Fusion shortcut: Heat plasma before compressing it

For more than 70 years, fusion researchers have relied on the Lawson criterion, a mathematical rule that defines when plasma is hot enough, dense enough and confined long enough to sustain fusion without external heating. But it does not say how to get there most efficiently.

Now a team at PPPL (the Princeton Plasma Physics Laboratory) has published a more comprehensive version of the criterion in the journal Physical Review Letters. Physicists Luis Delgado-Aparicio, Masayuki Ono and Jonathan Menard added four factors that affect plasma performance and mapped a route to ignition that could require substantially less energy.

"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 key is the order of operations. Many proposed strategies increase plasma density first and then apply the huge heat needed for ignition. The new calculations suggest heating the plasma before making it denser avoids the steepest energy demands. The researchers call the sweet spot the Cordey saddle, a relatively accessible point along the boundary between plasma that needs outside heating and self-sustaining plasma.

To compare routes, the team used Q, the ratio of fusion power produced to external heating power supplied. In an idealized pure-fuel plasma, the Cordey saddle sits at about Q = 5. Real systems are messier: contaminants and strong magnetic fields shift the saddle, raising the Q needed.

The tungsten problem

One striking finding concerns tungsten, a heat-resistant metal chosen for the interior walls of more than a dozen next-generation fusion machines. The team's calculations show that tungsten contamination at a concentration of just one part in 10,000 can roughly double the pressure required for fusion ignition. When the analysis was extended from two dimensions to three, the pressure needed could exceed the limits at which plasma remains stable. A seemingly tiny impurity could have major consequences for reactor feasibility.

Energy losses as a stabilizer

Counterintuitively, some processes that make ignition harder could also make fusion reactors safer. Scientists have long worried about thermal runaway, where fusion heat increases the reaction rate, creating a self-reinforcing cycle. The researchers found that energy losses within plasma naturally counteract that 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.

"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."

Practical paths forward

The study also identifies technologies that might help. One is coating reactor interiors with liquid lithium, which could prevent tungsten from entering the plasma while reducing heat losses. Another is spin-polarized fuel, where atomic nuclei are prepared with aligned spins before fusion, increasing the reaction rate.

What's next

The heat-first approach has not yet been tested experimentally. Existing fusion machines cannot reach the temperatures associated with the Cordey saddle. The PPPL team plans to run computer simulations to explore whether the predicted advantages hold. The work was supported by the DOE Office of Science, Office of Fusion Energy Sciences.

"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.

• DOE Office of Science
• fusion energy
• Jonathan Menard
• Lawson criterion
• Luis Delgado-Aparicio
• Masayuki Ono
• Physical Review Letters
• PPPL