Background
Cryogenics
Large Scale Applications
Electronics Applications
Background
Superconductor technology combining proper electrical, mechanical and thermal management of nano-engineered materials, allows solutions ranging from power components operating at current densities 100 times higher than copper to quantum-based electronic circuits. Key features are higher efficiencies, higher currents, fields and forces, higher power densities, smaller weight and size, higher resolutions, quantum-precision sensitivities or ultra-high speed. More detailed information for various devices can be found under SUPERCONDUCTIVITY. Such intrinsic performance advantages make superconductors a first choice for overcoming technological barriers.
Although superconductors in several respects offer the ultimate technical performance and unique functionalities on the device level, it is not at all straightforward to convert this into technical superiority on the system level. This requires not only application-specific developments for superconducting components, but also parallel developments for other system parts. Regarding the future customer, operation at cryogenic temperatures uses a technology which is unfamiliar and still needs further proof of reliability under rough daily operating conditions. Thus being a cross sectional technology, progress in more than only one technology is needed for systems with substantially new features providing additional benefits to the customer.
Considerable progress has been made within the past 20 year in advances in new materials and fabrication processing. Several successful system demonstrations and field tests have been carried out. In a couple of segments HTS-based components and systems are already well-established. The broad breakthrough, however, has not yet been achieved: In several cases the technical performance is not yet comparable with LTS and not yet sufficient for the anticipated application. In other cases the cost for the devices is still significantly higher than for LTS or conventional non-superconducting systems and thus the required investment outweighs the advantage of lower operation costs. In summary, these new materials hold great economic potential, but sustained efforts are still required in order to further increase the technical performance, and in order to further bring down process complexity and cost. It may be noted that time scales of about 20 years from the discovery of a novel physical phenomenon to first practical prototypes in pilot applications are quite common. Realistic forecasts have to account for such time scales in technology and market development. LTS magnets, which were first wound in the early sixties, only came to be used on a large scale in the eighties when MRI took off. Information about status and perspective of market development is presented under MARKET.
Cryogenics
One key issue for the broader implementation of superconductive components is cryogenics i.e. the cryogenic packaging or insulation, the reliable supply of liquid cryogens like Helium (LHe, 4.2K), Hydrogen (LH2, 20K), Neon (LNe, 27K) and Nitrogen (LN2, 77K) or, alternatively, the reliable operation of cryocoolers adequate for the specific application. Another issue is that superconducting systems not only need optimised cryogenic facilities, but also (non-superconducting) materials and components which concerning their mechanical, thermal and electrical properties have to be qualified for low temperature operation.
increase of power requirements for cooling with decreasing operating temperature
Although time intervals for the refill of LHe tanks used in MRI or NMR, are already in the range of several years, for many applications only cryocoolers are acceptable for customers. Maintenance-free operation for several years has been demonstrated for cryocoolers as well, but these specially developed refrigerators are up to now, produced in small series only and are therefore rather costly. Thus cost-effective, reliable long-life cryocoolers are essentially "only" a question of quantities or the corresponding market pull to bring prices down. In this respect the Pulse Tube Cryocooler is generally seen as a particularly promising candidate for a reliable and low-cost cryocooler.
examples for cryocoolers of the Stirling type and the Gifford McMahon type (one- and two-stage cryocooler)
Large Scale Applications
Large scale applications are mainly based on conductors i.e. wires, tapes and cables, and to a smaller extent on sheets, coatings and bulk parts. The field may be roughly divided into two groups: magnets as well as other high-field components with perfomances otherwise technically not achievable, and low-to-medium-field high-current components which are in economic competition with normalconducting solutions.
Nearly all LTS applications utilize wires and cables based on NbTi, Nb3Sn or other A15 compounds. Different applications which essentially are all magnet applications require a variety of different types of conductors. In general, LTS wires represent a mature viable technology today providing a solid basis for magnet applications in science, research and technological development (RTD) incl. Nuclear Magnetic Resonance Spectroscopy (NMR), Magnetic Resonance Imaging (MRI) and new emerging, mostly industrial applications. Due to the sometimes very tough requirements as regards the conductor technology, these high-current high-field applications will essentially remain LTS-based for the next several years.
some examples of multi-filament LTS wires
Traditionally the first significant market for superconductivity were magnets for science, research and technological development (RTD) which covers a broad range of different types of coils: from rather small laboratory magnets up to huge and sometimes quite complex structures for big science projects in high-energy physics like high-energy particle colliders or fusion experiments. Based on the mature LTS conductor technology a great variety of different shapes and sizes for high-field coils are available today.
some examples for superconducting coils of sometimes rather complex shape and significant size
Nuclear Magnetic Resonance Spectroscopy (NMR) requires the currently highest magnetic fields with ultimate spatial homogeneity and temporal stability. NMR then allows to monitor e.g. organic macro molecules with highest spectral resolution thereby providing an increasingly important analytical tool for the pharmaceutical industry and other life sciences. This has resulted in very significant growth rates for very high-field superconducting magnets.
