Superconducting magnets are the core enabling technology for Tokamak Fusion Reactors and their performance and reliability are paramount for the reactor operations. The materials are very expensive and typically the magnets coils, with associated systems, amount to nearly half the cost of the reactor.

Tokamaks employ complex magnetic fields to confine and circulate the plasma. Three, principle, sets of magnet coils are used: toroidal; poloidal and solenoidal, as shown in the figure below.

ITER, as well as DEMO plans, use Niobium-Tin and Niobium-Titanium superconductors which necessitate maintaining the magnet coils at 4°K (-269°C) with liquid helium cooling inside a cryostat. 

A feature of superconducting magnets is that if certain threshold conditions are not respected (cryogenic temperatures, current density, magnetic field), the zero conductivity characteristic is lost and the conductors become resistive. This transition from superconducting to resistive is referred to as a "quench". If a quench occurs there is a risk that large amounts of energy are dumped in the coils, causing damage, and so a sensitive quench protection system is required. These systems divert the current, following an alert from a network of sensors, to an external bank of resistors , an event referred to Fast Safety Discharge (FSD).

Another requirement to protect the superconductor, is shielding outside the breeder blanket to eliminate any flux of neutrons which could generate heat or cause radiation damage and activation. 


Experience with Superconducting Magnet

Although the use of superconducting magnet is now common in medical imaging, MRI, equipment, several pioneering scientific devices have experienced serious problems with the technology. Some examples are given in this section.

Tore Supra was the first large tokamak to employ superconducting magnets. The machine was operational from 1988-2010 with niobium-titanium conductors for the toroidal coils, designed with a maximum field in the coils of 9T. Tore Supra holds the record for the longest stored plasma at 6.5 minutes. Complete acceptance tests were performed before assembly on the coils at the nominal current of 1400A giving a reduced field in the coils of 5.4T because of configuration.

After the assembly of Tore Supra there were some problems with superconducting coils. The first fault occurred in a toroid coil during the commissioning phase at a reduced current of 600 A. The coil developed a short circuit and had to be replaced, an operation which took 6 months of work. Later during the commissioning at nominal power, test FSD were initiated which caused abnormal heating in three coils and in the coil with the most severe overheating a limited short circuit was diagnosed, which later in operations disappeared.

In the operation of Tore Supra, most of the FSD were not due to quenches but occurred for other reasons and only caused some limited down time to recover. After the initial problems, Tore Supra operated successfully for 18 years. A paper describing this experience, Tore Supra Superconducting Toroidal Magnetic Field System, concludes emphasizing the importance of the pre-assembly acceptance tests in superconducting conditions.

The Large Hadron Collider at CERN is the world's most powerful particle accelerator. The accelerator has 1232 dipole superconducting magnets, with niobium-tin conductors, each 15 m long with  field of 8 T. It uses cryogenic technologies demonstrated in Tore Supra.

Very early in LHC operations there was a major fault with a superconducting which caused a massive helium release and mechanical damage : Analysis of LHC incident .


ITER Magnet System