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Superconducting magnet

A superconducting magnet is an electromagnet that is built using superconducting coils. They must be cooled to cryogenic temperatures during operation. Their advantages are that they can produce stronger fields than ordinary iron-core electromagnets, and can be cheaper to operate, since no power is lost to ohmic resistance in the windings.

Liquid helium is used as a coolant for superconducting windings with critical temperatures around its boiling point of 4.2 K. The magnet and coolant are contained in a thermally insulated container (dewar) called a cryostat. To keep the helium from boiling away, the cryostat is usually constructed with an outer jacket containing (significantly cheaper) liquid nitrogen at 77 K.

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 Tc of 18 K. When operating at 4.2 K 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.

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Use of superconducting magnet

Superconducting magnets have a number of advantages over resistive electromagnets. They can achieve an order of magnitude stronger field than ordinary ferromagnetic-core electromagnets, which are limited to fields of around 2 T. 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 the cryostat), and for large magnets consume much less power - in fact, power consumption is negligible in the steady field state. Higher fields, however can be achieved with special cooled resistive and hybrid magnets, as the superconducting coils will enter the normal (non-superconducting) state at high fields.

Superconducting magnets are widely used in MRI machines, and also in bubble chamber magnets and particle accelerator magnets, and soon in tokamak fusion reactors.

One of the most challenging use of SC magnets is in the LHC particle accelerator. The niobium-titanium (Nb-Ti) magnets will operate at 1.9 K to allow them to run safely at 8.3 T. Each magnet will store 7 MJ. In total the magnets will store 10.4 GJ. Once or twice a day, as the protons are accelerated from 450 GeV to 7 TeV, the field of the superconducting bending magnets will be increased from 0.54 T to 8.3 T.

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 will carry 46 kA and produce a field of 13.5 Tesla. The 18 Toroidal Field coils at max field of 11.8 T will store 41 GJ. They have been tested at a record 80 kA. Other lower field ITER magnets (PF and CC) will use niobium-titanium. Most of the ITER magnets will have their field varied many times per hour.

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