Scientists may have found a surprising shortcut to nuclear fusion that could dramatically cut the energy needed to ignite the power of the Sun.
Scientists at the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) have identified a potentially much more efficient way to achieve nuclear fusion, the process that powers the Sun. Their calculations suggest that changing the order in which plasma is heated and compressed could dramatically reduce the energy needed to reach ignition, a crucial milestone in the pursuit of practical fusion power.
For more than seven decades, scientists have relied on an equation called the Lawson criterion to determine whether plasma can remain sufficiently hot and dense for long enough to sustain fusion reactions without additional external heating. Developed in the 1950s, this equation establishes the conditions required for ignition, but it does not explain the most efficient way to achieve them.
Now, PPPL physicists Luis Delgado-Aparicio, Masayuki Ono, and Jonathan Menard have updated that decades-old framework. By incorporating four additional factors that influence the behavior of fusion plasma, they have developed a more comprehensive model for determining how fusion systems should heat and maintain their fuel.
Their findings were published in the journal Physical Review Letters.
To understand the significance of the discovery, it helps to know what plasma is. Often called the fourth state of matter, plasma is a gas so hot that its particles become electrically charged. When plasma reaches ignition, the energy released by fusion reactions produces enough heat to sustain the process without further external heating. Achieving this condition efficiently is one of the central challenges facing fusion energy researchers.
A Surprising Shortcut to Fusion Ignition
Delgado-Aparicio compares the team’s discovery to finding an easier route through a mountain range. Imagine that the energy required to ignite fusion is represented by a landscape, with the destination lying beyond a towering mountain.
Many proposed fusion strategies take the equivalent of climbing directly over that mountain. They first increase the plasma’s density and then supply the heat needed to reach ignition. However, the researchers’ calculations point to a less demanding route: raise the plasma’s temperature first, then increase its density.
“A lot of companies want to climb the mountain head-on and spend enormous energy to get there,” Delgado-Aparicio said. “Go around the peak instead. You reach the same place in a much smarter way, and you use far less energy.”
The mathematical explanation involves a region called the Cordey saddle. Like a mountain pass, this represents the lowest point along a barrier separating plasma that still needs outside heating from plasma capable of sustaining its own fusion reactions.
To evaluate possible routes, the researchers used a measurement known as Q, which compares the power generated by fusion with the external heating power supplied. In an ideal plasma containing only fusion fuel, the Cordey saddle occurs when fusion produces approximately five times the heating power being supplied.
Conditions become more complicated in actual fusion machines. Contaminants within the plasma and extremely powerful magnetic fields can shift the location of this critical point, increasing the Q value needed to reach it.
A New Roadmap for Future Fusion Reactors
The findings could have important implications for the design of fusion machines called tokamaks and stellarators.
Researchers and private companies are investigating several technologies that could eventually generate electricity for the power grid through nuclear fusion. Tokamaks are among the leading approaches. These doughnut-shaped devices use powerful magnetic fields to confine plasma while it is heated and brought to the density needed for fusion ignition.
Identifying a more efficient path to ignition could help engineers make better decisions about how these machines are designed and operated. The work also supports PPPL’s broader efforts to address scientific and technological obstacles standing in the way of commercial fusion power and future fusion pilot plants.
One major advantage of the new model is that it considers several important physical processes simultaneously. Previous studies often examined these effects separately, making it harder to understand how they interact under realistic operating conditions.
The researchers incorporated four additional influences that can change the requirements for sustaining fusion:
- Helium ash: Helium left behind by fusion reactions can accumulate inside the plasma, reducing the concentration of usable fuel.
- Plasma impurities: Both light and heavy elements released from the reactor’s inner walls can contaminate the plasma and interfere with fusion.
- Synchrotron radiation: Moving electrically charged particles, particularly electrons, can release energy through radiation, contributing to energy losses.
- Heat escaping from the plasma: Energy can flow out of the confined plasma, with these losses becoming more significant as temperatures rise.
Considering these effects together produces a more realistic picture of the challenges that fusion reactors must overcome.
