Levitated dipole fusion reactor clears neutron hurdle

A working levitated dipole confines plasma around a freestanding core magnet rather than the fixed coil structures used in tokamak designs

(Image courtesy of OpenStar Technologies)

Fusion company OpenStar Technologies has published a peer-reviewed study addressing the neutron shielding problem that has long excluded levitated dipole reactors from deuterium-tritium fuel cycle designs. It specifies two distinct reactor concepts, Reactor A and Reactor B, and the smaller of the two, not the ITER-comparable design drawing initial coverage, carries a lower capital cost profile that could make it the more appealing first-of-a-kind candidate.

Two dipole reactor designs, not one

The larger of the two, Reactor A, is a 208 MWe plant producing 667 MW of fusion power, putting it in the same output class as ITER and Commonwealth Fusion Systems’ 2016 ARC concept. Reactor B is smaller, at 74.5 MWe and 237 MW of fusion power, and better suited to industrial use than grid-scale power. It’s also cheaper to build, and the study stops just short of saying outright that it’s the one OpenStar would actually build first.

Both designs came out of the same optimization process rather than separate studies. The team searched a fourteen-dimensional design space using a differential evolution algorithm, chosen over brute-force or gradient-based methods for its balance of flexibility and computational practicality. Plasma equilibrium was modeled using the DipolEQ code, neutron transport through OpenMC, and fast alpha-particle losses through ASCOT5. Reactor A and Reactor B emerged from the same toolchain under two different capital cost ceilings, with Reactor B constrained to less than half of Reactor A’s overnight cost.

Neutron shielding anchors the dipole magnet design

The core engineering challenge here is the 14.1 MeV neutrons produced by the DT reaction, which damage the levitating superconducting core magnet and have historically ruled dipole reactors out of DT fuel cycle designs. A layered, radiatively cooled tungsten and boron carbide shield deposits roughly 90 percent of incident neutron energy in its outer tungsten layer before it reaches the REBCO coil.

The core magnet in Reactor A reaches a peak field of 23.0 T while keeping peak tensile strain in the REBCO tape at 0.35 percent, below the 0.4 percent material limit. A sacrificial portion of the coil absorbs the highest neutron flux. When it reaches its wear limit, the entire core magnet assembly comes out of the reactor as a single unit and is replaced with a refurbished one, a swap estimated at under two weeks of downtime. The damaged magnet is then repaired separately in an external hot cell. A minimum one-year lifespan for the sacrificial section is built into the design as a constraint, and a scenario modeling annual replacement puts plant availability above 95 percent.

Dennis Whyte, professor of nuclear science and engineering at MIT’s Plasma Science and Fusion Center, has called simpler, faster component replacement one of the central unsolved challenges in commercial fusion. OpenStar’s maintenance approach is one attempt at answering that problem, one Whyte has called distinct and credible.

The magnet’s duty cycle depends as much on cooling as on shielding. The core magnet operates at 30 K, cooled by a solid-liquid neon slush reservoir that melts at a constant 24.6 K, letting heat get pumped out at the speed of a fluid rather than the slower pace of thermal diffusion through a solid magnet. That lets the magnet levitate for more than 45 minutes before it needs to dock and recharge for around 5 minutes, giving both reactors a core magnet duty cycle above 90 percent.

The method used to shape the magnet’s low-field region, which houses onboard superconducting power supply electronics away from the main confining field, has already been demonstrated on OpenStar’s Junior device, a 14-coil REBCO levitated dipole. Both reactor designs also lean on projected performance from Faraday Factory’s new “Mirai” REBCO tape line, expected to deliver engineering current densities above 1,000 A per square millimeter, roughly 30 percent above the company’s current generation product. That figure is a projection layered onto existing tape data, not a validated production spec, so it’s one to watch rather than take as settled.

Vessel design and fuel breeding choices

The outer vacuum vessel in both designs is a thick reinforced concrete dome, providing a rough vacuum and bearing the structural load of the magnets, an approach the study compares in scale and function to NASA Glenn Research Center’s Space Power Facility. The inner vessel, which forms the bulk of the tritium breeding blanket, uses a thin Inconel 718 wall with a tungsten coating on the plasma-facing side. A two-metre maintenance cavity sits between the blanket and the outer wall.

Both reactors target a tritium breeding ratio of 1.1 using a lithium oxide ceramic blanket. Liquid metal blankets such as lithium-lead can reach similarly high breeding ratios, but only with thick layers that would make material costs prohibitive across a vessel this size. Ceramic blankets are typically avoided elsewhere in fusion design because they need more frequent maintenance, but the dipole’s open vessel structure removes that penalty by making the blanket easy to access.

Plasma heating in the current design assumes ion cyclotron resonance heating as the baseline. The study weighs it against electron cyclotron and neutral beam alternatives. ECRH has a more established track record on dipole experiments but suffers from gyrotron wall-plug efficiencies of only 30 to 40 percent. ICRH offers roughly 70 percent efficiency and a more mature industrial supply chain, at the cost of more complex antenna engineering. OpenStar’s future experimental devices will put these trade-offs to the test rather than leave them to the models.

Economics and what’s still open

OpenStar hasn’t published capital cost or levelized cost of electricity figures for either reactor. Its cost model is still under development, and for now it’s presenting relative cost ratios rather than hard numbers, useful for comparing the two designs against each other, less so for anyone trying to pin down an actual investment timeline.

The cost model and a more detailed tritium breeding blanket design are both left for future work. Top magnet and limiter designs sit outside this study’s scope entirely. And neither the study nor OpenStar has said which of the two reactors it actually intends to build first.

As lead author Tom Simpson put it, this marks the first time a closed engineering solution for a DT-fuelled levitated dipole has been published.

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