At 4 a.m., while most of New Jersey slept, a Princeton Plasma Physics Laboratory (PPPL) physicist sat at his computer connected to a control room 3,500 miles away in Oxford, England. Years of experience running fusion experiments in the U.S. helped guide the U.K. team through delicate adjustments as they worked together to coax particles of plasma - the fourth state of matter - to temperatures that match those found at the heart of the sun.
This late-night, intercontinental collaboration happened many times from 2019 to 2024 during critical experiments at Tokamak Energy's ST40 facility. It's just one example of how PPPL is meeting the moment, leading collaborative efforts with private companies and other public institutions to make fusion power practical. Fusion, the process of combining atoms to release energy, could be the source of a nearly inexhaustible supply of electricity. But there are still challenging scientific and engineering issues to overcome in the quest for power. That's why scientists are increasingly working together to take fusion further.
In this new landscape, researchers at the U.S. Department of Energy's (DOE) PPPL have emerged as leaders, and the Lab itself has become a crucial hub for fusion research. With experiments dating back to the 1950s and the earliest days of fusion energy, PPPL's depth of experience makes it an ideal partner.
"The public sphere and the private sphere have these incredible complementary synergies, and I think this will be a really critical combination to get us to that pilot plant," said Laura Berzak Hopkins, associate laboratory director for strategies and partnerships and deputy chief research officer at PPPL. "On the public side - which includes PPPL as a national laboratory - we have this depth of expertise and enabling capabilities, and it's not just toward one particular magnetic confinement configuration."
The DOE's recently released Fusion Science and Technology (FS&T) Roadmap outlines a national strategy to accelerate the development of commercial fusion energy over the next decade by aligning public and private investment. The roadmap highlights critical gaps that innovative partnerships can fill to deliver the public infrastructure needed for the private sector's expansion into fusion in the 2030s.
"The Fusion Science and Technology Roadmap brings unprecedented coordination across America's fusion enterprise," said DOE Under Secretary for Science Darío Gil in a press release. "For the first time, DOE, industry and our national labs will be aligned with a shared purpose - to accelerate the path to commercial fusion power and strengthen America's leadership in energy innovation."
Bringing U.S. fusion leadership to fusion systems around the world
After seven decades of fundamental research, mature scientific understanding has combined with interest from private investors to create unprecedented momentum. Today, dozens of approaches to fusion are advancing in parallel. PPPL's expertise aids many in plasma physics theory, computer simulations and advanced measurement tools, known as diagnostics.
"We are not picking the winners and losers. We are enabling the fusion community as a whole," said Jack Berkery, deputy director of the National Spherical Torus Experiment-Upgrade (NSTX-U) research program at PPPL. Like many of the physicists at PPPL, Berkery is a globe-trotter, working closely with fusion researchers in many countries, including Japan, Spain and the United Kingdom.
This approach recently resulted in a remarkable achievement. When Tokamak Energy's ST40 fusion system achieved ion temperatures exceeding 100 million degrees Celsius, which is hot enough for commercial fusion, it wasn't only a victory for the private U.K. company. It was confirmed by computer models developed at PPPL and, importantly, the result validated public research into spherical tokamaks.
VIDEO: What's at the heart of NSTX-U?
A tokamak is one of several types of fusion systems scientists are currently exploring. Tokamaks use magnetic fields to hold plasma, in an effort to get the particles to fuse and release energy. There are approximately 60 tokamaks operating today. All of them are experimental, meaning they serve as test beds to search for the ideal mix of materials, features and fuels. Once a team of researchers thinks they have the ideal configuration, the next step will be to build a fusion pilot plant before a commercial fusion system can bring fusion power to the grid. But first, the recipe must be perfected, and experience is key to making meaningful progress.
"PPPL scientists participated remotely in experiments on ST40. They were actively involved in the operation of the machine, helping them get to these plasmas where they could produce the 100 million degrees Celsius," said Stanley Kaye, director of PPPL's NSTX-U research program and co-author of a journal article in Fusion Science and Technology exploring this collaboration. "We were also instrumental in helping them set up the data acquisition for one of their critical diagnostic systems."
