it was the fortuitous availability of high magnetic fields in both dc and pulsed modes that sustained the pace of the investigation of these materials. Table 7.1 gives the upper critical temperature and upper critical field for selected iron-based superconductors.
The upper critical field phase diagram of several superconductors having potential for commercialization in high-field magnets, including the LTS and HTS conductors already in production, are shown in Figure 7.4.
Superconducting magnet design also must take into account electrothermal stability, ac losses (magnetic hysteresis) if cycled, quench detection and protection, and stress management. A magnet design that resolves all these issues simultaneously and in an integrated fashion requires a high level of engineering and manufacturing sophistication. This becomes increasingly more difficult as magnetic fields are pushed ever higher. This is fundamental because all of these issues scale with the magnetic field B, or with the magnetic pressure B2.
Clearly, the critical current density Jc decreases with increasing field B. This then requires the use of more superconductor at lower overall winding current density, leading to use of more materials and higher cost. As the size of the coil winding increases, the conductor turns are placed at larger radius, decreasing the effectiveness for generating axial field in the magnet bore. Operation at these high fields also increases the probability and consequences of unstable behavior as operating margins are reduced.
Two of the most significant impacts, though, are (1) difficulty in protecting the magnet from damage in event of a quench and (2) management of the coil stresses from the Lorentz forces. Quench protection becomes more difficult because the
TABLE 7.1 Critical Temperature, Upper Critical Field, and Structure Classification for Iron-Based Superconductors
||Jc(Local) at Temp (K)||Jc(Global) at Temp (K)||Structure Classification||Source|
|LaFeAsO0.89F0.11||26 K||60-63 T||1111||1|
|CeFeAsO0.88F0.12||30 K||48.8 T (31.61 K)||1.5 × 106 A/cm2 (5 K)||1111||3|
|SmFeAsO0.85||53.25 K||30.48 T (36.9 K)||7.3 × 106 A/cm2 (5 K)
1.0 × 105 A/cm2 (53 K)
|3,850 A/cm2 (5.4 K)||1111||2, 10|
|NdFeAsO0.94F0.06||53 K||45 T (33 K)||6.7 × 106 A/cm2 (5 K)
1.08 x 105 A/cm2 (49 K)
|2,090 A/cm2 (4.75 K)||1111||2, 10|
|(Ba,K)Fe2As2||34 K||90 T (20 K)||68 T (20 K)||5 × 105 A/cm2 (5 K)||122||4|
|Sr(Fe,Co)2As2||20 K||42 T (8 K)||38 T||122||5|
|FeSe0.5Te0.5||14 K||14 T (14 K)||105-106 A/cm2 (4.2 K)||111||6, 8|
|FeSe||8 K||50 T||600 A/cm2 (4.2 K)||11||7,9|
SOURCE: (1) Hunte et al., 2008; (2) Jaroszynski et al., 2008; (3) Chong et al., 2008; (4) Weiss et al., 2012; (5) Y. Kohama et al., 2008; (6) Kawale et al., 2013; (7) Ding et al., 2012; (8) Tsukada et al., 2011; (9) Jung et al., 2010; (10) Putti et al., 2010.
FIGURE 7.4 Upper critical field phase diagram of several superconductors having potential for commercialization in high field magnets. SOURCE: Courtesy of David Larbalestier, Florida State University/NHMFL.
stored energy per unit volume of the magnet increases with B2, and along with it comes an increase of internal mechanical stresses. A quench in superconductor parlance refers to an unplanned transition of the conductor to its resistive state, which is generally initiated by an abrupt excursion of the superconductor beyond its critical surface.
For an NMR magnet that is operated in persistent mode, all quench energy must be safely dissipated within the winding pack at cryogenic temperature. Keeping stresses within acceptable design criteria may mean the addition of stronger materials and/or more structure with subsequent reduction of overall current density.
These issues require intensive engineering design and analysis. Implementation of any new materials or designs requires substantial R&D and testing. This type of expertise is available at only a few highly select laboratories in the United States and abroad, the NHMFL being premier among them.
Resistive magnet technology of the Bitter plate type (e.g., Florida Bitter) is highly developed. Presently generating CW fields to 33 T, the limit is primarily availability of cooling water and electric power, with the cost of power a major issue. This technology requires large infrastructure to operate and maintain and thus is available only in a limited number of laboratories such as NHMFL. Any increase in magnetic field will require upgrade of both cooling and electric power capability at significant cost. Increased magnet time availability would require either operation during more shifts (e.g., three shifts) or building more magnets and power supplies.
