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 fusion ignition, a major milestone in the pursuit of practical fusion energy. Their calculations suggest that changing the order in which plasma is heated and compressed could dramatically reduce the energy needed to reach a self-sustaining fusion reaction.
For more than seven decades, fusion researchers have relied on a mathematical rule to determine whether plasma can remain sufficiently hot and dense, for a long enough period, to sustain fusion without additional external heating. Known as the Lawson criterion, this equation defines the conditions required for ignition. However, it does not explain the most efficient way to achieve them.
Now, PPPL physicists Luis Delgado-Aparicio, Masayuki Ono, and Jonathan Menard have developed a more comprehensive version of the criterion. By incorporating four additional factors that influence fusion performance, they have mapped a promising route to ignition that could require substantially less energy than alternative approaches.
Their findings were published in the journal Physical Review Letters.
The research focuses on plasma, often called the fourth state of matter. Plasma forms when a gas becomes so energetic that its particles carry electrical charges. In fusion reactors, this extremely hot material contains the atomic nuclei that researchers hope to combine, releasing energy through nuclear fusion.
The ultimate goal is to produce a burning plasma, in which fusion reactions generate enough heat to sustain the process without continued external heating. Reaching this state efficiently remains one of the most important challenges in fusion energy research.
Scientists Find a Shortcut to Fusion Ignition
Delgado-Aparicio compares the discovery to finding a more efficient route through a mountain range.
Imagine that the energy requirements for fusion ignition form a landscape dominated by a towering mountain. The destination lies on the other side, but there are several possible routes to reach it.
Many proposed fusion strategies effectively attempt to climb directly over the mountain. They begin by increasing the density of the plasma and then supplying the enormous amount of heat needed to reach ignition.
The new calculations suggest that a different sequence could be considerably more efficient. Instead of increasing density first, researchers could heat the plasma before making it denser. This alternative route avoids the highest energy requirements while still reaching the desired conditions.
“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 Hidden Mountain Pass in Fusion Physics
The researchers identified a particularly important region of this mathematical landscape called the Cordey saddle.
In geographical terms, a saddle is a low passage between higher elevations. In fusion physics, the Cordey saddle represents a relatively accessible point along the boundary separating plasma that still needs outside heating from plasma capable of sustaining its own fusion reactions.
To evaluate different routes, the scientists used a measurement called Q, which compares the power produced by fusion reactions with the external heating power supplied to the plasma.
For example, a Q value of 5 means the fusion reactions produce five times as much power as the external heating system delivers.
In an idealized plasma containing only pure fusion fuel, the Cordey saddle occurs at approximately Q = 5. However, actual fusion systems are much more complicated. Contaminants within the plasma and extremely strong magnetic fields can shift the location of this passage, increasing the Q value needed to reach it.
Understanding how these conditions change is essential for determining whether a proposed fusion reactor can realistically achieve ignition.
A More Realistic Roadmap for Future Fusion Reactors
The findings could influence the design of two major types of magnetic fusion devices: tokamaks and stellarators.
Both approaches use powerful magnetic fields to confine plasma while it is heated to the extraordinary temperatures needed for fusion. Tokamaks, which have a characteristic doughnut shape, are among the leading technologies being investigated for future fusion power plants.
Research institutions and private companies are pursuing several different ways to generate electricity from fusion and eventually deliver that power to the electrical grid. Identifying an efficient route to ignition could help engineers design more practical systems while avoiding costly mistakes.
For PPPL, this work is part of a broader effort to address the scientific and technological challenges that must be overcome before commercial fusion power becomes possible, including the development of future fusion pilot plants.
One of the study’s major advances is that it evaluates several important physical processes together rather than examining them separately, as much previous research has done.
The researchers incorporated four additional factors that can influence whether a plasma reaches and maintains ignition:
- Helium ash buildup: Fusion reactions produce helium as a byproduct. As this spent fuel accumulates, it can dilute the remaining fusion fuel and reduce performance.
- Plasma contamination: Light and heavy impurities can enter the plasma from materials lining the inside of the reactor, interfering with fusion conditions.
- Synchrotron radiation: Electrons traveling through magnetic fields emit radiation, carrying energy away from the plasma.
- Heat loss: Energy continuously escapes from the plasma, and these losses can become greater as its temperature rises.
Accounting for these effects produces a more realistic picture of the conditions future reactors must achieve.
“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 more reliable, widely applicable model that scientists can use to evaluate fusion reactor designs before construction begins.
Treating the Lawson criterion as a single unchanging threshold overlooks physical processes that can substantially alter the requirements for ignition. Including those effects early in the design process could prevent expensive problems later.
“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
One particularly striking finding concerns tungsten, an extremely heat-resistant metal selected for the interior walls of more than a dozen next-generation fusion machines.
Tungsten is attractive because fusion reactors must withstand conditions involving plasma temperatures greater than those found at the center of the sun. However, the material also presents a significant challenge if even tiny quantities enter the plasma.
The team’s calculations show that tungsten contamination at a concentration of just one part in 10,000 can approximately double the pressure required to achieve fusion ignition.
That is a substantial increase caused by an exceptionally small amount of material.
The calculations were initially developed in two dimensions. When the researchers extended their analysis to three dimensions, they found that the pressure needed for ignition could exceed the limits at which the plasma remains stable.
This suggests that seemingly minor contamination could have major consequences for the feasibility of certain fusion reactor designs.
How Energy Losses Could Help Stabilize Fusion
Surprisingly, some of the processes that make ignition more difficult may also help fusion reactors operate more safely and consistently.
Scientists have long investigated the possibility of thermal “runaway” instability. This occurs when heat from fusion reactions increases the reaction rate, producing still more heat and potentially creating a self-reinforcing cycle.
The researchers found that energy losses within the plasma can naturally counteract this effect.
Although those losses increase the difficulty of reaching ignition, they also provide a balancing influence that could allow a burning plasma to maintain a stable operating state without constant adjustments.
This balance could be valuable for future reactor designs.
(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.)
Liquid Lithium and Advanced Fuel Could Improve Fusion Efficiency
The study also identifies technologies that might help overcome some of the obstacles revealed by the calculations.
One possibility involves coating the inside of fusion reactors with liquid lithium, a technique PPPL researchers have investigated for years.
These coatings could help prevent tungsten from entering the plasma while reducing heat losses, making it easier to maintain the extreme conditions necessary for fusion.
Another promising approach uses spin-polarized fuel. This involves preparing the atomic nuclei so that their spins are aligned before fusion occurs, which can increase the fusion reaction rate.
Together, these strategies could improve the performance of future reactors and make the path to ignition more achievable.
What Comes Next for the New Fusion Strategy?
Despite the promising findings, the proposed heat-first approach has not yet been demonstrated experimentally.
The research is theoretical and relies on mathematical calculations rather than direct measurements. Existing fusion experiments have not reached the temperatures associated with the Cordey saddle, preventing scientists from testing the predicted route under those conditions.
The PPPL team plans to investigate the approach further through digital experiments, using computer-based simulations to explore whether heating plasma before increasing its density can deliver the expected advantages.
If future studies confirm the calculations, the findings could provide an important foundation for designing more efficient fusion systems and help researchers make better decisions about reactor construction.
“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 was supported by the DOE Office of Science, Office of Fusion Energy Sciences, under contract DE-AC02-09CH11466. Additional support came through 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.
