International Landscape of
High-Magnetic-Field Facilities
SCALE OF HIGH-MAGNETIC-FIELD FACILITIES
This chapter describes the international landscape of large, high-magnetic-field facilities, with a discussion at the end on opportunities for collaboration among these facilities.
High magnetic field installations are expensive and therefore rare. A rough estimate of the investment needed to finance the construction of a facility providing dc fields similar to the U.S. National High Magnetic Field Laboratory (NHMFL) in Tallahassee is several hundred million U.S. dollars. This would include the costs for basic infractructure, specialized hybrid magnets, and full instrumentation for conducting experiments. A pulsed-field facility similar to NHMFL’s laboratory at Los Alamos National Laboratory would cost roughly half this amount. Annual operating costs for such facilities depend significantly on the intensity of their use and on how much of the personnel costs are included but are nevertheless in the tens of millions of U.S. dollars. The development of specialized new magnets such as hybrids or high-temperature superconducting coils is estimated to cost between $10 million and $20 million each.
Because of their size and the costs of building and operating such installations, only a few of these large facilities are in operation worldwide, and most function as user facilities that host external guest researchers. Typically their funding sources are more stable and of a longer-term nature than other projects that might be funded locally or whose funding is project-related. Access to these facilities normally entails submitting a proposal for evaluation by an external project review
panel. Selection criteria include the scientific merit (innovativeness and excellence) of the proposed project and whether the unique resources provided by the facility are needed for the planned experiments.
Appendix G summarizes the main characteristics of large research facilities for high magnetic fields available worldwide. Also included in that appendix are the yearly number of projects executed, the number of different projects executed, and the number of external researchers involved. These numbers provide some indication of the worldwide demand for these fields. It can be seen that the number of such facilities worldwide is quite limited and that their characteristics are similar. All facilities that are in full operation (primarily located in Europe and the United States) have a successful, oversubscribed user program and produce many excellent groundbreaking publications every year.
TECHNICAL CHALLENGES IN MAKING THE HIGHEST MAGNETIC FIELDS
In addition to the financial constraints associated with constructing and operating such facilities, these research resources are limited because of the technological difficulty of producing the highest possible magnetic fields.
Producing such magnetic fields requires not only world-class engineering but also the development of rare materials that must combine strength with good electrical conductance. The demands on magnets due to the Lorentz force are huge, and controlling them in all parts of the magnets—the winding body, the housing, and other components such as current leads—is a major design challenge. Design requirements are further complicated by the need to have the largest possible currents, and hence the largest forces, in a small volume near the magnet’s central bore. The design of dc magnets is further complicated by the need to use some of the space in the magnet body for the flow of coolants to uniformly counter the heat generated by the currents flowing through the magnet. In the design of superconducting magnets, one must take into account not only the constraints imposed by the Lorentz force, which are the same as in resistive magnets, but also the very poor mechanical properties of the superconducting material. This requires including in the design an external reinforcing structure that sufficiently strengthens the magnet against the substantial Lorentz forces. Furthermore, because superconducting magnets must be operated at low temperatures, they must be able to withstand thermal cycling over a large temperature range, where thermal expansion of materials can be significant.
For all magnets, regardless of which technology is used, the amount of energy stored in the magnet depends on natural constants (the permeability of free space), the square of the magnetic field, and the volume where this field is present. Therefore, in all high-field magnets with a reasonable measuring volume the amount
of energy stored is enormous. For larger magnets such as hybrids, 100-200 MJ of stored energy is not uncommon. This enormous amount of stored energy requires that important safety precautions be implemented in the magnet’s operations, since release of that stored energy over a short time period is comparable to the energy released by a small bomb. This energy is of particular concern for superconducting magnets since their relatively large self-inductance means they are unable to rapidly carry away that stored energy.