All very high field pulsed magnets are resistive. They are also limited by the availability of cooling and electric power. With stresses increasing with B2, engineering design and materials properties are major issues. The conductor resistance (usually a copper alloy) increases with temperature and magnetic field, depending on the alloy composition. Resistive magnets require high power to reach high field and high stored energy in the pulsed power source to achieve long, peak field durations. Here 100 ms is considered a long time. This relatively short duration pulse requires advanced methods for instrumentation and diagnostics of the sample under test and methods to prevent excessive heating of the test sample. Now only a few laboratories worldwide are available to provide this type of magnet system. The most powerful system is located at NHMFL/LANL, which takes advantage of a major electrical power generator as the power and energy storage source, coupled with a large capacitor bank and switching circuits. Recently, a world record 100 T pulse was achieved at LANL.
Presently available pulsed field magnets include the following:
• Capacitor driven—Field strength: 50-70 T, duration: 20-800 ms (total pulse length, including decay, available now at NHMFL);
• AC power driven (long-pulse, adjustable pulse shape)—Field strength: 40-60 T, duration: 2 s (>100 ms duration at constant field (flat-top), available now at NHMFL);
• Capacitor + ac power—Field strength: 80-100 T, duration: 20 ms (100 T recently achieved at NHMFL); and
• Destructive—Capacitor + chemical 100 T - 250 T.
The highest magnetic field values (up to 1,000 T) can be obtained during
microsecond pulses with destructive techniques. Fields up to 200 T can be reached with the so-called single-turn coils. A very steep high-voltage pulse is provided to a relatively simple and cheap copper single-turn coil, inducing a large current in it. During the pulse the copper is evaporated, and the current is carried by the ionized plasma, which is pushed outward by the Lorentz force. Due to inertia, during a few microseconds the high current density is maintained near the center, and a high-field pulse of the same duration is experienced by the sample. The coil is destroyed, but since the debris is projected outwards, it is usually possible to save the sample and additional experimental infrastructure. At the LNCMI in Toulouse (France) and the MegaGauss laboratory in Kashiwa (Japan), this technique is routinely used in optical and simple transport experiments. Even higher fields can be obtained using flux compression. Here, with a relatively slow pulse in an outer coil, a small seed field is fed into a small inner copper ring (the liner). Subsequently the flux in this liner is compressed, thus increasing the field in the center. The flux compression can be achieved with explosives, as was done in the Dirac experiment in Los Alamos, where field values of 1,000 T have been reported. Alternatively, flux compression can also be achieved by applying a second, huge electrical pulse to the outer coil, which is destroyed in the process but generates a short-duration high-field pulse on the inner coil, which compresses it and which generates the high field by flux compression. This technique has been pioneered by the MegaGauss laboratory in Kashiwa, which has also reported scientific data from such experiments up to several hundreds of tesla. This facility is being used to study magnetic phases of spinel oxides such as ZnCr2O4 at ultrahigh magnetic fields (Miyata et al., 2012). Obviously, the flux compression technique is destructive for the outer coil, the liner, the sample, and any apparatus near the field center.
The amount of information that one can extract from a pulsed experiment is proportional to the product of the pulse duration, the bandwidth of the experiment, and the binary logarithm of the signal-to-noise ratio (SNR). Obviously, all things being equal, less information can be extracted in shorter times. Fast data acquisition can only partially alleviate these problems, because increasing the bandwidth also will increase noise and thus a deterioration of the SNR. Furthermore, the technique is limited to physical effects that have shorter timescales than the pulse duration. Therefore no extensive and precise data at these high fields may be expected, and not all experiments are suitable. Nevertheless, very useful data might be obtained under these unique conditions, and exploring this field is very promising.
The 2005 NRC report of the Committee on Opportunities in High Magnetic Field Science (COHMAG) included recommendations for specific magnet
development goals. Among them was the goal of developing a 30 T superconducting, high-resolution, small-bore (54 mm) magnet for nuclear magnetic resonance (NMR). Recently the NHMFL demonstrated an HTS (YBCO) coil operating at 35.4 T (Trociewitz et al., 2011). The test coil comprised an insert HTS coil generating 4.2 T at 1.8 K, within the bore of a resistive magnet generating a 31.2 T background field. Although this magnet system was not all superconducting, it served to demonstrate that coated conductor YBCO tape has sufficient engineering current density as well as mechanical strength to be used as the high-field insert coil of a future, graded, all-superconducting small 32 T research magnet (Weijers et al., 2010).