900 MHz superconducting NMR system for studies of various biological macromolecules at Yokohama City University
The by far biggest market for superconductivity today is Magnetic Resonance Imaging (MRI) which started off at the beginning of the eighties. It has become a well established diagnostic tool routinely used in hospitals. In addition to the use of whole-body systems using quite big solenoid coils, also smaller open systems based on split coils have raised growing interest over the last years. These diagnostic systems allow free access to the patient e.g. during an operation.
MRI systems routinely used as a standard diagnostic tool in hospitals and several doctors' practices
For quite a while efforts have been made to open up new markets for superconducting magnets. A field that especially due to deregulations of electric utilities has recently found increased interest is the Superconducting Magnetic Energy Storage (SMES) e.g. for uninterruptable power sources at customer sites or to stabilize fluctuations of the electric grid. Other emerging new businesses are seen in the fields of industrial processing like magnetic separation, transportation based on magnetic levitation, and new medical applications like Positron Emission Tomography or cancer treatment using proton therapy.
installed 2MJ LTS-SMES (left) and mini undulator for increasing the brightness of synchrotron radiation sources (right)
concept (left) and realized 250 MeV proton cyclotron for proton therapy (right)
Another major LTS large scale application which is not based on wires, but on Nb metal sheets or coatings, are high-frequency cavities and systems. The ultra-high quality factors and the excellent power-handling capability of these Nb resonators make them the first choice for transmitting high microwave power to electrons, protons and ion beams in a variety of accelerators e.g. used in huge particle colliders of high-energy physics, in synchrotron radiation sources or in Free Electron Lasers.
superconducting cavities for accelerators of the TESLA type (left) and assembled module for CERN / LEP (right)
In addition to these existing LTS applications, also several electric power components like transmission cables, current limiters, transformers or generators were fabricated with LTS wires and, from the technical point of view, successfully tested already quite a time ago. Cost, however, especially related to the very low operating temperature of LHe, prevented a practical implementation of these LTS devices. For this reason it is anticipated that most of the emerging new businesses will be based on HTS given that these new materials can be developed to techno-economic maturity. Depending on the specific application, different materials are currently developed. Essentially two classes of HTS are pursued for large scale applications: Bismuth-based HTS (BSCCO) for melt-cast bulk parts and for wires and tapes, and 123-HTS with YBCO as the most prominent representative for melt-textured bulk parts and for coated conductors. New options are also seen for MgB2 which concerning operating temperature and manufacturing complexity lies somewhere between LTS and HTS.
Starting from powders, rods, tubes and melt-textured bulk parts have been fabricated and are available for applications like targets for thin film deposition, current leads, magnetic bearings, fault current limiters etc.
from powder to shaped bulk parts based on BSCCO, YBCO and MgB2
One application that was introduced fairly early are HTS current leads. They are used in LTS magnet systems to reduce the thermal load which is related to feeding high currents to the LTS windings.
BSCCO rods for current leads in LTS magnet systems (left) and completed 13 kA current lead for CERN / LHC (right)
One application that has been pursued with rather different materials and fabrication technologies is the fault current limiter. As a self-switching and self-recovering device it offers, compared with existing solutions, a new functionality of network operation i.e. for controlling short-circuits in electric grids.
HTS fault current limiter based on melt-cast BSSCO: system design and HTS components for voltages up to 110 kV
Melt-textured 123-HTS bulk parts, in particular YBCO, have been used in demonstrations for magnetic bearings which due to their self-stabilization do not require any active position control. YBCO bulk parts have also been employed as rotor parts of very compact electric motors thereby contributing to a size and weight reduction of about 2-3. Both are usually operated at LN2 temperatures.
contactless bearing for a 4 MVA HTS generator: mounting of 270 HTS cylinders (left) and cryogenic housing (right)
40kW (left) and 200 kW (right) 3000 rpm reluctance motors with YBCO bulk parts in the rotor (diameter 70mm / 148 mm)
For a couple of years BSSCO tapes have been commercially available in long lengths with different specifications for different purposes, but the conductor cost is still a strong issue for several applications. Coated conductors based on 123-HTS could withstand higher magnetic fields at higher operating temperatures than BSCCO, but are still in an earlier development stage, and cost is the major issue, too. For moderate magnetic fields and operating temperatures also MgB2 could become a cost-effective alternative.
