Tokamak Energy's compact spherical tokamak in Oxfordshire, which achieved 100 million degrees Celsius in 2022, setting a record for privately funded fusion devices and advancing the case for high-field spherical tokamaks.
ST40 is a compact spherical tokamak designed and operated by Tokamak Energy, a private fusion company headquartered at Milton Park near Oxford, United Kingdom. In March 2022, the company announced that ST40 had achieved ion temperatures of 100 million degrees Celsius (approximately 9 keV)—a threshold often cited as necessary for commercial fusion—making it the first privately funded fusion device to reach that milestone.1
ST40 embodies Tokamak Energy's core thesis: that spherical tokamaks (STs), with their tight aspect ratios and naturally high plasma beta, offer a more compact and cost-effective path to fusion energy than conventional large-bore tokamaks. The spherical tokamak configuration, where the plasma cross-section resembles a cored apple rather than a doughnut, allows stronger magnetic confinement per unit of magnet mass and reduces the overall size of the machine needed to reach a given plasma performance.2
ST40 uses merging-compression start-up, a technique in which two plasma rings (spheromaks) are formed separately and then merged at the machine's midplane. The magnetic reconnection during merging heats the plasma rapidly and efficiently, eliminating the need for a large central solenoid—a significant advantage for compact reactor designs where space inside the central column is at a premium.3
The 100 million degree achievement, verified by neutron diagnostics and spectroscopic measurements, demonstrated that a relatively small, privately funded device could access the temperature regime required for deuterium-tritium fusion. While the plasma density and confinement time in ST40 remain far below what would be needed for net energy production, the temperature milestone validated key aspects of the spherical tokamak approach and attracted significant investor attention.1
ST40 uses conventional copper magnets, which limit pulse lengths but allowed rapid construction and iteration. The machine has served as a physics testbed, building confidence in the plasma behavior, stability, and heating mechanisms of high-field spherical tokamaks before Tokamak Energy commits to its next-generation devices using high-temperature superconducting (HTS) magnets.
Tokamak Energy's roadmap positions ST40 as one step in a staged development program. The company's strategy combines the spherical tokamak geometry with HTS magnets—specifically rare-earth barium copper oxide (REBCO) tape—to create compact, high-field fusion devices. HTS magnets can produce stronger fields in smaller volumes and operate at higher temperatures than conventional low-temperature superconductors, potentially enabling fusion reactors compact enough for distributed power generation.4
The company has raised over $250 million in funding and has demonstrated HTS magnet prototypes producing fields relevant to fusion. Its next planned machine, ST80-HTS, aims to demonstrate plasma performance in a fully superconducting spherical tokamak, building toward an eventual pilot plant concept. ST40's results, combined with parallel advances in HTS magnet technology at institutions worldwide, have reinforced the spherical tokamak as a serious contender in the race toward practical fusion energy.5