Superconducting magnet

Superconducting magnets are electromagnets that are built using superconducting coils.

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Construction

Composition

Coil windings

The coil windings of a superconducting magnet are made of wires of type II superconductors (e.g.niobium-titanium).

Coil

The coil itself is made of tiny filaments (about 20 micrometers thick) of a type II superconductor in a copper matrix. The copper is needed for adding mechanical stability, and thermal stability in case the temperature rises above T_c or the current rises above I_c at which point superconductivity is lost. These filaments need to be this small because in this type of superconductor the current only flows skin-deep. The coil must be carefully designed to withstand (or counteract) magnetic pressure and Lorentz forces that could otherwise cause wire fracture or crushing of insulation between adjacent turns.

Cooling

Liquid helium is used as a coolant for superconducting materials with critical temperatures around 4.2 K. Liquid nitrogen is used for higher critical temperatures, or (being significantly cheaper) to cool a jacket around the helium.

Materials

The superconducting portions of most such magnets are composed of niobium-titanium. This material has critical temperature of 10 kelvins and remains in this state until about 15 teslas. More expensive magnets can be made of niobium-tin (Nb₃Sn). These have a T_c of 18 kelvins. When operating at 4.2 kelvins they are able to withstand a much higher magnetic field intensity, up to 25 to 30 teslas. 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 Nb₃Ti for the lower field sections is used. High temperature superconductors (BSCCO or YBCO) may be used for high-field inserts when magnetic fields are required which are higher than Nb₃Sn can manage. BSCCO, YBCO or magnesium diboride may also be used for current leads, conducting high currents from room temperature into the cold magnet without an accompanying large heat leak.

Use

Superconducting magnets have a number of advantages over resistive electromagnets. The field is generally more stable, resulting in less noisy measurements. They can be smaller, allowing more freedom in the configuration of the rest of the device (such as a cryostat), and consume much less power - in fact, power consumption is negligible in the steady field state. Higher fields, however can be achieved with cooled resistive and hybrid magnets, as the superconducting coils will enter the normal (non-superconducting) state (see quench, below) at high fields.

Magnet quench

A quench occurs when part of the superconducting coil enters the normal state. This can be because the field inside the magnet is too great, the rate of change of field is too great (causing eddy currents and resultant heating in the copper support matrix), or a combination of the two. More rarely a defect in the magnet can cause a quench. When this happens, that particular spot is subject to rapid joule heating, which raises the temperature of the surrounding regions. This pushes these into the normal state as well, which leads to more heating. The entire magnet rapidly (in less than a second) becomes normal. This is accompanied by a loud bang and rapid boil-off of the cryogenic fluid. Permanent damage to the magnet is rare, but components can be damaged by localised heating or large mechanical forces.

See also

Fault current limiter