Partnering to reach record temperatures
The story of how ST40 reached fusion-relevant temperatures illustrates how PPPL has the scientific know-how and engineering capabilities to make new industry ideas a reality. Tokamak Energy had built an impressive machine, secured funding and assembled a talented team. But when it came time to push their fusion system to its limits, they wanted the depth of knowledge accumulated at PPPL through nearly 75 years of plasma-focused research.
"PPPL physicists had worked on PPPL's Tokamak Fusion Test Reactor and the National Spherical Torus Experiment, predecessors to NSTX-U. So a lot of their experience from those machines and understanding of how to optimize these plasmas was really valuable in achieving the high temperature regime on ST40," said Steven McNamara, lead scientist for Tokamak Energy's fusion power plant program. "The expertise from the PPPL team and the Oak Ridge National Laboratory team really enabled us to accelerate our diagnostic development."
PPPL's contribution went beyond operational expertise. When ST40's diagnostics struggled to measure the extreme temperatures accurately, PPPL scientists helped redesign the tool. When the team needed to confirm their record-breaking achievement, they turned to TRANSP: a sophisticated simulation code developed at PPPL and used worldwide. And when some of the plasma particles became unstable and cooled, PPPL theorists identified the cause.
"The physicists in the public institutions have the latitude, the flexibility to really explore results and understand the physics of what's going on," said Kaye.
The key to managing an effective, combined and aligned team lay in the project's open research approach, which meant that everyone involved had full access to all the data produced and the freedom to publish results. "PPPL remains committed to enabling community-wide knowledge building and key peer-review evaluation of challenges and accomplishments," said Berzak Hopkins.
VIDEO: What is a diagnostic?
Cementing the evidence for spherical tokamaks
While PPPL scientists were supporting ST40 to reach new temperatures, another remarkable collaboration was expanding between PPPL and the U.K. Atomic Energy Authority's Mega Amp Spherical Tokamak-Upgrade (MAST-U).
ST-40, MAST-U and PPPL's NSTX-U are all spherical tokamaks, meaning the plasma inside the device is shaped more like an apple with the core removed than the squatter, doughnut shape of plasmas in conventional tokamaks. The shape of the plasma affects how easily it can be held in place, and a growing body of research suggests the spherical shape might be best for generating electricity reliably on a commercial scale. However, there are still important physics and engineering questions left to answer, and partnerships between facilities with similar tokamaks are key.
NSTX-U is expected to resume operations in 2026. The new machine is the largest spherical tokamak in the U.S. and is designed to be the most powerful spherical tokamak on Earth. This user facility will help scientists from around the world answer key scientific questions about the scalability of the spherical tokamak for a fusion pilot plant and eventually commercial fusion. Its predecessor, NSTX, began operations at PPPL in 1999 as part of DOE's efforts to study cost-effective designs for fusion systems and had a decade of successful experiments.
"You want to verify that what you see on one machine is universal. Having data from both machines gives you a lot more confidence than having data from one," said Berkery, who was a key member of the collaborative team and lead author of a journal article in Plasma Physics and Controlled Fusion about the project.
VIDEO: Take a virtual tour of NSTX-U
When NSTX-U shut down for major repairs in 2016, MAST-U was starting operations but lacked sufficient staff to fully explore its capabilities. The solution seemed almost too simple: share resources and knowledge. The repairs to NSTX-U would take time, so it made sense to work more collaboratively.
The collaboration has already yielded crucial findings. Both machines confirmed that energy confinement scales more favorably to the expected operating regime in fusion pilot plants in spherical tokamaks than in conventional designs: a finding that could determine the shape of future commercial fusion plants. Now, as NSTX-U prepares to resume operations in 2026, there are plans for the exchange to reverse, with U.K. scientists coming to Princeton.
"The experiments in NSTX-U and MAST-U are really necessary to inform future spherical tokamaks," said Berkery. "Private companies are making projections from what we think we know, and it's a big step to those devices. The more experiments we can do, the better sense we'll have of how practical or optimistic the projections are."