Because of these challenging conditions, magnet technology for high-field magnets requires very specialized engineering, and the safe use and maintenance of high fields for research requires having a dedicated and skilled staff. These rather unique conditions are another reason why only a few high-magnetic-field laboratories exist. In fact, the continuous increase in maximum fields over the last decades has gone hand-in-hand with the concentration of those very high field-producing magnets in only a few large facilities in the United States, Japan, Europe, and, recently, China. As described below, interactions between these laboratories are quite extensive, and there is a general awareness that future-generation magnets can be developed only by fostering a widespread expertise in magnet technology worldwide.
OPPORTUNITIES FOR COLLABORATION
Interactions among the large magnet laboratories around the world are ongoing, with laboratory representatives informally exchanging information during frequent visits to each others’ laboratories or at conferences. Personnel are also frequently exchanged (even between laboratories located on different continents), and most laboratories employ people who have worked in one of the other laboratories. For instance, the NHMFL at Tallahassee has hired staff who previously worked at the Laboratoire National des Champs Magnétiques Intenses (LNCMI-G) in Grenoble, France, and key engineers at the new magnet laboratories in China— the Chinese High Magnetic Field Laboratory (CHMFL) in Hefei and the Wuhan High Magnetic Field Center (WHMFC) in Wuhan—were trained in the United States and Europe. Furthermore, new magnet designs from one laboratory often are reviewed by chief engineers at other laboratories in order to profit from all of the expertise available. Regular mutual visits ensure the sharing of developments in technology and the layout of new systems. It is therefore not surprising that key parameters of the larger installations, such as the amount of energy stored, the capacitive voltage for pulsed installations, maximum currents and voltages, and the hydraulic parameters in cooling circuits for dc magnets, are rather similar in all the laboratories. Such similarities also make the exchange of magnet systems between laboratories rather easy. Technology for making magnets is shared on a
regular basis, and magnet parts, and at times entire magnets, are provided by one laboratory to another.
The dc laboratory at the NHMFL plays a particularly important role among large-scale magnet laboratories. The laboratory has closely collaborated with LNCMI-G from the time that NHMFL opened. The earliest NHMFL dc magnets were provided by LNCMI-G, while NHMFL later developed its own very successful resistive magnets based on what has come to be known as Florida-Bitter technology. This technique has been adopted at the High Field Magnet Laboratory (HFML) in Nijmegen, Netherlands, and the Tsukuba Magnet Laboratory (TML) in Tsukuba, Japan, which have acquired magnets from the NHMFL. This technology in Nijmegen was developed further and improvements have found their way back to NHMFL. Rigid housings that can handle the forces and withstand the high water pressure while still having low vibrations are complex engineering objects. Working together, NHMFL and LNCMI-G developed a 24 MW housing for dc magnets for which each laboratory then constructed its own inserts. NHMFL pioneered the cable in conduit conductor (CICC) for use in superconducting outserts for hybrid magnets. It also built a 25 T hybrid magnet with conical access for installation at the Helmholtz-Zentrum Berlin (HZB), which is designed to be operated in combination with a neutron beam. Representatives from NHMFL have also provided advice on the building of the infrastructure at HZB. At the time this report was being written, Nijmegen and NHMFL were entering into a collaboration in which the Nijmegen hybrid magnet will be jointly designed and the cold body (the actual magnet inside the cryostat) will be produced at NHMFL’s facility in Tallahassee. This choice is largely based on the experience NHMFL has developed in this technology. As mentioned earlier, key engineers in laboratories in China earlier worked at the NHMFL and in European laboratories, and the experience these engineers acquired during their stays at those laboratories is now being used to build the Hefei and Wuhan laboratories.
