Superconducting magnet
A superconducting magnet is an
Construction
Cooling
During operation, the magnet windings must be cooled below their
Liquid-cooled
Mechanical cooling
Because of increasing cost and the dwindling availability of liquid helium, many superconducting systems are cooled using two stage mechanical refrigeration. In general two types of mechanical cryocoolers are employed which have sufficient cooling power to maintain magnets below their critical temperature. The Gifford–McMahon Cryocooler has been commercially available since the 1960s and has found widespread application.[1][2][3][4] The G-M regenerator cycle in a cryocooler operates using a piston type displacer and heat exchanger. Alternatively, 1999 marked the first commercial application using a pulse tube cryocooler. This design of cryocooler has become increasingly common due to low vibration and long service interval as pulse tube designs use an acoustic process in lieu of mechanical displacement. In a typical two-stage refrigerator, the first stage will offer higher cooling capacity but at higher temperature (≈77 K) with the second stage reaching ≈4.2 K and < 2.0 W of cooling power. In use, the first stage is used primarily for ancillary cooling of the cryostat with the second stage used primarily for cooling the magnet.
Coil winding materials
The maximal magnetic field achievable in a superconducting magnet is limited by the field at which the winding material ceases to be superconducting, its "critical field", Hc, which for
The superconducting portions of most current magnets are composed of niobium–titanium. This material has critical temperature of 10 K and can superconduct at up to about 15 T. More expensive magnets can be made of niobium–tin (Nb3Sn). These have a Tc of 18 K. When operating at 4.2 K they are able to withstand a much higher magnetic field intensity, up to 25 T to 30 T. Unfortunately, it is far more difficult to make the required filaments from this material. This is why sometimes a combination of Nb3Sn for the high-field sections and NbTi for the lower-field sections is used. Vanadium–gallium is another material used for the high-field inserts.
Conductor structure
The coil windings of a superconducting
Operation
Power supply
The current to the coil windings is provided by a high current, very low voltage DC power supply, since in steady state the only voltage across the magnet is due to the resistance of the feeder wires. Any change to the current through the magnet must be done very slowly, first because electrically the magnet is a large inductor and an abrupt current change will result in a large voltage spike across the windings, and more importantly because fast changes in current can cause eddy currents and mechanical stresses in the windings that can precipitate a quench (see below). So the power supply is usually microprocessor-controlled, programmed to accomplish current changes gradually, in gentle ramps. It usually takes several minutes to energize or de-energize a laboratory-sized magnet.
Persistent mode
An alternate operating mode used by most superconducting magnets is to
where is a small residual resistance in the superconducting windings due to joints or a phenomenon called flux motion resistance. Nearly all commercial superconducting magnets are equipped with persistent switches.
Magnet quench
A quench is an abnormal termination of magnet operation that occurs when part of the superconducting coil enters the normal (
A large section of the superconducting magnets in CERN's Large Hadron Collider unexpectedly quenched during start-up operations in 2008, necessitating the replacement of a number of magnets.[6] In order to mitigate against potentially destructive quenches, the superconducting magnets that form the LHC are equipped with fast-ramping heaters that are activated once a quench event is detected by the complex quench protection system. As the dipole bending magnets are connected in series, each power circuit includes 154 individual magnets, and should a quench event occur, the entire combined stored energy of these magnets must be dumped at once. This energy is transferred into dumps that are massive blocks of metal which heat up to several hundreds of degrees Celsius due to the resistive heating in a matter of seconds. Although undesirable, a magnet quench is a "fairly routine event" during the operation of a particle accelerator.[7]
Magnet "training"
In certain cases, superconducting magnets designed for very high currents require extensive bedding in, to enable the magnets to function at their full planned currents and fields. This is known as "training" the magnet, and involves a type of material memory effect. One situation this is required in is the case of
History
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Although the idea of making electromagnets with superconducting wire was proposed by Heike Kamerlingh Onnes shortly after he discovered superconductivity in 1911, a practical superconducting electromagnet had to await the discovery of superconducting materials that could support large critical supercurrent densities in high magnetic fields. The first successful superconducting magnet was built by G.B. Yntema in 1955 using niobium wire and achieved a field of 0.7 T at 4.2 K.[10] Then, in 1961, J.E. Kunzler, E. Buehler, F.S.L. Hsu, and J.H. Wernick made the discovery that a compound of niobium and tin could support critical-supercurrent densities greater than 100,000 amperes per square centimetre in magnetic fields of 8.8 teslas.[11] Despite its brittle nature, niobium–tin has since proved extremely useful in supermagnets generating magnetic fields up to 20 T.
