In this chapter, the committee reviews briefly several additional areas where high magnetic field science and technology is of great importance, or might be in the future. Two of these, high-energy physics and controlled nuclear fusion, are areas where magnetic fields already play a very large role. Particle accelerators and detectors, and devices for generation and control of hot hydrogen plasmas, require enormous magnets, with stringent demands on field geometries as well as requirements for the highest practical field strengths. In the United States, design and development of such magnets have largely been carried out at laboratories supported by the Department of Energy (DOE), but collaborations with the National Institute of Standards and Technology (NIST), universities, and the National High Magnetic Field Laboratory (NHMFL) have also been involved. A detailed discussion of the requirements of these magnets and the challenges they pose for high-field magnet technology will be discussed in Chapter 7 of this report. Here, we present a brief reminder of the scientific motivation for these projects.
In particle astrophysics, large magnets with strong magnetic fields have been employed in several ground-based experiments designed to look for axions, or axion-like particles, of solar or cosmological origin. Space-based experiments have, so far, made use of only magnetic fields that are rather weak relative to the scale considered in this report. However, improvements in the design of superconducting magnets, particularly a reduction in their consumption of helium and an extension of field lifetime, could give them an important role at some time in the future.
Particle accelerators using superconducting magnets could also play a role in health care in instruments for radiotherapy. In this chapter, the committee summarizes briefly the advantages that could be gained from such accelerators. The challenges involved are discussed more fully in Chapter 7, Magnet Technology Development.
Reports of the recent success of the Large Hadron Collider (LHC) in facilitating the discovery of the long-sought-after Higgs boson of the Standard Model of particle physics have garnered much interest among physicists, in addition to capturing the imagination of many nonscientists worldwide. This achievement stands as a prominent example of the triumph of superconducting magnet technology. High magnetic fields are used both in the particle accelerator itself and at various places in the detectors used to observe the collision products. Magnets at the LHC are all superconducting, as the power and cooling requirements for resistive magnets would be entirely prohibitive.
Accelerators employ bending magnets to keep the particle beams in a circular track and use focusing magnets of various kinds to prevent the beams from spreading out in space as they circulate. The bending magnets use uniform magnetic fields, oriented in the vertical dimension, while the focusing magnets require nonuniform fields, with carefully designed gradients, in order to function properly. High fields for the bending magnets are necessary to achieve the highest possible beam energies for a given accelerator radius.
Although the magnetic field strengths of superconducting magnets for high-energy and nuclear physics applications do not approach the present level of laboratory research magnets, i.e., 1 GHz NMR, they do require mostly nonsolenoidal geometry to be useful for charged-particle beam bending (dipoles), focusing (quadrupoles), and error field correction (sextupoles and higher order). They have bore tube diameters on the order of 50 mm but with magnetic field lengths approaching 20 m. The largest detector magnets are often solenoids, but of unprecedented scale, complexity, and stored energy.
Accelerator physicists have already begun to envision the next generation of particle accelerators in the form of a muon collider enabled by high-temperature superconducting magnet technology. Such a device would require solenoidal magnets with a central field as high as 50 T. Several collaborations have been established, involving industry, universities, and government laboratories, to further the development of magnets necessary for such a project. An outline of these efforts may be found in Chapter 7.
High magnetic fields could be useful in space-based detectors designed to analyze high-energy charged particles in cosmic rays. The experiment AMS-2, which was deployed on the International Space Station (ISS) in May 2011, employs a 1,200 kg Nd2Fe14B permanent magnet with a field strength of around 0.125 T. Original plans called for a superconducting magnet using NbTi wire, which would have had a field strength about five times higher and which would have enabled the study of particles with proportionately higher energy. However, difficulties were encountered with the superconducting magnet, particularly a poorly understood heating effect, which would have increased the cryogenic cooling load and shortened the running time for the experiment. Consequently, the decision was made to employ the permanent magnet instead.
Presumably, the development of improved magnets, perhaps at much higher fields, could significantly increase the capability of a future space-based detector. However, a new detector is unlikely to be undertaken in the near future, for reasons unrelated to the magnet issue. The power requirements for the experiment are beyond the capabilities of any existing space platform other than the ISS, and AMS-02 was already too large to be carried by any vehicle other than the U.S. space shuttle, now retired.