NMR magnets to 950 MHz are presently commercially available. The first 1 GHz NMR magnet was brought into service by Bruker in 2009 at the European Center for High Field NMR, at the University of Lyon. The central field of this magnet is 23.5 T, achieved with a NbTi outsert and a Nb3Sn insert cooled to ~1.5 K (Bruker Biospin Corporation, 2009). Several organizations are now designing 1.3 GHz small-bore NMR magnets, including the National Institute for Materials Science in Tsukuba, Japan (Otsuka et al., 2010). This magnet requires generating a central field of 30.5 T and thus will require the use of a YBCO insert coil. Iwasa and co-workers at the Francis Bitter Magnet laboratory at MIT are designing and building a prototype 1.3 GHz (30.5 T) NMR magnet based on using BSCCO-2223 tape for a 600 MHz insert coil in combination with a 700 MHz LTS coil (Bascuñán et al., 2011).
The status of hybrid magnet development is summarized in Table 7.2.
The NHMFL Magnet Technology Division is presently constructing the NHMFL II, the Berlin, and Nijmegen III hybrids.
With the major goal of bringing the Large Hadron Collider (LHC) into scientific operation having been achieved several years ago, magnet technology R&D programs have been reoriented to achieving even more powerful superconducting beam bending and focusing magnets. The dipole magnets will likely operate with peak fields well beyond 12 T and strong focusing magnets with high-field gradients operating at similarly higher peak fields in the windings. This next step requires a change in focus from magnets using NbTi superconductor at 4.2 K and 2.0 K, to
TABLE 7.2 Resistive-Superconducting Hybrid Magnets Built, in Operation, or Under Construction
|Magnet||Total Field (T)||Year of First Operation||Outsert Field (T)||Energy (M)||Technology|
|MIT I||20||1972||5.8||Ventilated, cryostable?|
|Oxford I||16-25||1973||6.5||Stabilized NbTi|
|Moscow||25||1973||6.3||4||Ventilated multifilamentary strip|
|Nijmegen I||25-30||1977||8.5||Ventilated, cryostable|
|MIT II||25-30||1981||7.5||3.5||Ventilated, cryostable|
|Sendai I||20||1983||8||1||No ventilation|
|Nijmegen II||30||1985||10.5||10||Ventilated, cryostable|
|Tsukuba||31-37||1995||15||63||“Fully stable” monolithic|
|Grenoble II||42+||2016||8.5||76||Quench-shield, RCOCC|
NOTE: CICC, cable in conduit conductor; RCOCC, Rutherford Cable on Conduit Conductor.
SOURCE: Courtesy of the National High Magnetic Field Laboratory.
an Nb3Sn superconductor operating at 4.5 K. Unlike NbTi, which is a ductile alloy, Nb3Sn is a brittle compound, requiring a high-temperature reaction heat treatment to form the superconducting phase. This imposes a significant additional step in the integrated magnet fabrication of the conductor, coil, and insulation, which requires important changes in manufacturing processes and handling from those used for past accelerators. Operation at higher magnetic fields leads also to increased stored energy and magnetic forces necessitating stronger and better-integrated structural reinforcing materials. In the United States a new program in applying HTS conductors to accelerator magnets requires R&D to accommodate the complexity of working with this new material.
A laboratory-university-industry collaboration has been established for the development of magnets with fields >22 T. This Very High Field Superconducting Magnet Collaboration (VHFSMC) includes Fermi National Accelerator Laboratory (FNAL), Lawrence Berkeley National Laboratory (LBNL), LANL, Brookhaven National Laboratory (BNL), NHMFL, North Carolina State University, Texas A&M, and National Institute of Standards and Technology (NIST) (Clements, 2009). The focus of the research is to design and build HEP relevant magnets based on round-wire, multifilament Bi-2212 to complement other ongoing work with YBCO-coated conductors.
The U.S. LHC Accelerator Research Program (US-LARP) was established as a consortium of U.S. national laboratories, BNL, FNAL, LBNL, and SLAC, to collaborate with the European Organization for Nuclear Research (CERN) on development of accelerator technology to increase the luminosity of the LHC and to upgrade the interaction regions, through advanced superconducting magnet technology (Gourlay et al., 2006). US-LARP also serves as a programmatic vehicle to advance U.S. accelerator science and technology, including forefront accelerator research, improving capabilities and skills, and preparing the U.S. scientists to design the next generation of particle colliders. Present research is focused on using advanced, very high critical current density Nb3Sn strands and cables to design, build, and operate high-gradient focusing quadrupole magnets.