The persistent switch was invented in 1960 by Dwight Adams while a postdoctoral associate at Stanford University. The second persistent switch was constructed at the University of Florida by M.S. student R.D. Lichti in 1963. It has been preserved in a showcase in the UF Physics Building.
In 1962, T.G. Berlincourt and R.R. Hake[12] discovered the high-critical-magnetic-field, high-critical-supercurrent-density properties of niobium–titanium alloys. Although niobium–titanium alloys possess less spectacular superconducting properties than niobium–tin, they are highly ductile, easily fabricated, and economical. Useful in supermagnets generating magnetic fields up to 10 teslas, niobium–titanium alloys are the most widely used supermagnet materials.
In 1986, the discovery of
In 2007, a magnet with windings of
Globally in 2014, about five billion euros worth of economic activity resulted from which superconductivity is indispensable. MRI systems, most of which employ niobium–titanium, accounted for about 80% of that total.[14]
In 2016, Yoon et al. reported a 26 T no-insulation superconducting magnet that they built out of GdBa2Cu3O7–x,[15] using a technique which was previously reported in 2013.[16]
In 2017, a YBCO magnet created by the National High Magnetic Field Laboratory (NHMFL) broke the previous world record with a strength of 32 T. This is an all superconducting user magnet, designed to last for many decades. They hold the current record as of March 2018.
In 2019, a new world-record of 32.35 T with all-superconducting magnet is achieved by Institute of Electrical Engineering, Chinese Academy of Sciences (IEE, CAS).[17] No-insulation technique for the HTS insert magnet is also used.
In 2019, the NHMFL also developed a non-insulated YBCO test coil combined with a resistive magnet and broke the lab's own world record for highest continuous magnetic field for any configuration of magnet at 45.5 T.[18][19]
A 1.2 GHz (28.2 T) NMR magnet[20] was achieved in 2020 using an HTS magnet.[21]
In 2022, the Hefei Institutes of Physical Science, Chinese Academy of Sciences (HFIPS, CAS) claims new world record for strongest steady magnetic field of 45.22 T reached,[22][23] while the previous NHMFL 45.5 T record in 2019 was actually reached when the magnet failed immediately in a quench.
Uses
Superconducting magnets have a number of advantages over
Superconducting magnets are widely used in
Rail transport
In Japan, after decades of research and development into superconducting maglev by Japanese National Railways and later Central Japan Railway Company (JR Central), the Japanese government gave permission to JR Central to build the Chūō Shinkansen, linking Tokyo to Nagoya and later to Osaka.[citation needed]
Particle accelerator
One of the most challenging uses of SC magnets is in the
Fusion reactor
The central solenoid and toroidal field superconducting magnets designed for the ITER fusion reactor use niobium–tin (Nb3Sn) as a superconductor. The Central Solenoid coil carries 46 kA and produce a field of 13.5 T. The 18 toroidal field coils at a maximum field of 11.8 T store 41 GJ (total?).[clarification needed] They have been tested at a record 80 kA. Other lower field ITER magnets (PF and CC) use niobium–titanium. Most of the ITER magnets have their field varied many times per hour.
