The ITER magnet system comprises 18 superconducting toroidal field and 6 poloidal field coils, a central solenoid, and a set of correction coils that magnetically confine, shape and control the plasma inside the vacuum vessel. Additional coils will be implemented to mitigate Edge Localized Modes (ELMs), which are highly energetic outbursts near the plasma edge that, if left uncontrolled, cause the plasma to lose part of its energy.
The power of the magnetic fields required to confine the plasma in the ITER vacuum vessel is extreme. For maximum efficiency and to limit energy consumption, ITER uses superconducting magnets that lose their resistance when cooled down to very low temperatures. The toroidal and poloidal field coils lie between the vacuum vessel and the cryostat, where they are cooled and shielded from the heat generating neutrons of the fusion reaction.
Superconducting cable being spooled after production at ASIPP, Institute for Plasma Physics, Hefei, China. Photo: Peter Ginter
The superconducting material for both the central solenoid and the toroidal field coils is designed to achieve operation at high magnetic field (13 Tesla), and is a special alloy made of niobium and tin (Nb3Sn). The poloidal field coils and the correction coils use a different, niobium-titanium (NbTi) alloy. In order to achieve superconductivity, all coils are cooled with supercritical helium in the range of 4 Kelvin (-269°C). Superconductivity offers an attractive ratio of power consumption to cost for the long plasma pulses envisaged for the ITER machine.
Toroidal Field System
The 18 toroidal field (TF) magnets produce a magnetic field around the torus, whose primary function is to confine the plasma particles. The ITER toroidal field coils are designed to have a total magnetic energy of 41 gigajoules and a maximum magnetic field of 11.8 tesla. The coils will weigh 6,540 tons total; besides the vacuum vessel, they are the biggest components of the ITER machine.
The coils will be made of cable-in-conduit superconductors, in which a bundle of superconducting strands is cabled together and cooled by flowing helium, and contained in a structural jacket. The strands necessary for the ITER toroidal field coils have a total length of 80,000 kilometres.
Poloidal Field System
The poloidal field coil system consists of six independent coils placed outside the toroidal magnet structure.
The poloidal field (PF) magnets pinch the plasma away from the walls and contribute in this way to maintaining the plasma's shape and stability. The poloidal field is induced both by the magnets and by the current drive in the plasma itself.
The poloidal field coil system consists of six horizontal coils placed outside the toroidal magnet structure. Due to their size, the actual winding of five of the six poloidal field coils will take place in a dedicated, 257-metre-long coil winding building on the ITER site in Cadarache. The smallest of the poloidal field coils will be manufactured offsite and delivered finished.
The ITER poloidal field coils are also made of cable-in-conduit conductors. Two different types of strands are used according to operating requirements, each displaying differences in high-current and high-temperature behaviour.
The main plasma current is induced by the changing current in the central solenoid which is essentially a large transformer, and the 'backbone' of the magnet system. It contributes to the inductive flux that drives the plasma, to the shaping of the field lines in the divertor region, and to vertical stability control. The central solenoid is made of six independent coil packs that use a niobium-tin (Nb3Sn) cable-in-conduit superconducting conductor, held together by a vertical precompression structure. This design enables ITER to access a wide operating window of plasma parameters, enabling the testing of different operating scenarios up to 17 MA and covering inductive and non-inductive operation.
Each coil is based on a stack of multiple pancake winding units that minimizes joints. A glass-polyimide electrical insulation, impregnated with epoxy resin, gives a high voltage operating capability, tested up to 29 kV. The conductor jacket material has to resist the large electromagnetic forces arising during operation and be able to demonstrate good fatigue behaviour. The conductor will be produced in unit lengths up to 910 metres.