Physicists have been working to design fusion reactors, technologies that can generate energy via nuclear fusion processes, for decades. The successful construction of commercial fusion reactors relies on the ability to effectively confine charged particles with magnetic fields, because this in turn enables the control of high-energy plasma.

Researchers at Laboratorio Nacional de Fusión–CIEMAT in Madrid have developed a new family of magnetic fields that could be more efficient for confining particles in these devices without the need for complex equipment configurations. Their paper was published in Physical Review Letters. The work could be an important step toward the successful realization of fusion reactors.

José Luis Velasco is the first author of the paper. He said, “In the last years, there have been many initiatives proposing the design and construction of new experimental fusion devices and reactor prototypes. When these projects design the magnetic field that will confine the fusion plasma, practically all of them try to make the field ‘omnigenous.’ The fact that inspired our research is that the fusion community actually knew that it is possible to have magnetic fields that are quite far from being omnigenous but still display good plasma confinement (e.g., the Large Helical Device, an experimental device operating in Japan, and some old and recent numerical experiments in U.S.).”

In recent years, many physicists conducting research on nuclear fusion have focused their attention on omnigenous magnetic fields. Properties of these fields are well-documented. Velasco and his colleagues set about to investigate less understood magnetic fields that could inform the design of future stellarator reactors.

Velasco explained, “Our intuition was that, in these outliers, there was something interesting and useful to be learned about stellarator reactor design”.

In a stellarator, the electric currents passing through the coils generate a magnetic field organized into nested magnetic surfaces in the shape of a deformed doughnut. This magnetic field confines a plasma composed of deuterium and tritium hydrogen isotopes, as well as the charged alpha particles resulting from fusion.

For fusion reactions to occur, the plasma inside the stellarators needs to be hot enough. To raise plasma to these high temperatures, physicists must carefully design the magnetic fields used to confine particles. This process is known as “optimizing the stellarator.

Velasco said, “Optimizing the stellarator to make it omnigenous ensures that the particles that make up the plasma stay, along their trajectories, close to the same magnetic surface. Nevertheless, to achieve omnigenity, it is necessary to optimize the stellarator ‘as a whole.” In our work we have found that similarly good confinement properties are obtained if one ‘splits’ each magnetic surface of the stellarator into several pieces and optimizes each of them separately. Hence the name ‘piecewise omnigenous.’”

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The approach for optimizing stellarators proposed by Velasco and his colleagues could help to generate optimized magnetic fields for nuclear reactors more effectively. In contrast with previously proposed approaches, it also does not rely on complex plasma configuration and the use of sophisticated coils.

Velasco explained, “Designing and building an omnigenous field is not easy. In some cases, it may require complicated and expensive coils, which could endanger the whole project. An unfortunately extreme example of this was the National Compact Stellarator Experiment. Because there exists a vast variety of piecewise omnigenous magnetic fields, we are hopeful that some of them will be easier and/or cheaper to build.”

The recent work by this team of researchers may contribute to the future design of fusion reactors, by expanding the space of possible reactor configurations.

In their next studies, Velasco and his colleagues intend to systematically assess all the relevant physics properties of the piecewise omnigenous magnetic fields they uncovered, in order to determine whether they could compete with more conventional omnigenous magnetic fields.

Velasco added, “For instance, is the loss of energy due to turbulent processes too strong and how much simpler can we make the coils to comply with the technological requirements of a reactor? Answering these questions will require a lot of work, drawing on the expertise (in several areas: theory, experiment, technology, engineering, etc.) of many colleagues of the National Fusion Laboratory at CIEMAT and of other collaborators abroad.”

Laboratorio Nacional de Fusión–CIEMAT

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LeChien explained, “The IMG was inspired by the need for a more practical, efficient, and reliable architecture for commercial fusion. As with a traditional Marx generator, it charges capacitors in parallel and discharges them in series. In this case, its triggering matches the speed of electromagnetic waves, so energy is delivered with about 90 percent efficiency. This boosts performance while cutting the size of the fusion system in half. The IMG uses lower-voltage components and standard materials, making it safer, easier to assemble, and more cost effective. The goal was to improve fusion performance while keeping the system simple, scalable, and practical for real-world power use.”

Each pulser module consists of stages (thirty-two for the demonstration system) connected in a series along a pulse tube. Each of the circular stages features multiple ‘bricks’ (ten per stage for the demonstration system) positioned around the circumference of the stage. Each brick consists of two capacitors and a switch. The electricity stored in the capacitors is released in pulses that travel through metallic pulse tubes toward the fusion chamber. The energy from multiple modules is funneled into two electrodes, which drive current through the target and electromagnetically compress it to trigger fusion.

Each pulser module has a diameter of about six feet and can deliver about two terawatts of peak power in a single fast pulse. PF expects its demonstration system to store about eighty megajoules of electrical energy and deliver more than sixty megaamperes in about one hundred nanoseconds.

That energy is directed to centimeter-scale deuterium-tritium fuel capsules. This is similar to laser-driven inertial confinement fusion, but this system has the ability to ‘magnetically squeeze’ the fuel inside a meter-scale fusion chamber surrounded by a deionized water tank for neutron shielding. PF’s founders believe their approach can expand the range of pressure and confinement time conditions under which fusion can be achieved.

The total footprint of the planned demonstration system is about two hundred and forty feet by two hundred and sixty feet. LeChien added that for a hypothetical future power plant, “We can tailor power plant size and capacity across a wide range, primarily by varying the designed target yield and/or repetition rate. One interesting combination would let us produce about two hundred and fifty megawatts with a very compact footprint of twenty-five acres or less.”

PF published a technical paper earlier this month on arXiv that details the company’s case for “affordable, manageable, practical, and scalable (AMPS) high-yield and high-gain inertial fusion.”

LeChien said that PF’s goal is “to generate the world’s lowest-cost firm power” and he noted that the company’s modular technology is “amenable to low-cost mass manufacturing with highly scalable supply chains. We combine estimates from detailed quote-informed costs of our demonstration system with average cost estimates of other fusion-related systems—the blanket, for example—and balance of plant.”

It requires a great deal of money to design affordable, scalable power sources. For PF, that includes the nine hundred million dollars of committed capital from investors, including venture capital firm General Catalyst, that was announced when PF emerged from ‘stealth mode’ in October of 2024. Those nine hundred million dollars are being released in tranches as the company reaches milestones.

LeChien added, “We’re pursuing federal funding opportunities to support our research, development, and demonstration efforts. Public-private partnerships are essential to accelerating fusion energy and building U.S. leadership in this field.”

LeChien continued, “About half of our technical team comes from the U.S. national labs, bringing deep fusion expertise. As we grow, we’re combining that foundation with talent from a wide range of fast-moving hard technology industries like aerospace and automotive. Our focus is on building a team that can move quickly, scale systems, and deliver real-world energy solutions.”

General Atomics