Mass spectrometer
One high-resolution mass spectrometer planned to use a 21-tesla SC magnet.[25]
See also
References
- ^ Gifford, W. E.; Longsworth, R. C. (1964), Pulse tube refrigeration (PDF), Trans. ASME, J. Eng. Ind. 63, 264
- ^ Gifford, W. E.; Longsworth, R. C. (1965), Surface heat pumping, Adv. Cryog. Eng. 11, 171
- ^ Longsworth, R. C. (1967), An experimental investigation of pulse tube refrigeration heat pumping rate, Adv. Cryog. Eng. 12, 608
- Bibcode:2011TRACE..11...89M
- ^ 1. Adams, E.D.; Goodkind, J.M. (1963) "Cryostat for Investigations to Temperatures below 0.02 K". Cryogenics 3, 83 (1963)
- ^ "Interim Summary Report on the Analysis of the 19 September 2008 Incident at the LHC" (PDF). CERN.
- SLAC. Retrieved 15 February 2013.
- ^ Restarting the LHC: Why 13 Tev? | CERN. Home.web.cern.ch. Retrieved on 2015-12-19.
- ^ a b c First LHC magnets prepped for restart. symmetry magazine. Retrieved on 2015-12-19.
- ^ Yntema, G.B. (1955). "Superconducting winding for electromagnets". Physical Review. 98 (4). APS: 1197. .
- ^ Kunzler, J.E.; Buehler, E.; Hsu, F.S.L.; Wernick, J.H. (1961). "Superconductivity in Nb3Sn at High Current Density in a Magnetic Field of 88 kilogauss". Physical Review Letters. 6 (5). APS: 890. .
- ^ Berlincourt, T.G.; Hake, R.R. (1962). "Pulsed-Magnetic-Field Studies of Superconducting Transition Metal Alloys at High and Low Current Densities". Bulletin of the American Physical Society. II (7). APS: 408.
- ^ "New mag lab record promises more to come". News Release. National High Magnetic Field Laboratory, USA. August 7, 2007. Archived from the original on 2008-10-12. Retrieved 2008-10-23.
- ^ "Conectus – Market". www.conectus.org. Archived from the original on 2014-08-11. Retrieved 2015-06-22.
- S2CID 124134119.
- PMID 24255549.
- S2CID 213171620.
- ^ Larbalestier, David (June 12, 2019). "With mini magnet, National MagLab creates world-record magnetic field". News Release. National High Magnetic Field Laboratory, USA. Retrieved 2020-07-31.
- ^
Hahn, S. (June 12, 2019). "45.5-tesla direct-current magnetic field generated with a high-temperature superconducting magnet". Nature. 570 (7762): 496–499. S2CID 186207595. Retrieved 2020-07-31.
- ^ "GHz Class NMR | Ultra High Magnetic Field". www.bruker.com. Retrieved 2022-08-16.
- PMID 33927545.
- ^ Xiaomin, Huang; Shu, Zhou; Sciences, Chinese Academy of. "China claims new world record for strongest steady magnetic field". phys.org. Retrieved 2022-08-16.
- ^ "刷新世界纪录!国家稳态强磁场实验装置产生最高稳态磁场". m.thepaper.cn. Retrieved 2022-08-16.
- ^ Operational challenges of the LHC. cea.fr
- ^ "Bruker Daltonics Chosen to Build World's First 21.0 Tesla FT-ICR Magnet". 29 October 2010.
Further reading
- Martin N. Wilson, Superconducting Magnets (Monographs on Cryogenics), Oxford University Press, New edition (1987), ISBN 978-0-19-854810-2.
- Yukikazu Iwasa, Case Studies in Superconducting Magnets: Design and Operational Issues (Selected Topics in Superconductivity), Kluwer Academic / Plenum Publishers, (October 1994), ISBN 978-0-306-44881-2.
- Habibo Brechna, Superconducting magnet systems, New York, Springer-Verlag New York, Inc., 1973, ISBN 0-387-06103-7
External links
- Making Superconducting Magnets From the National High Magnetic Field Laboratory
- 1986 evaluation of NbTi and Nb3Sn for particle accelerator magnets.