A toroidal field coil is inserted into its structural steel case.

The ITER magnet system will be the largest and most integrated superconducting magnet system ever built. Its magnetic fields will initiate, confine, shape and control the plasma. With a combined stored magnetic energy of about 51 GJ, superconducting technology is essential for achieving the required field strengths and currents efficiently.

Material

The ITER coils are made from type-II superconductors, in which electrical resistance falls to essentially zero below a critical temperature while superconductivity can be maintained in strong magnetic fields. The magnets are cooled with supercritical helium to about 4 K (−269 °C). Superconducting magnets can carry very large currents and generate strong magnetic fields with much lower electrical losses than conventional resistive magnets.

ITER’s poloidal field coils use niobium–titanium (NbTi), while the toroidal field coils and central solenoid use niobium–tin (Nb₃Sn), which performs better in the higher magnetic fields required by these systems. NbTi is comparatively ductile and easier to manufacture, whereas Nb₃Sn is brittle and requires a more complex fabrication process.

Nb₃Sn is extremely brittle once formed, so the conductor cannot simply be wound after reaction. Instead, precursor strands containing niobium and tin are first wound into the required coil geometry and then heat-treated so that the elements react to form the superconducting Nb₃Sn compound. This “wind-and-react” process allows the complex coils to be manufactured before the conductor becomes brittle.

The superconducting cables also contain copper strands that act as electrical and thermal stabilisers. During a quench, when part of the conductor loses superconductivity, the copper temporarily carries part of the current and helps conduct heat away until the magnet protection system safely discharges the stored energy. ITER required about 500 tonnes of Nb₃Sn strand — more than 100,000 kilometres — produced by nine suppliers. This procurement programme increased global annual Nb₃Sn production capacity from about 15 to 100 tonnes.

Toroidal Field

Toroidal field coils with support structures.

The toroidal field system consists of 18 D-shaped coils arranged around the vacuum vessel. Their main purpose is to generate the toroidal magnetic field that confines charged plasma particles. Together, the coils store about 41 GJ of magnetic energy and produce a maximum field of 11.8 T on the coils. Each coil is about 17 metres high and 9 metres wide and weighs roughly 360 tonnes including its structural casing. The Nb₃Sn conductor is wound into double-pancake layers embedded in radial plates and enclosed in massive stainless-steel structures.

Poloidal Field

Poloidal field coils with support structures.

The six poloidal field coils form large horizontal rings around the outside of the toroidal field coil structure and are made from NbTi superconductor. Together, they store about 4 GJ of magnetic energy and generate fields of up to about 6 T. They shape and position the plasma and contribute to its equilibrium and stability. The largest coil is about 24 metres in diameter and the heaviest weighs around 400 tonnes. Four of the largest coils were manufactured in a dedicated on-site winding facility because they were too large for conventional transport.

Central Solenoid

Central solenoid.

The central solenoid acts as the primary winding of a transformer. By changing its magnetic flux, it induces a toroidal current of up to 15 MA in the plasma. This plasma current generates a poloidal magnetic field which, together with the toroidal field, produces the helical magnetic configuration required for confinement. The induced current also provides ohmic heating during the early stages of plasma formation.

The central solenoid is about 13 metres high, or 18 metres including its support structure, 4 metres in diameter and approximately 1,000 tonnes in mass. It consists of six independently powered Nb₃Sn modules. A maximum field of about 13 T will be reached at the centre of the stack, producing enormous electromagnetic forces between the modules.

A massive pre-compression structure of high-strength stainless steel holds the six modules together and must withstand electromagnetic forces of about 60 MN — roughly twice the thrust of a Space Shuttle at lift-off.

Other Coils

Eighteen correction coils compensate for small magnetic-field errors caused by manufacturing tolerances and assembly imperfections. They are arranged in three sets of six above, at and below the mid-plane of the vacuum vessel. Each coil is about 8 metres wide and carries a current of around 10 kA, providing fine control of the magnetic configuration.

ITER also includes two non-superconducting in-vessel coil systems for rapid plasma control. Two vertical stability coils above and below the mid-plane provide fast correction of vertical plasma motion. A set of 27 ELM control coils mounted inside the vacuum vessel can generate resonant magnetic perturbations to suppress or mitigate edge-localised modes and improve control of the plasma edge.

2 pictures

2 pictures