The NHMFL fulfills an important role in the development of high-energy particle (HEP) accelerator magnets, both by serving as a national resource for performing high magnetic field tests of advanced, state-of-the-art superconducting materials, conductors, and cables, and conducting research at the Applied Superconductivity Center (ASC). Research at ASC, in particular, and in collaboration with the Department of Energy-HEP laboratories, has been highly effective in advancing the development of round-wire, multifilament Bi-2212, making it a leading candidate for very high field accelerator magnets of the future.
Technologies for magnetic confinement fusion applications are designed in various shapes and sizes to provide plasma confinement, shaping, heating, and stabilization. Depending on the plasma configuration, the magnet geometries vary from simple rings and long solenoids, toroids, helical coils, and, sometimes, three-dimensional twisted shapes. Although most of the magnetic fusion experiments built in the past and still operating use pulsed, adiabatic resistive magnets recooled with either water or liquid nitrogen, several new superconducting devices have come into operation in the past few years. A working fusion power reactor has always been envisaged to require the use of large-scale superconducting magnets, since creating very high fields over large volumes would make resistive magnets
impractical because of their large electrical and cooling power requirements. The international fusion community has put great effort into development of large-scale NbTi and Nb3Sn superconducting magnets over the past four decades. Fusion magnet research programs in the United States, Europe, and Asia are now beginning to focus magnet development on using HTS conductors for some plasma confinement coils.
Since the COHMAG report was written, two new, all-superconducting fusion experimental systems have been built and are operational. These are both of the tokamak configuration that is presently demonstrating the most advanced plasma confinement. The EAST tokamak in Hefei, China, is made from all-NbTi cable-in-conduit conductors (CICC) (Wan et al., 2006; Wei et al., 2010). The KSTAR tokamak in Taejon, South Korea, is also made from all-CICC conductors, but the toroidal field (TF) magnets and the central solenoid (CS) use Nb3Sn strand while the poloidal field (PF) coils employ NbTi strand (Kim et al., 2005). Although the peak magnetic field in these machines is relatively low, ~ 7 T, compared with the most advanced high-field research and NMR magnets, the magnetic field volumes are very large, on the order of cubic meters. Thus the stored energy of the magnet system is 1-2 orders of magnitude greater than, say, a 1 GHz (23.5 T) NMR magnet. This results in very high Lorentz forces and mechanical stresses that in most cases require the use of external supporting structure. The very high stored energy also makes quench detection more difficult, forcing magnet designers to use high current (~10’s kA) conductors and external quench protection circuits operating at ~5-10 kV.
Although the tokamak configuration offers the most progress to date in plasma confinement, there are several other magnetic configurations that have significant advantages worth pursuing. Of these, a helical magnetic field is foremost in technology development, including the all-superconducting Large Helical Device (LHD) now in operation at the National Institute for Fusion Studies (NIFS) (Satow and Motojima, 2002), in Toki, Japan, and the Wendelstein 7-X (W7-X) stellarator nearing completion of construction at the Max Planck Institute for Plasma Physics (MPIPP) in Greifswald, Germany (Rummel et al., 2012). Again, although both devices operate at relatively low magnetic field, 6-7 T, using NbTi, their challenge has been to construct large-scale superconducting magnets of very complex geometry.
The W-7X is shown during construction in Figure 7.5 as well as one of its convoluted superconducting magnets.
The largest and most significant large-scale application of superconducting magnets for magnetic confinement fusion is the ITER project. ITER is a large-scale scientific experiment that aims to demonstrate the feasibility of producing fusion
power in a sustained burning plasma, while also demonstrating some of the key technology components (ITER Project). It is probably the most ambitious international scientific project ever undertaken, both in the scale of the device and in the complexity of the international collaboration. Over half the world’s population is represented by the seven countries participating in the project, including China, the European Union, Japan, India, South Korea, Russia, and the United States. Among the goals of ITER is the production of 500 MW of fusion thermal power using deuterium-tritium fuel, while consuming no more than 50 MW of heating input power, to give a ratio of Q ≥ 10 for at least 300 s. Another goal is to demonstrate long pulse sustainment for at least 1,000 s with a Q ≥ 5.