Going further with infrastructure and expertise combined
On PPPL's campus is a large, concrete room. Its walls are 3-4 feet thick, designed to contain radiation from fusion experiments. For two decades, this space housed a tokamak called the Tokamak Fusion Test Reactor (TFTR). In the 1990s, TFTR set records for fusion power production with experiments yielding key research findings. After the fusion system was decommissioned in 2002, the room sat largely empty. Now, PPPL's Berzak Hopkins is leading the charge to reimagine the space as a hub for fusion innovation.
"The capabilities that we have at the Lab speak to fusion writ large. That takes the form of knowledge - a depth of expertise, a highly trained workforce, our computational capabilities with codes capturing the underlying physics algorithms, high-performance computing capabilities and the data our facilities and experiments are generating," said Berzak Hopkins.
The space, with its radiation shielding, 110-ton overhead crane and industrial-scale power and cooling systems, represents significant infrastructure that private fusion companies desperately need but can't afford to build. Jonathan Menard, the deputy director for research and chief research officer at PPPL, along with Berzak Hopkins and others at the Lab, are transforming it into the Fusion Research and Technology Hub (FuRTH). The new facility will serve as a multidevice fusion incubator where private companies could install and operate experimental fusion systems without the enormous capital expense of building from scratch.
The FuRTH space, a future hub for fusion innovation, is shown here with sketches of several different fusion experimental concepts superimposed. (Photo credit: Michael Livingston / PPPL Communications Department)
The concept addresses what Berzak Hopkins calls the "valley of death" - the gap between initial venture funding for proof-of-concept devices and the hundreds of millions needed for demonstration plants. Companies could design and build their devices off-site, then truck them to PPPL and plug them into the existing infrastructure. When they're ready to move on to larger machines, they could leave their devices for PPPL to operate for research and training, creating a living laboratory of different fusion approaches that advance solutions while overcoming barriers.
PPPL recently secured DOE funding for a six-month feasibility study to catalog the facility's capabilities and develop detailed plans for the transformation. Already, companies from Wisconsin to New Zealand have expressed interest.
"There's experience and knowledge within PPPL, which we're looking to tap into, and then there's existing infrastructure as well," said Kieran Furlong, CEO and co-founder of Realta Fusion of Madison, Wisconsin, which is exploring siting its magnetic mirror fusion device at FuRTH. "Right now, fusion is essentially graduating, making that transition from being basic research into applied research and commercialization."
PPPL is also working with the state of New Jersey to spur innovation, partnering with the New Jersey Economic Development Authority and the venture capital firm SOSV to establish the NJ HAX Plasma Forge, which aims to support startups and established companies focused on low-temperature plasma technology. Low-temperature plasmas are used in a variety of manufacturing processes, including many of the steps used to make computer chips.
A final location for the forge has not yet been announced, but it will be strategically located close to PPPL in central New Jersey. In addition to being in proximity to world-renowned experts on low-temperature plasmas, the forge will feature approximately 10,000 square feet of laboratory and co-working space, serving as a Strategic Innovation Center.
The DOE's Innovation Network for Fusion Energy (INFUSE) program provides another path for private companies and universities to access national laboratory capabilities. The program also provides funding, with each INFUSE project awarded between $100,000 and $500,000. In 2025 alone, more than $6 million was awarded via INFUSE, with three of the 20 project grants going to collaborations involving PPPL.
Partnership is a model that clearly works. ST40's temperature record, the confirmed scaling results from NSTX-U and MAST-U, and the growing queue of companies asking about space at FuRTH all suggest that the fusion community has found a formula that accelerates progress beyond what either public or private efforts could achieve alone.
"If we look back through the fusion history timeline, we've never had this depth of capabilities available," said Berzak Hopkins. "It's this really critical combination between those capabilities and private investment drive that makes this moment in time very different than any previous in fusion history."
This work was supported by DOE under contract numbers DE-AC02-09CH11466 and DE-AC05-00OR22725, as well as CRADA NFE-19-07769; by the EUROfusion Consortium via the Euratom Research and Training Programme (grant agreements 633053 and 101052200); and by the U.K. Engineering and Physical Sciences Research Council (EP/W006839/1, EP/T012250/1, EP/P012450/1, EP/I501045).
