Tokamak Energy has successfully completed a 14-month campaign on Demo4, its world-leading fusion magnet system, validating the company’s ability to design, build, integrate and operate complete high-temperature superconducting (HTS) systems
The expertise and learning gained through Demo4 is now being applied in Tokamak Energy’s work with UK Fusion Energy, supporting the development of the HTS magnet system for STEP Fusion.
The results from the campaign, which also support the development of commercial HTS systems for applications beyond fusion, were presented at this week’s Applied Superconductivity Conference in Pittsburgh.
Engineers pushed Demo4 to a peak magnetic field of 13.7 Tesla, ran toroidal field coils at 5,600 Amps, and cycled the system across a wide operating range from 77 Kelvin down to 17 K, while withstanding transverse mechanical stresses of 150 MPa – almost 1,500 times atmospheric pressure.
The magnet system – which is arranged in a fusion-relevant spherical tokamak configuration – operated for roughly 10,000 hours in various states of energisation, including extended periods at high field. Testing also included multiple forced discharges up to 12.5 T and high current, designed to stress-test the system’s resilience under fault conditions.
Warrick Matthews, CEO of Tokamak Energy, said: “Demo4 is a major technical and commercial milestone. It proves we can design, build and operate complete HTS magnet systems, and underpins our ambition to become the go-to HTS magnet provider for the fusion industry – building on our role as STEP’s Magnet Systems Partner and our place in the UK Infinity Fusion Consortium. The system endurance testing results provide excellent data to show the robustness of our HTS designs, which is relevant across all industrial applications for magnets and power systems.”
Liam Brennan, Director of TE Magnetics, Tokamak Energy’s HTS business division, added: “HTS magnet design is key to the performance and affordability of a fusion power plant. Demo4 proves our integrated magnet system can perform under demanding fusion-relevant configurations and we’re already putting this learning to work with UK Fusion Energy on the national STEP programme.”
A complete fusion magnet system
Demo4 is a complete HTS fusion magnet system demonstrating magnetic field strengths and angles, current densities, operational temperatures and mechanical loads together in a single, fully integrated device – validating not just the magnets themselves, but the cryogenics, energisation systems, instrumentation and controls needed to run a balanced set of interacting magnets.
The system comprises 44 HTS coils arranged in a spherical tokamak configuration: 14 toroidal field (TF) limbs, each made up of two partially insulated HTS coils, and two poloidal field (PF) coils, each built from eight fully insulated HTS coils.
The system is cooled by a fusion-relevant pressurised helium cooling system operating at up to 20 bar, and its behaviour was tracked by more than 600 sensors monitoring voltage, field, temperature and stress.
Importantly, the experimental data from the campaign continues to feed into the validation and improvement of Tokamak Energy’s models and control software. This enables advanced performance prediction capabilities which enable Tokamak Energy to design and build robust, reliable HTS systems.
Key achievements:
- Validated HTS performance up to 13.7 T, with critical current behaviour in line with expectations and improved critical-surface mapping across the full 77 K to 17 K range
- Proved the reliability of more than 90 HTS joints, with joint resistances meeting expectations and reliable current transfer demonstrated throughout
- Demonstrated HTS thermal stability, early transition detection and active cooling as an effective route to quench avoidance, with resilience confirmed by failure testing and a forced discharge at 12.5 T and high current with no hotspots or degradation observed
- Proved mechanical performance at 150 MPa transverse compressive pressure, deepening understanding of winding-pack behaviour
- Proved Tokamak Energy HTS magnet designs are robust against thermal cycling effects
- Demonstrated full-system integration – magnets, power, cryogenics, controls and quench protection working together – including validated HTS current leads and TF/PF coil interactions
The campaign also surfaced valuable engineering learning that is already shaping Tokamak Energy’s next generation of HTS magnet systems, including refined manufacturing and jointing methods, improved instrumentation, and a better understanding of operating margins. The campaign has also enabled further validation and improvement of Tokamak Energy’s in-house models and control software, feeding into its design processes for robust and reliable HTS systems.
Supporting STEP and commercial fusion
Tokamak Energy is the Magnet Systems Partner for the UK Government’s STEP fusion programme, having been awarded a £70 million contract by UK Fusion Energy (UKFE) to March 2029 in recognition of its specialist manufacturing and design capabilities.
The expertise and learning from Demo4 is now feeding directly into STEP model coil testing and work on power-plant-scale HTS magnet systems.
The company is also part of the UK Infinity Fusion Consortium alongside Type One Energy and AECOM. Supported by Barclays, the consortium is accelerating the development of a commercially viable fusion power plant in the UK.
Presenting at ASC 2026, Pittsburgh
Tokamak Energy presented results from the Demo4 campaign at the Applied Superconductivity Conference (ASC 2026), taking place this week at the David L. Lawrence Convention Center in Pittsburgh, Pennsylvania. It is the world’s leading gathering for applied superconductivity research and industry.