A drawing of the fusion tokamak core is shown in Figure 7.6. Its immense scale requires production of ~500 metric tons of Nb3Sn strand, supplied by most of the world’s LTS suppliers. This amount is more than 10 times the total integrated amount of Nb3Sn produced since first commercialization in the 1960s. It will also require about 250 metric tons of NbTi strand (Devred et al., 2012).
The ITER TF magnets generate a peak magnetic field of 11.8 T at an operating current of 68 kA and store 41 GJ of magnetic energy. The CS magnet generates a peak field of 13 T and operates with 40 kA current in a pulsed, fully bipolar cycle. It is over 13 m tall and is lifted as a single unit weighing over 900 metric tons (Mitchell et al., 2012). Although the magnitude of these magnetic fields does not approach that of the very highest fields required for NMR studies and scientific research, the huge scale of the magnets imposes extremely stringent design criteria and requires an extremely comprehensive and interdisciplinary engineering and technology approach.
The U.S. fusion base program for magnet technology is now focused on developing magnet technology for fusion reactors beyond ITER. The present magnet program is focused on developing high current, high field, and HTS conductors for fusion magnets. This requires development of 50 kA class conductors that can operate in magnetic fields in the range 16-20 T. Recent studies performed at the Massachusetts Institute of Technology indicate that the use of demountable magnets is a feasible option for future devices that incorporate magnets made with HTS conductors (Hartwig et al., 2012). This could have a major impact on the ability to maintain the machine and increase reliability and availability; however, much more development is required. Similar research is being done in Japan (Yanagi et al., 2012) and Germany (Schlachter et al., 2011). Among the innovations waiting to be explored are structural materials with strengths and elastic moduli much higher than present stainless steels and other alloys. Developing better structural materials is the only viable way to increase the overall magnet current density in a tokamak inner leg, with subsequent savings in machine cost and size.
FIGURE 7.5 (Left) Overview of the Wendelstein 7-X stellarator during construction. (Right) One of the superconducting magnets comprising the complicated main confinement magnetic field. SOURCE: Courtesy of Max-Planck-Institut für Plasmaphysik.
The largest commercial application of superconductors today is for NMR magnets in chemistry and biology and magnetic resonance imaging (MRI) magnets in life science. Two new medical applications that can benefit from using superconducting magnets are compact cyclotrons for charged-particle radiotherapy and compact cyclotrons for production of radionuclides used in nuclear medicine.
Radiotherapy using protons to deposit ionizing radiation in tumors has been shown to be very effective in the treatment of cancer. A major feature and advantage of proton therapy is that this method takes advantage of the Bragg peak of energy loss when transiting the body. An example of energy deposition versus depth is shown in Figure 7.7, which compares photon energy versus depth with proton energy versus depth (Yock and Tarbell, 2004). Depth of maximum energy deposition can be controlled by proton energy, and magnetic steering enables
precise targeting, thus the radiation dose can be precisely targeted to the tumor, minimizing damage to surrounding tissues (Kaderka et al., 2012), especially when the lateral energy deposition profiles are compared to those from photon radiations, as shown in Figure 7.8. Proton radiotherapy’s main use is for treating tumors where surrounding tissue has a low ionizing radiation tolerance. This is often the case for childhood cancers and for tumors near the eye, the spinal cord, or in the brain. Another potential benefit is reduced probability of secondary tumors resulting from the radiation treatment. A maximum proton energy of 250 MeV will penetrate tissue to a depth of ~30 cm, which is sufficient to treat deep tumors in a human patient.
Ernest O. Lawrence invented the cyclotron in 1930 at the University of California at Berkeley. It was one of the earliest types of particle accelerators for scientific research. In 1946 Robert R. Wilson first suggested using a proton cyclotron, then
FIGURE 7.6 A detailed model of the ITER device. The 18 D-shaped toroidal field magnets are 17.5 m tall by 9 m wide. The two largest outer ring poloidal field magnets are 24 m in diameter, each weighing 300 metric tons. SOURCE: © ITER Organization, http://www.iter.org/.
