The large coils which will generate the major magnetic field components in ITER will all be superconducting. The high currents and long pulse duration would generate too much heating in conventional coils. However, the construction of superconducting coils with the required performance is a major technological challenge, so working prototypes need to be built to prove it can be done. Dr Robert Aymar, the ITER  Director, recently announced the successful testing of a model of the central solenoid coil – the one which sits at the centre of the doughnut and acts as the primary winding of the transformer to drive the plasma current.

But this is no small model. The test coil is 2 m high, 3.6 m in diameter, carries a current of Test facility at Naka (Japan) for the model of the ITER central solenoid coil (courtesy of JAERI, Japan) 46000 amperes and produces a field of 13 tesla.

To put that in perspective, the magnets in the biggest hi-fi loudspeakers produce no more than about one tesla, over a volume more than a million times smaller!

The development and testing of this coil is an example of a remarkable international collaboration, involving industry as well as fusion labs. The superconducting strand was produced by companies in the EU, Japan and the USA (with support from the Russian Federation), the conductors were then manufactured by industry in the EU, the magnet was assembled from modules made in Japan and the USA, and the whole assembly was tested on a purpose built facility in Japan.

Dr Aymar says that the successful testing of the coil is “a major milestone of the ITER R&D programme and conveys a strong message about the quality of the ITER collaboration and the strength of industrial involvement”.