Helical Fusion’s HTS coil data reveal a fivefold margin to quench

The current lead and voltage tap wiring visible here are what let researchers isolate the 21 nanohm resistance increase from ordinary measurement noise

(Image courtesy of Helical Fusion)

New peer-reviewed data show Helical Fusion’s high-temperature superconducting coil has roughly five times the margin it needs before quenching. Published by the company and Japan’s National Institute for Fusion Science, the paper adds numbers that weren’t in the original test announcement we covered in January, an estimated critical current near 200 kA against the 40 kA operating point, a doubling of coil resistance under external field, and simulation results placing peak local field at 8.9 tesla. It also gives the conductor a documented lineage, tracing UROCOIC back through NIFS’s earlier WISE design, and provides a real basis for measuring the no-insulation approach against MIT and Commonwealth Fusion Systems’ SPARC magnet program.

From WISE to UROCOIC, the conductor behind Helical Fusion’s no-insulation coils

UROCOIC did not appear from nothing. NIFS and Helical Fusion tested an earlier design called WISE, Wound and Impregnated Stacked Elastic tapes, in a paper published in Plasma and Fusion Research earlier this year. That conductor was a straight, U-shaped stack of 60 REBCO tapes wrapped in titanium armor blocks and impregnated with a low-melting-point alloy. It carried 40 kA at 6 K and 8 tesla, matching UROCOIC’s later current, though the test facility’s current leads capped that hold at 8 seconds rather than the conductor itself reaching a limit.

The WISE tests also documented a training effect. Voltage spikes appeared during current ramp-up as REBCO tape shifted inside the conductor under electromagnetic force, then settled and quieted with repeated cycles. UROCOIC’s design responds to that finding directly. It adds BeCu reinforcement tapes and a stainless steel armor block around the REBCO stack, and the double-pancake coil sustained 40 kA for 280 seconds, thirty five times longer than WISE managed under its facility constraints, wound into an actual coil shape rather than a straight test article.

What the high-temperature superconducting coil data actually shows

Under self-field conditions at roughly 19 K, the coil carried 15 kA and produced 0.2 tesla with a total resistance of 10 nanohms. Once researchers added the 7 tesla external field and pushed current to 40 kA, that resistance rose to 21 nanohms, close to double. The paper attributes the increase to magnetoresistance and noise from the external magnet’s circuitry rather than any degradation in the conductor itself.

No-insulation coils are expected to carry some current outside the superconducting path, and the data back that up. NIFS calculated a shortcut current of 2.6 kA against a measured 1.9 kA, a gap the authors call reasonably consistent given the model’s assumptions. When the background magnet shut down abruptly, dropping the external field from 7.1 tesla to zero within about 10 seconds, the voltage spike exceeded 68 millivolts, and the coil held its temperature and structure throughout.

How Helical Fusion’s no-insulation coils compare to SPARC’s insulated tokamak design

MIT’s Plasma Science and Fusion Center and Commonwealth Fusion Systems ran the closest published comparison, the SPARC Toroidal Field Model Coil program. That coil reached 20.1 tesla peak field on conductor at 40.5 kA, with Lorentz loading of 815 kilonewtons per meter, more than double what UROCOIC experienced. It is also a genuinely reactor-relevant magnet, three meters across, sixteen pancakes, 270 kilometers of REBCO tape, weighing over ten tons.

The design philosophy differs in one specific way worth noting. SPARC solders REBCO tape and copper into machined channels within each of its sixteen pancakes, so there’s no insulation between turns inside a pancake. But the pancakes themselves sit behind insulating sheets, except at the joint pads that carry current from one pancake to the next. Helical Fusion and NIFS’s tested coil removes that boundary too, its two layers connect through the Faro shuffle joint with nothing insulating between them, one more insulation layer than SPARC’s design keeps, in a test article built at a fraction of SPARC’s scale. The paper describes one double-pancake coil, not a full multi-pancake winding pack, so it doesn’t show that a larger UROCOIC magnet would omit inter-pancake insulation entirely.

None of this puts UROCOIC at SPARC’s scale yet. The tested coil has a 245 mm average radius against SPARC’s three-meter magnet, and the field and force numbers reflect that gap. What the comparison shows is that helical stellarator and tokamak programs are solving their insulation and current-sharing tradeoffs differently, each shaped by their own coil geometry and operating conditions, which gives engineers evaluating HTS suppliers for either path a real basis for comparing insulation strategy against demonstrated scale.

Margin to quench sets the bar for Helix HARUKA’s superconducting magnets

The coil never quenched during testing, and the authors estimate its critical current at roughly 200 kA, about five times the 40 kA it actually carried. ANSYS simulations tied to the same run put the peak local field at 8.9 tesla on the coil’s inner side and the electromagnetic force at 356 kilonewtons per meter. The underlying test was funded in part through Japan’s SBIR Phase 3 program, which awarded Helical Fusion its largest grant to date, roughly JPY 2 billion, to develop HTS magnets and blanket and divertor systems alongside NIFS. Helical Fusion is now building Phase 1 of Helix HARUKA, the magnet demonstration stage of its integrated device, and the authors are explicit that further work is still needed to close in on the critical current and optimize performance before that scale-up goes further.

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