HTS SUPERCONDUCTING MAGNETS

HTS has played two roles in the development of superconducting magnets. HTS leads have made possible new classes of LTS magnets, and magnets employing HTS material in the windings have come on the market offering unique advantages.

Refrigerated LTS Magnets

HTS leads have served as an enabling technology for two new classes of LTS magnets, the "cryogen-free" conduction-cooled magnet, and the "zero helium consumption" magnet. These systems operate near 4 K yet consume no liquid cryogens. This mode of operation is particularly attractive in the Pacific Rim, where the cost of liquid helium is much higher than in the United States and Europe, and its availability is sometimes questionable. There are also operational advantages to these magnets: there is no need to train personnel in the handling of cryogens, there is no logistical problem associated with ensuring cryogen supply, and there are fewer safety concerns. Reports from Japanese universities where both conventional and cryogen-free superconducting magnets are installed indicate that the cryogen-free systems are much more heavily used.

As noted in the section above on HTS leads, the cryogen-free magnets require both HTS leads and a GM refrigerator capable of cooling the system to 4 K. The range of available refrigerators has limited the physical size of cryogen-free magnets available; at the time of this WTEC study, all fell in the class of research magnets, as opposed to larger MRI or industrial magnets. In Japan, systems are offered for sale by Kobe Steel, Toshiba, and Mitsubishi Electric Company (MELCO).

A "zero helium consumption" magnet more nearly resembles a conventional LTS magnet in that the coil is submerged in a bath of liquid helium. The cryostats on such magnets, however, are carefully designed to reduce heat leak into the system to a level that can be removed by GM refrigerators. The cryostats usually employ HTS current leads to achieve the necessary thermal performance. These systems normally employ two GM refrigerators. One, operating in the 20-50 K range intercepts radiation heat leak and conduction heat leak from the magnet supports, and it provides a high temperature heat station for the HTS leads. A second, lower-capacity refrigerator operating near 4 K is used to reliquefy the remaining helium boiloff. The major application of these systems has been for MRI magnets marketed in the Pacific Rim. Such magnets are made by MELCO for Shimadzu and by General Electric.

Kobe Steel

As reported in the site report (Appendix C), Kobe Steel in its joint venture with Magnex (Japan Magnet Technology) is addressing the broad spectrum of superconducting magnets for nuclear magnetic resonance (NMR), magnetic resonance imaging (MRI), and general research. This business is leveraged off Kobe Steel's basic strength in the manufacture of LTS conductors for NMR magnets. Fig. 4.20 shows its conduction-cooled magnet offering. This system is differentiated from that of other vendors by its ability to rotate on gimbals to provide both horizontal and vertical access to the room temperature bore. The technical significance of this is that GM refrigerators designed for operation at 4 K can often operate only in the vertical orientation.

Fig. 4.20. Conduction-cooled magnet offered by Kobe Steel, Ltd., and Japan Magnet Technology, Inc.

Toshiba

It appears that conduction-cooled magnets are less a business thrust for Toshiba than simply a logical extension of a very broad program in superconductivity and cryogenics. Toshiba, like Mitsubishi but unlike Kobe Steel, has developed its own 4 K GM refrigerator. With its LTS magnet and conductor experience and its HTS leads program, Toshiba has in-house all the technology needed to produce these systems. Its 1996 brochure lists 5 T and 10 T models (Fig. 4.21 and Table 4.2).

Fig. 4.21. Toshiba cryogen-free magnet.

Table 4.2
Typical Specifications, Toshiba Cryogen-Free Superconducting Magnets

Mitsubishi Electric Corporation (MELCO)

MELCO, like Toshiba, has an active program in GM refrigerator development and has applied this to conduction-cooled magnets. It appears to be marketing these magnets less aggressively than Kobe Steel to the retail market but has maintained a credible presence. The corporate literature (Fig. 4.22) features a 6 tesla Nb-Ti system, and during the site visit, the WTEC team was informed that MELCO has developed a klystron focusing magnet that operates at 3.8 K on a refrigerator. MELCO representatives also stated that they see conduction-cooled magnets as one of the fastest growing segments of the superconductivity market.

Fig. 4.22. Cryogen-free magnet from Mitsubishi Electric Corporation.

HTS Conduction-Cooled Magnets

While all of the conduction-cooled magnets described above offer the convenience of "dry" operation, they are very limited in the rate at which the magnetic field can be varied. Both the Kobe and Toshiba 10 T systems require one hour to reach rated field. While this is merely an inconvenience for many applications, it does preclude use of the magnets for applications where time-varying fields are required. This limitation, which arises from the fact that significant refrigeration capacity would be required to extract the heat generated in the superconductor, is unlikely to be relaxed in the near future, since 4 K GM refrigerators will always have significantly less cooling capacity than those operating at higher temperatures.

In contrast to the systems described above, Sumitomo has built a series of conduction-cooled all-HTS magnets that operate in the 20-30 K temperature range. These systems offer the advantages of very high operational stability and the ability to ramp very quickly (Fig. 4.23). The disadvantage at the present time is that the higher cost and lower performance of HTS material at 20 K compared to LTS material at 4 K increases the cost and reduces the maximum field of the HTS offerings. It appears that only two firms, Sumitomo in Japan and American Superconductor in the United States, produce cryocooled HTS magnet systems.

High Field NMR Magnets

Nuclear magnetic resonance is a widely used analytical tool in the medical, chemical, and biological industries and is seeing increasing application in industrial process control. NMR's ability to discriminate between different molecular structures is enhanced by operation at higher frequencies, i.e., at higher magnetic fields. As a result, there has been a steady increase in the operating field of both commercial and research NMR systems, and today commercial systems are available up to 750 MHz. Various vendors are addressing 800 MHz systems, and Oxford Instruments has a contract from the U.S. Department of Energy for the construction of an experimental 900 MHz system. This system is believed to be near the operational limits of doped Nb₃Sn conductor, and it is generally accepted that NMR above 900 MHz (~21.1 tesla) will require the use of HTS insert magnets. HTS conductor has higher current density than Nb₃Sn above about 16 tesla but has not demonstrated the ability to operate in a persistent mode. Several laboratories and companies are attempting to develop persistent mode systems while simultaneously placing into operation nonpersistent research magnets using HTS insert coils.

Fig. 4.23. Sumitomo conduction-cooled magnet (Nakahara 1996).

Perhaps the most well-delineated commercial strategy is that of Kobe Steel, which sees market opportunity up to 1.5 GHz for HTS magnets. This is discussed in detail in the Kobe Steel site report (Appendix C). The primary location for the development of high field magnets in Japan is the National Research Institute for Metals (NRIM). This is the site of the multicore program for development of 1 GHz NMR. Japan's Multicore Phase II program provides about ¥170 million per year until 1999 for the 1 GHz NMR project. The emphasis of this program has been on the development of Bi-2212 dip-coated and silver-sheathed tape with high critical current at high field, and on the development of persistent mode operation.

Hitachi Research Laboratory and Hitachi Cable, Ltd., are the collaborators for the silver-sheathed Bi-2212 effort, with measurements being performed at the new Tsukuba Magnet Laboratories of NRIM. The program involves measuring insert coils in the bore of a 21 tesla background magnet. The results shown in Fig. 4.24 indicate the ability to produce conductor J_c approaching 1,000 A/mm² and coil current density of ~130 A/mm² at 4.2 K in a background field of 21 tesla. According to Okada et al. (1996), these current densities are sufficient to allow fabrication of a 1 GHz NMR magnet.

Fig. 4.24. Performance of Hitachi silver-sheathed Bi-2212 multifilamentary conductor (Okada et al. 1996).