Fusion power on the high seas with Maritime Fusion’s Justin Cohen
Justin Cohen is the founder of Maritime Fusion, a company developing compact tokamak fusion reactors specifically designed to power large vessels.
Justin Cohen is the founder of Maritime Fusion, a company developing compact tokamak fusion reactors specifically designed to power large vessels.
What led you to start Maritime Fusion?
I’ve wanted to work in fusion since I was 10 years old and saw ‘Iron Man’. Like a lot of kids who saw that movie, I went home and Googled how to build an Iron Man suit. Most of the suit is challenging—the flight system, Jarvis, the materials—but the key enabling piece, the power source in his chest, is a fusion reactor. That’s what got me hooked.
I pursued nuclear engineering at North Carolina State University for undergrad, then did graduate work at Columbia University’s plasma physics program. Before starting this company, I was at SpaceX doing radiation effects and power electronics, and most recently at Tesla for almost four years doing battery design, electromagnetic design, motors, chargers, and converters.
When I decided to get back into fusion, I went shopping at the big fusion companies for jobs. While I found some approaches technically strong, what was lacking was a clear path to market adoption and proliferation of the technology and not just achieving break-even. That’s why we applied to Y Combinator earlier this year, which really got us off the ground. Now we’re building tokamaks for ships.
What is Maritime Fusion’s approach to fusion?
We believe the first-of-a-kind fusion reactors make more sense powering ships than powering the grid or data centers. There are two main pillars for this: engineering reasons and socioeconomic reasons.
We discovered this approach organically, but it mirrors the historical arc of fission technology. Many people don’t know that nuclear submarines like the USS Nautilus were operational several years before the first commercial nuclear power plants were putting megawatts on the grid.
From an engineering perspective, the biggest challenge facing grid-scale fusion is a materials science problem. In a tokamak—which is essentially a magnetic donut—the first wall faces the plasma and experiences super high heat flux and power loading. We simply don’t have materials that can survive that high power density at the hundreds of megawatts or gigawatt scale needed for centralized power production.
Ships, however, only need tens of megawatts, which is an order of magnitude less fusion power. This dramatically eases the materials science challenges around power handling, nuclear activation of structures, and tritium inventory requirements. While putting a reactor on a ship creates new challenges, the most difficult problems that we don’t have solutions for become much more manageable.
Why did you choose the tokamak design over other fusion approaches?
There’s a plot of triple product versus time for various fusion approaches, and tokamaks have been the strongest contender for decades with a really strong improvement curve. There are hundreds of tokamaks operating around the world with extensive literature and the highest technology readiness level for transitioning from energy production in pulses to steady-state power generation.
I view fusion approaches on a spectrum: tokamaks and stellarators have less physics risk but very challenging engineering problems since they’re huge and expensive. Other approaches like field-reversed configurations (FRCs) and pinch-type devices could be simpler to engineer and manufacture if the plasma physics were solved, but they carry more physics risk.
Given the choice between physics risk and engineering risk, I’d rather bet on engineering because it’s about doing the math and solving known problems. Physics risk involves more fundamental R&D with uncertain outcomes. We’re prioritizing getting reactor one to a paying customer, which led us to the low-physics-risk tokamak approach.
What about the economics of maritime fusion versus grid applications?
The socioeconomic rationale is equally compelling. These first fusion reactors will have very high capital expenditure. That’s just the reality of how expensive high-temperature superconducting magnets are, even as they become commoditized and we achieve economies of scale. It’s extremely challenging for first-of-a-kind fusion technology to compete on a levelized cost of electricity with solar, wind, batteries, or natural gas.
So we looked at markets willing to pay elevated electricity costs. Commercial shipping faces regulatory pressure to decarbonize, and their alternative fuels—primarily ammonia and hydrogen—are very expensive with high energy costs and significant infrastructure requirements. Even with the several hundred million to billion dollar capex of a fusion reactor, we can actually compete on a levelized cost basis with those decarbonized fuels. People are willing to put up with first-of-a-kind technology because they need the solution. That’s exactly what we’re targeting.
What technical milestones need to be achieved for maritime fusion to become reality?
The biggest external validation will come from SPARC, the tokamak being built by Commonwealth Fusion Systems and MIT outside Boston. It’s over 60 percent complete and will be the first fusion device to really leverage high-temperature superconducting magnet technology. When SPARC is expected to begin operations in 2027, according to Commonwealth Fusion Systems, there will be an unassuming Tuesday when it quietly achieves Q > 1 in a commercially relevant device finally ending the debate over whether fusion is always 10 years away. We’ll be able to point to a real machine consistently producing net energy from fusion.
Our device, which we’re calling Yinsen, is actually in some ways simpler than SPARC. It produces less power in a larger form factor but with longer pulses and a blanket. From a plasma physics standpoint, being further from known operational limits and lower power density makes some aspects easier to manage
Internally, we’re focused on three main technical areas: building our own high-temperature superconducting (HTS) magnets, detailing our reactor physics basis, design and operations, and designing and building auxiliary systems that can survive the marine and vibrational environment of a ship—things like high-current power supplies, radio frequency systems, cryogenics, steam extraction systems, and tritium handling systems.
Will the first maritime fusion reactor be for commercial or defense applications?
The beautiful thing about fusion, unlike fission, is that we build the exact same reactor for both applications with the same scale, size, fuel, and auxiliary systems. With fission, defense reactors often use highly enriched cores that can’t transition to commercial markets due to proliferation concerns. We don’t have that problem.
Both sectors have strong deployment strategies. If a major shipping company wants to be the first with a fusion-powered vessel, that’s an exciting commercial opportunity. But defense applications also make sense given their history with adopting new nuclear technology. Since we’re building the same reactor either way, we don’t need to decide immediately who gets the keys to the first one.
What makes high-temperature superconducting magnets a game-changer for fusion?
The material is called REBCO or rare earth barium copper oxide. It looks like copper tape, but the actual superconducting layer is only one micrometer thick. What’s revolutionary is its current density at higher temperatures. You can operate these systems at 20-30 kelvins and still generate 10-plus Tesla magnetic fields. This is the only material we know of that can do this.
When you look at fusion scaling laws, power density goes as magnetic field strength to the fourth power. Doubling the field strength gives you 16 times the fusion power density. We’ve known for a long time that stronger magnets would make fusion viable, but low-temperature superconductors would quench if pushed too hard.
REBCO was discovered in the 1980s, but what changed is the supply chain. Five to ten years ago, only a couple hundred meters existed. We need thousands of kilometers of this tape to build a tokamak. Now, if you have the capital, you can actually buy it. It’s like what silicon carbide did for power electronics. It’s a new material that enables entirely new capabilities.
What’s next for Maritime Fusion?
We wrapped up our seed round and are opening a lab in San Francisco. Over the next year, we’re focused on three main objectives: building our own HTS magnets, conducting detailed integration studies for putting reactors on ships including maintenance operations, and publishing our physics basis for peer review.
We want to be open with our design. Because our tokamak operates at lower power and somewhat lower field strength, it’s actually further from some known plasma physics limits, which we think is an advantage. We’re excited to put our approach in front of the fusion community and, most importantly, we’re hiring engineers to help us build the first fusion reactors that will actually power ships.
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