Today superconductors can be found in applications as commonplace as MRI systems installed in thousands of hospitals and cellular telephone base stations as well as monumental applications such as particle accelerators. In figure 1 we illustrate two major commercial applications, Magnetic Resonance Imaging, MRI, and high field Nuclear Magnetic Resonance, NMR. The difference in bore size is quite substantial. This is section is the largest section in the book and contains 26 articles. Many of the applications covered have perfectly viable conventional competitors that offer lower cost and higher consumer acceptance. A feature of this book is that we fully examine the conventional alternative. As superconductor properties continue to improve we can expect that more and more applications will find in superconductivity a technological or economic edge.

Figure 1 Low field MRI and high field NMR: a) GE Signa OpenSpeed MRI. This open MRI uses superconducting strand to create a highly stable 0.7 T field. Image courtesy of GE Medical Systems. b) 21.3 T Nb3Sn based superconducting magnet manufactured by Oxford Instruments plc. for a high resolution 900 MHz NMR system by Varian, Inc.. Image courtesy of Oxford Instruments, plc.

An excellent case in point would be the application of HTS microwave filters in cellular telephone base stations. Superconductor is almost lossless at microwave frequencies and extremely sharp microwave filters can be made, using large numbers of poles, without incurring significant insertion losses. This makes them ideal for use as preselect filters in wireless base stations (1). The combination of high T_c and improved advances in cryocooling means that microwave filter systems can be supplied for base stations in compact, hot-swappable rack- mount units (2). Because they produces clearer signals they not improve service quality but they can increase service area or allow base stations to be built further apart. The accelerated pace at which these units are being installed reflects the competitive economics of the new technology.

Figure 2. Small and large scale superconductors and their applications: a) YBa2Cu3O7-d based microwave filter for b) rack mount front-end receiver subsystem for cellular telephone base stations, images courtesy of Conductus, Inc.(now STI) c) 136 kg Nb-47wt.%Ti alloy billets are destined for accelerator application. Each billet is 750 mm tall by 200 mm diameter. Image courtesy of Wah Chang. d) A view of the superconducting magnets at Brookhaven National Laboratory's Relativistic Heavy Ion Collider (RHIC). The 1,740 Nb-Ti based magnets guide and focus gold particles along the collider's 4 km long tunnel at nearly the speed of light. Image courtesy of Brookhaven National Laboratory.

Superconducting current leads

The large currents the superconductors can carry must somehow be carried to the cooled superconductor from ambient temperature. Prior to HTS this has usually been the primary loss of heat for superconducting devices because metals with high enough electrical conductivity to carry the huge currents also have high thermal conductivity. This heat loss has, in the past, significantly reduced the cost effectiveness of superconductor applications. HTS represents a breakthrough in this area because ceramics are not good thermal conductors. Of course they cannot take supercurrent all the way to room temperatures (as of this writing) and account must be made of their sensitivity to magnetic field but they can be used as a high current thermal block. Combination normal/superconducting current leads are commercially available using BSCCO 2212 and 2223 and YBCO HTS superconductors. HTS leads make both LTS and HTS superconductor applications more economically viable.

And what of the future?

We have come to expect a continuous improvement in superconductor properties and nothing that we have seen over the last twenty years leads us to expect otherwise. This certainly impacts power and other applications where the competition is copper, which is very little improvement over the last fifty years. Increasing energy costs (and the effects of deregulation in the U.S.) will also benefit the competitiveness of superconductors in power applications such as generators (3). Environmental issues should also positively impact the application of superconducting power transmission cables. One such cable is already being installed at the Detroit Edison Frisbee substation in downtown Detroit and is expected to triple power throughput in an existing facility. For that project American Superconductor shipped 4.25 miles of HTS wire to Pirelli for cable assembly in August 2000. SMES (see: Power Quality, section: MicroSMES) is already commercially available today and is competitive with conventional systems for some applications. A 1000 hp superconducting is being developed using HTS technology (4). The development of 10 kWh superconducting flywheel system was initiated by Boeing and ANL in 1999. There are several investigation of HTS based fault current limiter technologies that look promising (see Superconducting Fault Current Limiters). High temperature superconducting (HTS) transformers are being developed including one by IGC, Waukesha Electric and ORNL which has so far resulted in a 3-phase, 1 MVA unit (5).

Improved properties developed in Nb₃Sn strand during the R&D production phases for the CS model coil of the International Thermonuclear Experimental Reactor, ITER, have made possible much lower cost alternatives to the original design that could make Fusion power a reality. The success of the of the ITER CS model coil shows that brittle superconductors, like A15 Nb₃Sn, can be made into very large scale magnets that can be used at high fields in demanding applications (see Figure 2).

Figure 2 a) The world's most powerful pulsed superconducting magnet, the150 ton 13 T (approximately 260 thousand times more powerful than the earth's magnetic field) ITER CS Model. The magnet consists of two modules, the inner module (b) fabricated in the US and the outer fabricated in Japan. The two coils were combined at the Naka Fusion Research Establishment test facility of the Japan Atomic Energy Research Institute, JAERI. Photos courtesy of and copyright retained by JAERI.

In 2005 the Large Hadron Collider (LHC) ring at CERN in Switzerland will contain of over 1000 superconducting bending magnets, each 13 metres long, manufactured using Nb-Ti superconductor. The magnets will be cooled down to 1.8 K and provide a field of 8.65 T. The designed collision energy will be 14 TeV. Already superconductors are being developed that can make a 100 TeV collider possible.

Bibliography

1.            M. J. Scharen, D. R. Chase, A. M. Ho, A. O'Baid, K. R. Raihn and R. J. Forse, Filter subsystems for wireless communications, IEEE-Trans. Appl. Superconductivity, 7 (2.3): 3744-9, 1997.

2.            Y. Ueno, N. Sakakibara, T. Yamada, M. Okazaki, and M. Aoki, High-temperature superconducting receiving filter subsystem for mobile telecommunication base station, IEICE Trans. Electronics, E82-C (7):1172-6, 1999.

3.            W. V. Hassenzahl, Applications of superconductivity to electric power systems, IEEE Power Engineering Review, 20 (5): 4-7, 2000. And  W. V. Hassenzahl, More applications of superconductivity to electric power systems, IEEE Power Engineering Review, 20 (6): 4-6, 2000.

4.            D. Driscoll, V. Dombrovski, and B. Zhang, Development Status of Superconducting Motors, IEEE Power Engineering Review, 20 (5): 12-15, 2000.

5.            B. W. McConnell, S. Mehta, and M. S. Walker, HTS Transformers, IEEE Power Engineering Review, 20 (6): 7-11, 2000.

6.            A history of MRI can be found at: http://www.isbe.man.ac.uk/personal/dellard/dje/history_mri/history%20of%20mri.htm

Excerpted from "Engineering Superconductivity," ed. Peter J. Lee, Wiley-Interscience, New York, 2001