World-first magnets set for fusion power plant testing

Fusion energy requires strong magnetic fields to confine and control extremely hot, positively-charged hydrogen fuel, which becomes a plasma several times hotter than the Sun.

Tokamak Energy’s new Demo4 facility – with a magnetic field strength of over 18 Tesla - will consist of 44 individual magnetic coils manufactured using 38km of HTS tape, which requires five times less cooling power than traditional superconducting materials.

Assembly at Tokamak Energy’s headquarters in Milton Park, near Oxford, will complete this year and testing will extend into 2024, informing designs and operational scenarios for its advanced prototype, ST80-HTS, and subsequent fusion power plant, ST-E1.

Dr Rod Bateman, HTS magnet development manager at Tokamak Energy, explained that the magnets enable the construction and operations of spherical tokamaks, with Demo4 facilitating the creation of substantial magnetic forces for test in relevant scenarios.

“The breakthrough to enable a compact spherical tokamak was the development of high temperature superconducting material which can operate in high magnetic fields,” he said. “Our co-founders, Dr David Kingham and Professor Mikhail Gryaznevich, were among the first pioneers to identify the opportunity to apply this technology to fusion energy – replacing copper magnets.”

Dr Bateman continued: “They need to be cooled down to -250˚C, but the magnets provide excellent performance at this temperature. This is much easier from a technology, cost and energy perspective than it is for low temperature superconducting material. We have tested this technology successfully in one of our early devices, ST25-HTS, and then decided to develop, in parallel to our spherical tokamak programme, a dedicated magnet programme.

Demo4, comprised of 14 toroidal field limbs and a pair of poloidal field coil stacks to form a cage-shaped structure, will need to be tested at minus 253^oC.

Strong magnetic fields are generated by passing large electrical currents through arrays of electromagnet coils that will surround the plasma in future power plants. The magnets are wound with precision from HTS tapes, which are multi-layered conductors made mostly of strong and conductive metals, but with an internal coating of rare earth barium copper oxide (REBCO) superconducting material.

“The magnets are instrumented with voltage taps, an array of hall probes to measure the magnetic field profile around the coil set, fibre Bragg gratings which are optical strain gauges and multiple temperature sensors to map temperature gradients around the coil set," said Dr Bateman. “This data is collected throughout the testing operation of the coil set and is correlated with the detailed digital models. From this information, we will understand exactly what is happening to each coil throughout the testing phase.”