Nine rings to cope with ITER’s powerful magnetic fields

Every legend has a ring somewhere in its narrative. Most of the times it is lost but when found it has the power to awake us, liberate us or even make us invincible. ITER, embarking on an epic journey to bring the energy of the sun to Earth, could be no exception. In fact, one ring will not suffice to “save” its impressive magnets from the electromagnetic loads as they confine the super-hot plasma. The biggest fusion device will require nine rings and Europe is responsible for delivering them.

ITER will rely on a sophisticated system of superconducting magnets consisting of the central solenoid, which can be described as its backbone; the correction coils, which will reduce the range of magnetic errors created by imperfections due to the location and geometry of other coils; six Poloidal Field coils responsible for the shape and stability of the plasma, and 18 Toroidal Field (TF) coils which will create a magnetic cage to entrap the hot gas. To cope with the fatigue exercised on the TFs, and with the deformation resulting from the powerful magnetic fields, three pre-compression rings will be placed on top of them and three below them. An extra set of three will be manufactured as spare in case there is a need in future to replace the lower set. The fiberglass composite rings, consisting of more than a billion miniscule glass fibers, will be glued together by a high performances epoxy resin. They will have a diameter of approximately 5 m, a cross-section of nearly 300 mm x 300 mm and will weigh slightly more than 3 T.

We visited the workshop of Airbus Defence and Space (Airbus D&S) in Madrid where teams of engineers and technicians are aiming to complete a full-scale prototype so that “real” production of pre-compression rings can kick-off in future. The components are manufactured using a well-established technology in the field of aerospace known as automated fiber placement. To reach this stage, three full-scale ring slices of fiberglass epoxy resin have been cured. Previously, one additional full-scale slice has been produced and has undergone the first stage of the qualification phase. F4E’s experts will soon examine the results of the full-scale prototype in detail before production advances.

Several kilometres away, another team of technicians is working at CNIM’s workshop in Toulon (FR) trying to develop the three spare pre-compression rings. They have been building on the work already performed by companies such as Exel (FI), Solvay (UK), CMF (IT), CMC (FR) and test laboratories such as Rescoll (FR), Etim (FR) and KIT (DE). As standard practice requires, they have started with the fabrication of a mock-up, which is roughly 1005 mm in diameter, almost 1/5 of the real size of the component. The material they have opted to use is pultruded laminate, which will be wound along the trajectory of the ring. As the tooling is slowly bending the material, a bonding tape is applied on its layer. This helical movement is repeated several times until the pre-compression ring mock-up consists of multiple layers which will then be cured. Afterwards, the mock-up is placed on an equipment to be machined to the final dimensions and to remove excessive material.

Next, it will go through nondestructive testing (NDT) and in the end it will be transported to ENEA’s facility where final destructive tests will be performed.  What is the state of play now? A series of mock-ups has been produced and qualification tests are being performed. It is estimated that by next year the “real” production of the spare pre-compression rings will begin.