some examples of multi-filament HTS tapes (top) and of wound HTS coils (bottom)
Superconducting cables and transformers offer not only reductions of losses, size and weight, but also oil-free operation. These aspects are of relevance e.g. in densely populated cities when the electric grid has to be upgraded, or in mobile applications like trains. Because of the low magnetic field these devices which have been realized with BSCCO so far, can be operated at LN2 temperatures.
concept and realization of a three-phase power cable, installation of cables
comparison of super- (left) and normalconducting railway-transformers
Superconducting motors and generators primarily aim at higher efficiencies, weight and size reductions, but they also offer a stiffer operational mode i.e. a reduced dependence on fluctuations of the supply grid (motors) or on load fluctuations on the customer side (generators).
BSCCO race-track coil for a 400 kW synchronous motor operated around LNe temperature
4 MVA BSCCO-HTS synchronous generator at 3600 rpm (in blue) designed for mobile installations like ships
In addition to power applications, HTS tapes have also been tested in other application fields e.g. magnetic separation or open MRI systems. For ultra-high resolution NMR systems (1000 GHz and higher) HTS insert coils are currently under development.
cryogenfree 0.2 T BSCCO HTS magnet for whole body open MRI
Electronics Applications
Electronics applications are based on superconducting thin films usually embedded in multi-layer structures, and active devices utilize Josephson Junctions (JJs) which represent highly non-linear contacts between two weakly coupled superconductors, as sensing and switching elements. Devices may be roughly divided into three groups: passive high-frequency and microwave devices, Superconducting Quantum Interference Devices (SQUIDs) and other analog devices based on Josephson Junctions and finally, integrated circuits for digital signal processing.
On the LTS side devices are mainly based of the advanced Nb technology, which today allows to fabricate circuits with tens of thousands of JJs. To a smaller extend also NbN with a somewhat more limited junction technology has been employed.
Single junction devices have been used as microwave frequency-mixers in radioastronomy, developments targeting e.g. X-ray detectors or far-infrared sensors have been undertaken with some success and due to their exceptional performance they are established in their special fields. The same is true for the Josephson voltage standards worldwide routinely used in metrological laboratories, which require already thousands of JJs. So far the overall markets for these high-precision devices, however, are very small.
SQUIDs with their otherwise non-achievable ultra-high magnetic field sensitivity have for a long time been in use for materials characterization, scientific instrumentation and some very special applications like geophysical exploration, but also for Magnetocardiography (MCG) and Magnetoencephalography (MEG). Up to now, the MCG and MEG systems installed in more than hundred hospitals all over the world, have been mainly used for clinical research on heart and brain. Clinical tests carried out recently, however, may soon lead to the official approval by authorities and health insurances, so that MCG or even MEG could become a routinely used diagnostic tool for hospitals and even doctors.
Integrated circuits for digital signal processing are also based on JJs as active switching elements and SQUID-like structures, but require, depending on the specific circuit, thousands or millions of them. The combination of ultra-high switching speed, ultra-low switching losses and nearly distortion-free signal transmission make them the perfect choice where semiconductors have reached the performance limits. Concrete developments target Analog-to-Digital-Converters (ADCs) especially for radio communication systems and routers for directing large data streams in communication networks.
LTS circuit: analog to digital converter
In the field of HTS 123-HTS and Thallium-based HTS have been successfully used to fabricate passive devices. The currently available JJ technology which is exclusively based on 123-HTS, has not yet reached the degree of reproducibility required for higher integrated circuits.
an early five pole elliptic HTS filter structure (left) and currently installed filter systems (right)
Passive high-frequency and microwave devices which utilize the ultra-low high-frequency losses of high-quality HTS thin films. Devices like high-frequency antennas as sensors in NMR or several microwave components have been manufactured. The potentially largest market in this segment are filter systems for ground- or satellite based wireless communication systems. Attractive features are an improved coverage in rural areas and better usage of limited transmission bandwidths due to reduced interferences in densely populated areas.
tower-mounted cryogenic front-ends of base stations for wireless communication
Although a JJ technology that allows to fabricate devices with several 100 JJs is not yet at hand, HTS SQUIDs based on 123-HTS have been fabricated and used e.g. for geophysical exploration, for non-destructive testing e.g. of turbines, steel structures in bridges, aircrafts etc., but also for MCG. Some first successful demonstrations of digital circuits have been made, too, and developments towards ADCs and routers are on the way.
Last Revision: 1 October 2006
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