Superconductor Magnet

a solenoid or electromagnet with a winding made from a superconductor. A winding in the superconducting state has zero DC resistance. If such a winding is short-circuited, the electric current that is induced in it is retained virtually indefinitely. The magnetic field of a sustained current circulating in the winding of a superconductor magnet is exceptionally stable and free of pulsations, which is important for a number of applications in research and technology.

The winding of a superconductor magnet loses its superconductivity when the temperature is increased above a critical temperature T_c of the superconductor or when a critical current T_c or a critical magnetic field H_c is reached in the winding. In view of this, the materials used in such windings have high values of T_e, I_c, and H_c (see Table 1).

To stabilize the current in the winding of a superconductor magnet (to prevent the loss of superconductivity in any of its parts), the superconducting winding materials are made in the form of wires and buses composed of thin strands of the superconductor in a matrix of a normal metal with high electric and thermal conductivity (for example, copper or aluminum). The strands are made not more than several dozen microns thick, thus reducing the release of heat in the winding when it is penetrated by a magnetic field that is increasing with the current. In addition, all the conductors are twisted along the axis during manufacture, so that the currents induced in the superconductive strands and shorted through the metal matrix can be reduced. Winding materials made of the brittle intermetallic compounds made of the brittle intermetallic compounds Nb₃Sn and V₃Ga are produced in the form of a niobium or vanadium tape 10-20 microns (jut) thick, with layers of the intermetallic compound 2-3 JUL thick on both sides. To stabilize the superconductor current and to provide strength, the tape is coated with a thin layer of copper or stainless steel.

Relatively small superconductor magnets (with magnetic field energies of up to several hundred kilojoules) are made with a closely wound coil with a superconductor content of 30-50 percent in the cross section. For large superconductor magnets (with field energies of tens to hundreds of megajoules), the conductors (buses) have a superconductor content of 5-10 percent in the cross section, and channels are provided in the winding to ensure reliable cooling of the turns with liquid helium.

The electromagnetic interaction of the turns of a solenoid creates mechanical stresses in the winding, which in the case of a long solenoid with a field of the order of 100 kilogauss (kG) is equivalent to an internal pressure of about 400 atmospheres (3.9 X 10⁷ newtons per sq m). Special bands are usually used to give the superconductor magnet the required mechanical strength. In principle, the mechanical stresses can be substantially reduced by placing the turns of the winding so that the flow lines of the current coincide with the lines of force of the magnetic field of the entire system as a whole (called the forceless winding configuration).

When an electric current of the required value is generated in the winding of a superconductor magnet, a heater located on a superconducting wire that short-circuits the winding is turned on at first. The heater increases the temperature of the shortcircuiting conductor above its T_c and the shunt circuit ceases to be superconductive. When the current in the solenoid reaches the required value, the heater is turned off. As the shunt circuit cools, it becomes superconducting, and when the supply current is reduced to zero, a sustained current begins to circulate through the winding of the superconductor magnet and its shortcircuiting conductor.

During operation, a superconductor magnet is usually placed in a cryostat filled with liquid helium (the temperature of boiling helium, 4.2°K, is below the T_c of superconductor winding materials). To prevent possible damage to the superconductor circuit and conserve liquid helium during release of the energy stored in the superconductor magnet, the circuit has a device to lead out the energy to a shunt-breaking resistor. In the final analysis, the maximum intensity of the magnetic field of a superconductor magnet depends on the properties of the materials used to manufacture the winding of the magnet (see Table 1).

Modern superconductors make possible the achievement of a field of up to 150 or 200 kG. The cost of large superconductor magnets having a field intensity of the order of tens of kG in a volume of several cubic meters is virtually indistinguishable from the expenditure for the construction of water-cooled solenoids having the same parameters, and the total expense for the electric power to supply a superconductor magnet and its cooling is approximately 500 times smaller than for the usual electromagnets. The operation of such a superconductor magnet requires about 100-150 kilowatts (kW), whereas about 40 to 60 MW would be needed to operate a similar water-cooled magnet.

A considerable number of the superconductor magnets that have been made are used to study the magnetic, electrical, and optical properties of substances and in experimental studies of plasma, atomic nuclei, and elementary particles. They are used extensively in communications technology and radar, and also as the magnetic field inductors for electrical machinery. Superconductivity opens fundamentally new possibilities in the development of superconductor magnets—inductive energy storage devices with a virtually unlimited storage time.