All magnet laboratories have close connections to various industries that develop systems for installation at the laboratories or provide specific materials needed to produce high-field magnets. The extreme conditions required to produce and use the highest fields (high energy or power densities, extreme mechanical loads, high current densities, high stability of the power source, etc.) often go beyond what is needed for standard industrial products and thus push the limits of the industries involved. Information about possible suppliers is shared among the laboratories, which often leads to orders for these industries. Parts like housings and cables for the CICC require specialized windings and jacketing facilities that often are fabricated on different continents and shipped back and forth between the laboratories. In summary, there is a very intense informal collaboration among the main high-magnetic-field facilities worldwide, and knowledge is shared very effectively. It is clear that even higher magnetic fields like 50 T or even 60 T hybrids
or pulsed fields higher than 100 T will constitute an enormous financial and technological challenge. It will be necessary to combine all global experience and financial clout in this field to succeed. Encouraging international collaboration as much as possible will help to ensure that the conditions needed to meet these challenges are in place.
To produce even higher fields than are now available will be very costly. Fields around 30 T that are produced by all-superconducting magnets should become available in the coming decade. Such magnets will cost around $10 million and will require a specialized staff and environment to operate them. Such magnets probably will be installed in existing facilities and possibly in some new regional facilities. It is not realistic to expect that such magnets will become affordable for many smaller research groups, in contrast to the case when the 20 T superconducting magnets became commercially available.
At present, purely resistive dc magnets typically use around 24 MW of power at maximum field. Increasing the power will only marginally increase the maximum attainable field, so this type of magnet is becoming economically unattractive. The highest dc fields will therefore be produced with hybrid magnets. The operation of hybrid magnets requires the same power supply and cooling as purely resistive dc magnets, but they are operated in combination with a superconducting outer coil. The record for the highest field produced using such a magnet (as of the time of this report) is 45 T using 28 MW of power at the NHMFL; a 45 T magnet expected to require 22 MW of power is being built at Nijmegen. Higher-field hybrid magnets capable of producing fields at 50 T or even 60 T could easily cost more than $100 million. This is a very rough guess, since no realistic design with cost estimates has been produced to date. It is clear that such magnets will be very large (a minimum of several meters in all dimensions) and will require enormous amounts of materials. The size of such a project presumably will require global cooperation among the large magnet facilities. Consequently, the highest dc magnetic fields will be available at only a few dedicated facilities. It is expected that while these facilities work on the development of even better performing magnets, they will run a guest program for external users providing both the field and the necessary infrastructure and will maintain their own research program in order to stay at the forefront of science. This last point is essential to guarantee the availability of the most advanced instrumentation and the scientific excellence of the work done at the facility.
Regional centers with lower fields, possibly with new high-Tc magnets in the 30 T range, may also be created, since these magnets are too costly and cumbersome to become a normal laboratory commodity but could still be provided in dispersed facilities.
In pulsed fields, the 100 T record has now been established at the Los Alamos facility of the NHMFL. Much higher fields are not expected to be reached in the near term using nondestructive techniques. Development in the near term will probably concentrate on increasing reliability and user accessibility so that these record fields will become more widely available. It is expected that these top-end pulsed magnetic fields will be found only in a few facilities worldwide. As with dc laboratories, the central pulsed facilities will continue to work on magnet technology, provide service to external users, and pursue their own research programs.
RECOMMENDATIONS AND CONCLUSION
Recommendation: High-field facilities worldwide should be encouraged to collaborate as much as possible to improve the quality of magnets and service for users. This can be accomplished through the establishment of a global forum for high magnetic fields that consists of representatives of large magnetic field facilities from all continents. Such a forum would further stimulate collaboration and the exchange of expertise and personnel, thereby providing better service to the scientific community and magnet technology development. The forum should establish a roadmap for future magnets and stimulate the realization of the defined targets on this roadmap.
Recommendation: Large high-magnetic-field facilities should also have strong collaborations with smaller regional centers, providing them with support and expertise. Users of these regional centers may need the higher fields available in the large facilities, while users of the large facilities could be referred to the regional centers if their proposed experiments are better suited for those centers.
Conclusion: Success requires that the large facilities have a threefold mission: (1) to generate the highest possible magnetic fields by developing new magnets needed to produce those fields (magnet technology); (2) to make these fields, together with experimental support and expertise, available to qualified external users (act as a facility); and (3) to perform world-class research led by the facility staff.
