A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

SUPERCONDUCTING MAGNETS

DOI: 10.1615/AtoZ.s.superconducting_magnets

Superconducting magnets are electromagnets wound with superconductors. SM's are used to generate high magnetic fields.

The SM windings need to be cooled below certain temperatures specific to particular superconductors and known as their critical temperatures. Most often they are cooled to 4.2 K (boiling point of liquid helium under atmospheric pressure), although on some occasions the temperature can be lowered down to 1.8 K.

One of the characteristic features of an SM’s that partially determine their operating parameters is the possibility of the superconducting coils going to the "normal" or resistive state (i.e., having some finite resistance). Such transitions can be triggered by some external interactions (vibrations, abrupt current changes, etc.) but, what is more important, they can arise as a result of the growth of unstable perturbations inside the windings proper. The heat generated inside the resistive zone warms nearby portions of the windings driving them also to become resistive so that, at the end, entire windings or a significant portion of their) can be overheated. Serious damage can result from this overheating or from other assosiated processes.

Stabilization techniques have been devised to provide stable and reliable operation of SM's. The use of so called "cryostatic stabilization" allows almost complete exclusion of the possibility of superconducting windings going normal. Large quantities (sometimes even more than 90%) of resistive metals (usually copper or aluminum) are used in this case within so-called composite conductors along with superconducting wires. The resistivity of such metals at low temperatures is rather small so that Joule losses that can be the result of eventual normal transitions must be quite low. Sufficient heat removal rate from the windings must also be provided so that such eventual transitions would not lead to the uncontrolled growth of the normal region. The overall current density that determines the dimensions of the coils can unfortunately be quite low in such magnets that can lead to their large sizes and masses. More intricate methods are also being developed when normal transitions are permitted but they are arranged such that no significant damage would be produced.

Solid state controlled rectifiers are most widely used as current supplies for SM's, though DC generators and accumulator batteries also find a more limited application. The zero resistance of SM windings allows the use of some peculiar low voltage devices as current supplies, e.g., the so-called flux pump, topological generators, cryotrons (superconducting analog of SCR), etc. Very peculiar operating modes are sometimes used with an SM, e.g., to provide extreme current and field stability, when after being charged with current from an external source the windings of the SM can then be shunted by an entirely superconducting circuit so that the magnetic flux becomes effectively "frozen-in". The current supply can then be turned off and it is possible, furthermore, to remove external current leads (e.g., in order to lessen heat losses). It is also possible to generate AC fields as modern technologies permit the reduction of the inevitable AC losses to a reasonable level.

The largest SM's have been built for experimental physics devices, such as accelerators, controlled nuclear fusion installations, particle detectors, etc. The maximum flux density obtained with SM is of the order of 20T and DC fields of 35T level have been generated in a combination of an external SM and an internal water cooled electromagnet.

The most extensive field of SM application is now medical diagnostics (in nuclear resonance imaging devices for NMR tomographs) where the field level is rather moderate (of the order of 1T). A few examples of successful use of SM in industrial ore separation technology are now known (e.g., in the xaoline purification process). Some prospects for use of SM in power engineering, transport (magnetic levitation) and in other industrial technologies are now under an intensive investigation. An extensive range of superconducting wires, cables, tapes and other forms of composite conductors is now supplied by industry to be used in the windings of SM's. Great number of superconducting thin wires embedded in copper matrix can be used in such conductors though some other forms of thin film or sponge-like superconducting structures are also encountered.

The most widely known compound to form these wires is now Nb—Ti alloy sometimes doped with other minor additives. It is used in SM's built to obtain maximum fields with flux densities of the order of 10T. About twice as high fields can be generated with conductors using superconducting compound Nb3Sn while other materials find only very limited applications.

SM's are now widely used in different branches of scientific research and in industrial technologies where it is necessary to obtain high magnetic fields. Energy requirements for SM's can be quite moderate as only refrigerating power is consumed in the DC mode to compensate for heat losses. On some rare occasions SM’s have also been applied to more promising prospects at first forseen immediately after the discovery (1987) of so-called high temperature superconductors that need not to be cooled to such extremely low temperatures. However, in spite of very large efforts, practical development of suitable forms of conductors is still awaited and only very small pilot electromagnets using such superconductors have been demonstrated.

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