Energy startup Hephaestus raises £12.8m led by CiRi Ventures with Prescience Ventures
This article covers Hephaestus, a London-founded energy startup, which has exited stealth after raising £12.8m in a series A to commercialise rare-earth-free high temperature superconductors (HTS) and negative triangularity (NT) tokamak designs. The funding is intended to accelerate development of lower-cost HTS magnets and NT reactor designs that target fusion developers, investors and the wider energy startup ecosystem.
Hephaestus, a London-founded energy startup, has exited stealth after raising £12.8m in a series A funding round to commercialise rare-earth-free high temperature superconductors (HTS) and negative triangularity (NT) tokamak designs. The raise aims to tackle two major cost and supply-chain issues that make fusion expensive today: the HTS magnets that confine plasma and the edge-localised mode instabilities that complicate reactor designs.
Why it matters
Fusion proponents have long argued the technology could supply large-scale low-carbon power, but cost and material bottlenecks remain major barriers to commercial deployment. Hephaestus is targeting both: cheaper, more scalable HTS conductor technology to reduce capital costs and a plasma configuration, negative triangularity, that appears to avoid the damaging ELM instabilities that drive complexity in conventional tokamak designs.
The company’s focus on replacing REBCO — the rare-earth-based superconductor used in most current fusion HTS magnets — speaks to a wider concern about geopolitical concentration in critical materials and the scaling limits of existing supply chains. If validated at scale, a cheaper, rare-earth-free HTS could become a supply-chain alternative for the broader fusion sector.
Inside the product
Hephaestus applies an AI-first approach across materials and plasma design. Its early AI plasma solver explored tokamak parameter space without being constrained by legacy designs and flagged negative triangularity — an inverted D-shaped plasma — as promising for lower-cost reactors. Experimental programmes at major facilities including DIII-D, JET and SMART have since produced results showing NT can deliver strong performance without ELM instabilities.
On the hardware side, the company completed Anvil 0 in 2024, described as the first tokamak designed for NT from inception. For magnets, Hephaestus uses a proprietary materials-discovery platform, MaDi-AI, to identify and prototype novel HTS materials; several candidates have been manufactured and characterised at lab scale and validated by the UK’s National Physical Laboratory.
Planned milestones funded by the round are concrete: begin HTS manufacturing at a 70,000 ft² facility in New Jersey from 2027, and demonstrate NT’s commercial advantages in a net-gain (Q>1) demonstration reactor by 2030. The company says it is in early-access discussions with fusion developers seeking HTS supply and is hiring across its New Jersey, London and Rome sites.
Who are the investors?
The £12.8m series A was led by CiRi Ventures. Other backers named include Prescience Ventures, Teampact Ventures, Climate Capital and Unruly Capital.
The investment is intended to accelerate Hephaestus’s dual strategy of HTS manufacturing scale-up and NT reactor demonstration. That combination positions the company both as a potential supplier of conductor technology to other fusion projects and as a developer pursuing its own reactor path.
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Founder perspective
Antoine Pochelon joined Hephaestus in February 2021 as co-founder and Head of the Experimental Campaign; he spent decades at EPFL and is credited with many foundational experimental results for negative triangularity. Jack Zamattio, CEO and founder of Hephaestus, framed the company’s objectives directly in the announcement.
In the announcement, Jack Zamattio, CEO and founder of Hephaestus, said:
Fusion has the potential of adding tens of GW’s per year of clean stable power to the grid, but that only happens if it becomes affordable and scalable. That’s why we chose negative triangularity, and why we're manufacturing our own novel HTS magnets, to realise fusion’s full industrial potential.
The bigger picture
Hephaestus’s raise and technical focus reflect two broader trends in the fusion and energy startup landscape: efforts to de-risk and domesticise critical material supply chains, and the application of AI to shorten invention cycles in both materials and device design. The UK’s National Physical Laboratory validating lab-scale HTS samples reinforces the role of national metrology and test labs in translating academic discoveries to industry-ready technologies.
Founded in London in 2019, with offices in London, Rome and New York and a production facility in New Jersey, Hephaestus sits at the intersection of UK research strength and international capital and manufacturing capacity. Its progress will be watched by investors and policymakers interested in whether startups can address the cost and supply challenges that currently limit fusion’s path to the grid.
They've invested in Hephaestus
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