under construction at the Harvard Cyclotron Laboratory (HCL) to treat tumors (Wilson, 1946; Geisler et al., 2005). The first treatment of patients occurred in the 1950s using existing scientific research accelerators. While working at HCL, Wilson began a collaboration with the Massachusetts General Hospital (MGH) in Boston to treat patients using the cyclotron at Harvard. The major proton and heavier ion treatments commenced at the Donner Laboratory of the University of California and the Lawrence Berkeley National Laboratory, now supported by the Department of Energy and the National Institutes of Health (NIH). Treatments by radiation ablation of the pituitary gland were successful for acromegaly (gigantism) and Cushing’s disease. That work from 1950s to 1990s was moved to Loma Linda Medical Center in California for the first hospital-based proton treatment in 1990. The radiation source was not a cyclotron, but rather a synchrotron designed and constructed by scientists from FNAL. The Northeast Proton Therapy Center
FIGURE 7.7 Comparison of relative photon dose with proton dose versus tissue depth. The Bragg peak occurs at the end of proton travel. By varying proton energy to generate different Bragg peaks and superposing them, the radiation dose can be constrained to the tumor volume. SOURCE: Adapted from R.R. Wilson, 1946, Radiological use of fast protons, Radiology 47:487-491.
FIGURE 7.8 Overview of the lateral dose profiles measured for radiation types and delivery techniques. SOURCE: R. Kaderka, D. Schardt, M. Durante, T. Berger, U. Ramm, J. Licher, and C. LaTessa, 2012, Out-of-field dose measurements in a water phantom using different radiotherapy modalities, Physics in Medicine and Biology 57:5059, © Institute of Physics and Engineering in Medicine, published on behalf of IPEM by IOP Publishing Ltd., all rights reserved.
opened at MGH in 2001 using a conventional (resistive) proton cyclotron. This center was partially funded by the National Cancer Institute. Since then, more than 36 proton radiotherapy centers have gone into operation worldwide, 24 of which are cyclotrons and 12, synchrotrons. Most new facilities that are opening now use cyclotron accelerators (Krischel, 2012).
A major impediment to rapid expansion of this highly effective treatment is the $100 million to $200 million cost of a treatment center that includes the necessary infrastructure. A significant part of this high cost is due to the very large size, mass, and cost of the conventional cyclotron, the long beam transport system, and the huge rotatable gantry required to direct the proton beam to the patient. The use of superconducting cyclotrons begins to address the size and cost issue by reducing
the size of the cyclotron and potentially leading to easier beam transport solutions and substantially less shielding and siting costs.
This reduction is a consequence of the inverse relationship between the radius of the cyclotron and the magnetic field, as shown below:
Ef ≈ Kr2B2
where Ef is energy, K is a constant, r is the cyclotron radius, and B is the magnetic field. Thus cyclotrons can be made very compact by going to high magnetic fields. Currently two superconducting cyclotrons have been built for proton radiotherapy and are treating patients on a regular basis (MSU, 1993; Miyata et al., 2012). One machine is at the Paul Scherrer Institute in Villigen, Switzerland (PROSCAN) and the other is installed at the Reinecker Proton Therapy Center in Munich, Germany. Both machines are isochronous cyclotrons built by the same company and based on a 1993 design done by the National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University. They are cooled with liquid helium but maintained cold by cryocoolers in a closed cryogenic system, similar to the methods used to cool MRI magnets. These machines are more compact than resistive cyclotrons, reducing the size and weight from 4.3 m and 220 metric tons down to 3.1 m diameter and 90 metric tons.
Although this is a factor of 2 reduction in weight, these machines used NbTi superconductor and limited the central gap field to 2.4 T. Newer designs by other organizations are capitalizing on the very high current density and high critical field of Nb3Sn to develop much more compact synchrocyclotrons. The most advanced of these designs is the Mevion S250 proton synchrocyclotron built by Mevion Medical Systems, Inc., based on technology licensed by MIT. The concept developed at MIT is based on using a Nb3Sn magnet generating 9 T at the pole gap with a peak field at the windings of ~11 T. This design takes advantage of a very high current density superconducting wire developed by U.S. industry using funding from the U.S. high-energy physics research program.
The device built by Mevion Medical Systems has a diameter of only 1.8 m and weighs about 20 metric tons. It is small enough and light enough to be placed on the treatment gantry so that the entire cyclotron rotates around the patient, as shown in Figure 7.9. The compact size and light weight of the cyclotron not only reduce cost but also eliminate the stationary beam transport system as well as the heavy gantry-mounted beam transport magnets, thus reducing the gantry weight as well. These systems can be installed as individual treatment machines instead of the contemporary device, which uses a single accelerator with beam line transport of protons to multiple rooms. Thus the initial capital investment in establishing a center that can scale to multiple treatment rooms is reduced by a factor of 10. This
FIGURE 7.9 Mevion S250 compact superconducting proton synchrocyclotron mounted on a rotating gantry for a single treatment room. SOURCE: Courtesy of Mevion Medical Systems.
lower cost makes introduction of this treatment therapy more widely available to medical centers and patients.
