Tokamak Energy concluded a 14-month test campaign on September 9 for Demo4, an integrated magnet system that operated 44 high-temperature superconducting coils at 13.7 tesla. The milestone highlights how magnetic endurance under extreme mechanical stress and cryogenic cooling shapes the commercial feasibility of future fusion power plants.
How Demo4 Survived 10,000 Hours at 13.7 Tesla
Hospitals typically run standard MRI machines between 1.5 and 3 tesla, requiring strict screening for any stray metal objects in the room. When Tokamak Energy pushed its spherical tokamak magnet assembly to a peak field of 13.7 tesla, the system generated roughly 270,000 times the Earth’s natural magnetic field. Over its 14-month testing window, Demo4 remained energized for approximately 10,000 hours, proving that multi-coil arrays can sustain high fields over extended periods rather than just brief laboratory demonstrations.
Operating at this scale creates immense structural challenges. Nuclear Engineering International documented that the coils endured sideways transverse mechanical stresses reaching about 21,750 psi, or 150 megapascals, equivalent to nearly 1,500 times atmospheric pressure. These forces actively try to push the conductors out of their structural supports. To prevent failure, the engineering team deliberately triggered multiple forced discharges at fields up to 12.5 tesla, testing how the system handles sudden energy dumps during simulated faults.
The Reality of High-Temperature Superconducting Tape
Despite carrying the industry label of “high-temperature” superconductors, Demo4’s HTS tape coils required intense chilling to maintain zero electrical resistance. During the test campaign, the magnets operated at temperatures between 321 and 429 degrees below zero Fahrenheit, spanning 77 down to 17 kelvin. This qualifies as warm only when compared against older superconducting wires found in conventional MRI systems, which must sit closer to 452 below zero to function. Every degree spared from cryogenic cooling reduces operating costs for a commercial utility.
The cooling apparatus relied on pressurized helium circulating at up to 290 psi, or 20 bar. Instead of holding the system at a single static temperature, engineers actively cycled the assemblies across that cold operating range. Meanwhile, the toroidal field coils carried 5,600 amps of electrical current. Liam Brennan, who runs TE Magnetics at Tokamak Energy, stated that Demo4 proves their integrated magnet system can perform under demanding fusion-relevant configurations.
Competing Magnet Architectures for Commercial Fusion
Magnet Design Approaches Compared
- Tokamak Energy (Demo4): Uses 44 HTS coils arranged for a spherical tokamak, achieving 13.7 tesla during its recent 14-month test run.
- Thea Energy: Employs stacks of identical flat coils configured for a stellarator, reaching a tested peak of 6 tesla.
Different firms are pursuing distinct paths to solve the engineering hurdles of magnetic confinement. While Tokamak Energy focuses on spherical tokamaks, Princeton spinout Thea Energy recently tested a stellarator magnet configuration composed of flat coils that reached 6 tesla. Developers have yet to standardize a single magnet architecture for the first wave of commercial power plants.
The data harvested from Demo4 feeds directly into STEP. Tokamak Energy acts as the magnet systems partner for this UK government prototype project alongside UK Fusion Energy. STEP is planned for West Burton on the site of a former coal power station in Nottinghamshire, placing advanced fusion hardware on land historically dedicated to fossil fuels.
Frequently Asked Questions
What was the peak magnetic field achieved by Demo4?
Demo4 reached a peak magnetic field of 13.7 tesla during its 14-month test campaign.
What temperature do HTS coils operate at in Demo4?
The high-temperature superconducting coils operated between 321 and 429 degrees below zero Fahrenheit, cooled by helium pressurized up to 290 psi.
Where is the STEP fusion prototype planned to be built?
STEP is planned for West Burton in Nottinghamshire, utilizing the site of a former coal power station.
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