High magnetic fields may also have a role in ground-based experiments to search for axion or axion-like particles as a possible constituent of cold dark matter. Strong magnetic fields are supposed to convert a small fraction of axions into observable photons. The Axion Dark Matter Experiment (ADMX), based at the University of Washington, employs a superconducting magnet that generates 8 T in a region that is 1 m long and 0.5 m in diameter. The experiment is designed to convert axions into microwave photons and would be sensitive to axions with a mass in the range of 2 to 20 μeV. Planned upgrades to this experiment, which should greatly increase sensitivity, do not involve stronger magnetic fields but rather involve improvements in the design of the microwave detectors and lowering of the temperature of the microwave cavity, by means of a dilution refrigeration system. However, stronger magnetic fields could have an important role in future experiments of this kind.
Another experiment, the CERN Axion Solar Telescope (CAST), is designed to look for axions produced in the core of the sun, with masses up to 104 eV. This experiment, which started operating at CERN in 2002, employs a magnet approximately 10 meters long, with a maximum field of 9.6 T, originally designed for the LHC (Aalseth et al., 2002). For masses below 0.02 eV, CAST has set an upper limit to the axion-photon coupling constant gαγ of <8.8 × 10-11 GeV-1, with larger values for higher masses (Collar et al., 2012). Again, stronger magnetic fields will be important for achieving greater sensitivity in future experiments.
If magnetic confinement fusion reactors are to become a viable source of energy for the future, this technology will require the development of magnets capable of generating large fields while being compact, lightweight, and, in some cases, of a novel geometry for special plasma field shaping. Fusion reactors that are currently operating or are in the planning or construction stages employ a variety of strategies, with different requirements on magnet design, as will be discussed further in Chapter 7.
Charged particles have been used for radiotherapy since the first proposal of Wilson at MIT in the 1940s and the implementation at Lawrence Berkeley National Laboratory at about the same time. The reason high magnetic field technologies are relevant to this health science program is that magnetic fields are used in the source beams of charged particles as well as in steering the beams to the patient.
Proton beams, the most commonly used mode, have the advantage over conventional photon radiotherapy in that the beams can be focused precisely to small tumors and the depth into tissue can be controlled by energy selection, thus allowing normal surrounding tissues to be spared radiation effects. Over the past 70 years, proton therapy systems have been commercialized throughout the world. The use of other charged particles such as helium, carbon, and neon has been shown to be efficacious in the treatment of cancers of the lung, liver, and prostate, while beams of protons, helium, and neon have successfully treated pituitary disorders and arteriovenous malformations. For example, current data, mostly from Japanese studies, show carbon ions are superior to protons in precise dose delivery and in the reduction in the number of times the patient must return for treatment (Kamada, 2012).
The installation of a patient-based accelerator and the supporting treatment devices and rooms is very expensive (approximately $200 million), so these facilities are found only in a few large medical centers. Superconducting technologies can play a major role in reducing the physical size and installation costs of the required particle accelerators and beam transport, as well as the operating costs.
Aalseth, C.E., E. Arik., Autiero, D., F.T. Avignone III, K. Barth, S.M. Bowyer, H. Brauninger, R.L. Brodzinski, J.M. Carmona, S. Cebrian, G. Celebi, et al. 2002. The CERN Axion Solar Telescope (CAST). Nuclear Physics B (Proceedings Supplements) 110:85.
Collar, J.I., R. Essig, and J.A. Jaros. 2011. New light, weakly coupled particles. Chapter 6 in Fundamental Physics at the Intensity Frontier, Report of the Workshop held December 2011 in Rockville, Md. (J.L. Hewitt and H. Weerts, workshop chairs). arXiv:1205.2671. Available at http://xxx.lanl.gov/abs/1205.2671.
Kamada, T. 2012. “Current Status and Future Prospects for Carbon Ion Therapy at NIRS-HIMAC National Institute of Radiological Sciences, Research Center for Charged Particle Therapy, Chiba, Japan.” Presentation at the CAARI 2012 conference, August 7, Ft. Worth, Tex.