Although proton beam radiotherapy is expanding in clinical use, other charged particles such as helium, carbon, and neon have also been used for the treatment of cancers. These charged particles have heavier mass than a single proton and thus require more powerful particle accelerators to achieve effective treatment energies. Ongoing activities include efforts in Japan at the Heavy-Ion Medical Accelerator in Chiba (HIMAC), which uses a range of charged particles for cancer therapy, and studies at high-energy-physics laboratories in Europe that have used carbon beams. Several organizations in the United States are considering using carbon and other heavy ions for radiotherapy, but there is no existing and mature commercially available accelerator technology ready to satisfy this wish. It is possible that advances in superconducting technology can be used to develop a medically and economically feasible solution, but this will require a substantial investment in accelerator technology. At present, NIH continues to provide research and development funds for the purpose of developing and installing more powerful MRI magnet systems, and it seems reasonable to extrapolate that support to the development of advanced heavy ion accelerators for radiotherapy applications. This
support would be especially efficacious at this early stage of exploratory medical research since it could affect the future direction of application and development of this technology.
Nuclear medicine radionuclide production for research and clinical studies depends for the most part on accelerator and reactor facilities that are remote from clinics and research institutions. This has severely limited the application of the short-lived nuclides 11C, 13N, 15O to those institutions with a local cyclotron that usually operates with a 1 to 1.5 T resistive magnet. The siting costs are dominated by shielding requirements and the size of the installation. These costs as well as cyclotron costs typically necessitate a $4 million investment. Thus for studies that take advantage of positron emission tomography, long half-life radionuclides are used, but even these cannot achieve the needed specificity to enable clinical studies in addiction, aging, heart disease, and some cancers where radionuclides such as 11C, 14N,15O, and 89Zr must be produced locally using a particle accelerator. To overcome this problem, superconducting cyclotron technology is being employed in the production of small cyclotrons at 5 to 9 T with modern cryostats and turnkey operations. A commercial prototype is being developed by Ionetix, Inc., with expected clinical installations in 2014.
High fields define a scientific frontier, and the new phases that are discovered as higher fields are made available are the feedstock for new materials and devices that reproduce these new behaviors at low or even zero field. Increased field strength inevitably leads to enhanced sensitivity and new experimental techniques that in turn increase the tempo of scientific discovery. Each breakthrough in magnet technology and experimental capability leads to a new flurry of scientific revelation and discovery, which in turn enables the next round of technological breakthrough. This virtuous cycle is nowhere more evident than in the bootstrap process by which new magnets are themselves developed, where access to higher magnetic fields provides the means for testing and improving the new concepts and components that will make possible the next generation of magnets.
It is imperative that magnet technology be constantly challenged—and also supported!—to provide the innovation that enables the ever higher fields that fuel these discoveries. It is in this spirit that the committee recommends here three magnet development goals. Each is a novel and first-in-class project, and significant development efforts will be required to reach the stated goals. These magnets also represent significant investments in the national research infrastructure, because
in some cases new research and funding partnerships must be formed in order to take full advantage of these new capabilities. The committee anticipates that it may take as long as a decade until these magnets become available for researchers. As discussed in Chapter 2 of this report, access to higher magnetic fields, both in dc and pulsed modes, will be crucial for progress in many aspects of condensed matter and material physics.
An additional recommendation calling for a design and feasibility study for a 20 T magnet for use in MRI studies of humans was discussed earlier, at the end of Chapter 4, and recommendations for the development and installation of new types of magnets for use at X-ray and neutron scattering facilities is presented in Chapter 6.
Finding: Recent advances in high-temperature superconductor (HTS) magnet technology are an important step forward, with the potential for making possible a new generation of all-superconducting high-field magnets that would be transformational in many research areas.
Such magnets will enable steady-state physics measurements at very high magnetic fields without the constraints and attendant costs of huge power supplies and a large-scale cooling facility. This means that significant reductions in both the construction and operation costs of a 40-T class magnet can be envisioned, making it possible to locate these magnets in regional centers, built around teams of users with specific measurement needs. The improved accessibility to the 40-T class magnet will greatly facilitate the advancement of sciences that require steady-state measurements in magnetic fields significantly higher than the ordinary laboratory fields; furthermore, all-superconducting magnets can be used in the persistent-current mode, which provides a noise-free environment and makes it possible to perform ultrahigh-sensitivity measurements that have not been possible in hybrid-type magnets. It seems likely that the availability of these magnets would significantly change the mix of users at NHMFL-Tallahassee and would free that facility to develop new and complementary capabilities that cannot be reproduced elsewhere.