“When you leave these effects out, you say the design will work fine,” Ono said. “When you put them in, the picture changes, and it becomes quite important.”
According to Ono, the goal is to establish a common framework that scientists can use to evaluate proposed fusion systems before committing to construction. Treating the Lawson criterion as a single, unchanging threshold can overlook physical effects that substantially alter the conditions required for ignition.
“Fusion experiments cost a great deal of money, and you do not want to make mistakes you could have caught beforehand,” he said.
Tiny Amounts of Tungsten Could Make Fusion Much Harder
The analysis also highlights a potentially serious challenge involving the materials used to build fusion reactors.
More than a dozen advanced fusion machines are being designed with tungsten lining their inner walls. Engineers favor this metal because it can tolerate the extraordinary heat associated with fusion plasmas, which must reach temperatures exceeding those at the center of the Sun.
However, tungsten can create problems if even a small amount enters the plasma.
The researchers found that a tungsten concentration of just one part in 10,000 could approximately double the pressure needed to achieve ignition. This means that an extremely small quantity of material released from the reactor walls could significantly increase the difficulty of sustaining fusion.
Those calculations were based on a two-dimensional analysis. When the model is extended to three dimensions, the required pressure can rise beyond the limit at which the plasma remains stable.
The results demonstrate why seemingly minor details in reactor design can have major consequences for fusion performance.
An Unexpected Benefit of Fusion Energy Losses
Although energy losses make ignition harder, the researchers discovered that these same effects could also help prevent a different problem.
Fusion scientists have long considered the possibility of thermal “runaway” instability. This occurs when heat from fusion reactions accelerates additional fusion, producing still more heat and potentially creating a self-reinforcing cycle.
The new calculations indicate that the processes responsible for draining energy from plasma can also limit this feedback. Working together, these effects could allow a burning plasma to maintain a stable operating condition without continuous external heating.
(Unlike a fission power plant, a fusion system holds only seconds of fuel at a time, so losing control means the plasma cools and the burn stops rather than anything worse.)
This finding suggests that some physical processes traditionally viewed as obstacles to ignition may also contribute to the stability of a working fusion reactor.
Liquid Lithium and Advanced Fuels Could Improve Fusion Efficiency
The research identifies potential ways to overcome some of the problems revealed by the new model.
One promising option involves coating the inner walls of fusion systems with liquid lithium, a technology PPPL has investigated for years. Such coatings could reduce the amount of tungsten entering the plasma while also helping it retain heat.
Another possibility is the use of spin-polarized fuel. This approach involves aligning the spins of fuel nuclei before fusion occurs, increasing the likelihood of fusion reactions and potentially improving the system’s performance.
Together, these strategies could help engineers develop reactors that reach ignition more efficiently while maintaining stable conditions.
For now, the proposed heat-first approach remains theoretical. The findings come from mathematical calculations rather than direct experimental measurements, since existing experiments have not reached the temperatures associated with the Cordey saddle.
The researchers plan to investigate their proposed strategy further through computer-based experiments, which will allow them to test the heat-first route under simulated fusion conditions.
“While more study is needed, we are excited by these results, and they suggest a clear path forward for future research in this area,” said Menard.
The work offers a new perspective on one of fusion energy’s most persistent challenges. Rather than simply pushing plasma to ever-higher densities and temperatures, scientists may be able to reach the same critical destination by choosing a more efficient route.
Reference: “Generalized Lawson-Cordey-Mills Accessibility of Fusion Ignition” by Luis F. Delgado-Aparicio, Masayuki Ono and Jonathan E. Menard, 10 September 2026, Physical Review Letters.
DOI: 10.1103/mmc9-nzfx
The research was supported by the DOE Office of Science, Office of Fusion Energy Sciences, under contract DE-AC02-09CH11466. Additional support included a 2015 Fusion Energy Sciences Early Career Award, a 2018 Diagnostic Innovation and Development award, and the 2021 and 2025 Long-Pulse Tokamak Research Programs.
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