The central core is made up of 24 inner elements of toroidal field coils which each needed to be insulated. This was achieved by applying a glass fibre prepreg/Kapton polyimide film/glass fibre prepreg insulating layer to each of the core elements.
Rockwood used a silicon intensifier consolidation system to accurately control the position and pressure during the cure. This enabled air and resin to be progressively squeezed out from under the Kapton film and ensure that the Kapton tightly bonded to the toroidal field coil to give a very uniform and consistent insulation layer.
The next challenge was to bond all the insulated column components together with extreme accuracy. This was achieved by using a dry glass fibre fabric to control the bond line thickness, and a dispersed adhesive system which provided a consistent 0.1 mm bonded joint.
Rockwood applied the same insulation approach to the solenoid winding, which creates one of the many huge magnetic fields in ST40. Again, glass fibre prepreg and Kapton was applied, this time in a helical overlapping manner during the winding process and applied between each of the coils. Finally, the whole solenoid was overwrapped with glass prepreg.
UK-based Rockwood developed this project from first principles, as no off-the-shelf solution was available. The company also supplied the entire cryogenic suspension system for the ST40 system, comprising a large number of bespoke carbon fibre bands. The technology is also being used in the ITER, the world’s largest fusion experiment involving engineers and scientists from 35 countries.
Rockwood established this process as a means of bonding large structures together very accurately over large distances. It is now developing processing solutions for high temperature super conductors, which is an important element in the challenging work of small scale tokamaks.
“Fusion devices are built to create one of the most extreme environments you can get – more extreme temperatures than anywhere in the solar system,” says Mark Crouchen, Managing Director of Rockwood Composites. “The properties of composites are instrumental in enabling the ST40 to achieve these extremely high temperatures – hotter than the centre of the sun. This was a technical challenge that we relished engaging with. We knew we had the knowledge, materials and processing expertise to provide a technical and elegant solution. We’re now working with Tokamak Energy on the next stage of development.”
“We were faced with a real problem in the manufacture of the central core of our high magnetic field tokamak,” explains Graham Dunbar, ST40 Project Manager. “We approached Rockwood Composites and their fantastic team of engineers were able to find the best solution through an innovative use of composites. I am enjoying working with the team as we develop the technology further.”
The ST40 is the first world-class tokamak controlled fusion device to have been designed, built and operated by a private venture. It is designed to show that fusion temperatures – 100 million °C – are possible in compact, cost-effective devices. This has the potential to allow fusion power to be developed for commercial deployment in years, not decades.
Tokamak Energy is an Oxfordshire, UK-based company that grew out of the Culham Centre for Fusion Energy and was established in 2009 to design and develop compact fusion power plants. Tokamak Energy’s aim is to put fusion power into the grid by 2030.