As the committee has discussed elsewhere in this report, the United States has largely ceded leadership in constructing high-field superconducting magnets for NMR to Europe, where there is a closer relationship between the national labs that provide the required technology and the companies that will build these magnets. In part, this reflects a long-term underfunding of both magnet technology research in this country and the research in high-strength materials that underlies this important area. Surmounting the technological challenges associated with realizing a 40 T all-superconducting magnet will be a big step toward making U.S.
industry competitive in the production of NMR/MRI magnets and the associated superconducting wires and cables.
Recommendation: A 40 T all-superconducting magnet should be designed and constructed, building on recent advances in high-temperature superconducting magnet technology.
Finding: The veritable explosion of new materials with new functionalities that we have witnessed in the past decade is a potent driving force for the need to push experimentation to higher fields, where new phases and new behaviors are invariably found. Although pulsed fields will always provide the highest peak fields, many of the most revealing measurement techniques have inherent timescales or sensitivity requirements that make them practical only in constant magnetic fields.
Techniques requiring dc magnetic fields include ultrasensitive voltage measurements that allow high-precision parametric studies of electrical resistance, heat capacity, susceptibility, and thermopower; scanned probe microscopies that provide both atomic-scale imaging and spectroscopic information; and optical spectroscopies performed over a wide range of frequencies. The ability to carry out these measurements, already proven in zero field to provide crucial information, has the potential to open up whole new fields of research and technology. Some examples include the exploration of the normal state that precedes the unconventional superconductivity in the cuprates and iron pnictides and chalcogenides, the manipulation of symmetry-broken phases and unconventional quantum Hall effects in single-layer and few-layer graphene, and the investigation of the interplay between topological insulators and superconductivity. The ability to bring these measurements to new generations of materials and devices in increasingly high magnetic fields would define a world-leading capability and confer a distinct advantage to the researchers who can exploit them.
Recommendation: A 60 T dc hybrid magnet should be designed and built that will capitalize on the success of the current 45 T hybrid magnet at the NHMFL-Tallahassee.
Finding: Many crucial measurements requiring the highest attainable fields can be performed in pulsed magnetic fields with durations on the order of 10 ms. Similar measurements at higher fields than are currently available would allow investigation of phenomena that are now beyond reach.
With the March 2012 attainment of 100 T in a nondestructive 15 ms pulse at
the pulsed-field facility of the NHMFL in Los Alamos, new terrain in high-field research has been opened up to researchers. The ability to routinely access 100 T fields will enable unprecedented research in topological insulators, quantum matter, and electronic structure determination.
Fields much higher than 100 T have been achieved in very short pulsed field magnets (microseconds duration), which destroy the magnet coil and in many cases also the sample. However, the types of measurements that can be performed on microsecond timescales are much too limited to provide the type of information needed for elucidating the most pressing research problems.
From a scientific point of view, a desirable long-term goal would be the ability to extend the suite of measurements now available at 100 T to fields on the order of 200 T or beyond. As one measure of magnetic field strengths, it can be noted that at 225 T, the energy difference between the two states of an electron’s spin is equal to the thermal energy at room temperature. Fields of this magnitude would allow direct investigations of unusual phases and phase transitions in quantum spin systems with strong exchange couplings. They would also enable investigations of some of the most important high-temperature superconductors by allowing field-induced suppression of superconductivity in the ground state of these materials.
Unfortunately, no clear route currently exists for producing nondestructive fields as high as 200 T. Among other limitations, magnets of this strength would have to sustain forces well beyond the yield strengths of any known material. Nevertheless, important advantages could be obtained already by extending the availability of nondestructive pulsed fields in a series of smaller steps, perhaps achieving 150 T by the year 2023. Higher-field magnet technology might be developed hand-in-hand with the ability to make required measurements on smaller samples and in shorter times.
Recommendation: Higher-field pulsed magnets should be developed, together with the necessary instrumentation, in a series of steps, to provide facilities available to users that might eventually extend the current suite of thermal, transport, and optical measurements to fields of 150 T and beyond.
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