For decades, fusion projects such as the International Thermonuclear Experimental Reactor (ITER) have relied on 316LN or JK2LB cryogenic stainless steels. Those alloys hold up well at liquid-helium temperatures, but their yield strengths top out near 0.9–1.1 gigapascals (GPa) at 4.2 K (cryogenic temperatures) and begin to lose ductility under repeated cycling, limits that cap the magnetic field of ITER at 11.8 tesla and force a large, expensive machine design
Chinese planners, intent on building smaller power-producing tokamaks, concluded they needed a jacket that could survive far higher stresses without cracking.
That requirement sparked a 12-year materials sprint that many overseas experts said was “absolutely impossible”. The result, CHSN01 (China High-Strength, Low-Temperature Steel No. 1), was certified to withstand 20 tesla fields and 1.3 GPa combined electromagnetic stress while still stretching about 30 percent before breaking.
Just as important, it keeps those properties after 60,000 on, off cycles, the lifetime workload of China’s Burning-Plasma Experimental Superconducting Tokamak (BEST).
What’s inside the alloy
The alloy’s appeal lies in how a handful of carefully tuned ingredients turn an ordinary stainless base into something that behaves almost elastically at liquid-helium temperatures. Engineers began with Nitronic-50, a nitrogen-strengthened austenitic steel, and stripped its carbon to below 0.01 per cent.
That ultra-low level keeps brittle carbides from forming during years of service at 4 K. Next, they lifted the alloy’s nitrogen to roughly 0.30 per cent, high for stainless steel, while also edging up the nickel. Nitrogen and nickel together hold the metal in the tough, ductile austenite phase even when the temperature plunges to –269 °C.
A trace addition of vanadium, introduced once the carbon–nitrogen balance had been fixed, precipitates vanadium-nitride particles only a few nanometres across. These pin dislocations and boost strength without the usual penalty in toughness. Finally, the team imposed foundry-grade cleanliness limits on oxygen, phosphorus, and sulphur (all below 0.02 per cent), so no stray inclusions can act as crack starters under magnet loads.
Those subtle chemical edits translate into headline-ready numbers: at 4.2 K, CHSN01 sustains about 1.5 gigapascals of yield stress while still stretching more than 30 percent before fracture, roughly 40 percent stronger than the 316LN jackets chosen for ITER, yet just as crack-resistant.
Why stronger jackets matter
Superconducting magnets are the pulsing heart of a tokamak. When operators ramp the current, Lorentz forces try to balloon the helically wound conductor. Engineers can fight those forces either by adding bulk (ITER’s approach) or by encasing the cables in a jacket so strong that it simply doesn’t yield. With CHSN01, China has chosen the second path.
Finite-element and crack-growth models show the jacket can start life with flaws up to 6 mm², well above the 0.5 mm² NDT detection limit, and still survive the 60,000 pulses BEST will run. In practical terms, that means fabricators no longer have to over-polish or oversize the conductors, shaving weight, cost, and assembly time.
Stronger jackets also open the door to higher magnetic fields. Going from ITER’s 11.8 T to 20 T roughly quadruples the confining pressure on the plasma, letting physicists design a machine that is one-third the volume yet achieves a fusion power gain (Q) greater than one. Smaller reactors are easier to shield, cheaper to build, and could be clustered like modular fission units.
Built-in fatigue insurance
High strength alone is not enough because fusion magnets pulse for years. To prove CHSN01’s staying power, researchers measured its fatigue-crack-growth rate at 4.2 K and used a Paris-law model to predict life under real load spectra.
At a conservative 99% confidence level, jackets can start with an initial flaw area of 1 mm² and still last the full mission without reaching critical crack length. Those numbers give inspectors a hard, testable threshold for nondestructive evaluation, something previous alloys could not offer.
Scaling from lab to yard
Because CHSN01 is based on existing Nitronic production routes, Chinese mills were able to scale quickly. By mid-2025, 500 tonnes of conductor jackets had been delivered to the BEST construction site in Hefei. According to SCMP, project physicist Li Laifeng believes that the volume proves the alloy is “ready for industry, not just the lab.”
Beyond fusion
Zhao Zhongxian, the cryogenics pioneer who urged the team on in 2020, argues that CHSN01’s impact will reach well past tokamaks. MRI scanners, particle accelerators, maglev trains, and even quantum-computing dilution refrigerators rely on structures that face the same cold-plus-stress dilemma. Swapping in a steel that is both stronger and tougher could shrink magnet footprints or extend service intervals across the board.
A quiet but pivotal advance
Fusion often grabs headlines with record plasma shots or exotic reactor concepts. Yet history shows that big energy technologies live or die on materials science. By pushing cryogenic stainless steel into the 1.5 GPa class without sacrificing toughness, Chinese researchers have supplied the mechanical “keystone” that high-field magnets were missing.
Whether BEST achieves its 40–200 MW power target later this decade, every fusion team now has a benchmark. And if your conductor jacket cannot match CHSN01’s numbers, it may be time to call the metallurgists back to the furnace.
