Heat first, compress later: New fusion strategy flips the script to reach ignition

A tiny amount of tungsten inside fusion plasma could roughly double the pressure needed to achieve ignition, according to a new theoretical model from the U.S. Department of Energy’s Princeton Plasma Physics Laboratory (PPPL). The researchers found that tungsten at a concentration of just one part in 10,000 can significantly raise the conditions required for a plasma to sustain fusion reactions without continued external heating. The study updates the Lawson criterion, a roughly 70-year-old benchmark used to determine whether a fusion plasma is hot and dense enough, for long enough, to reach ignition. The researchers incorporated several real-world effects that the original idealized criterion does not capture. Their calculations also point to a potentially more energy-efficient route toward ignition. Instead of first increasing plasma density and then heating it, the model suggests heating the plasma first and raising its density afterward could provide a more efficient path. Heating before adding density “A lot of companies want to climb the mountain head-on and spend enormous energy to get there,” said PPPL physicist Luis Delgado-Aparicio. “Go around the peak instead. You reach the same place in a much smarter way, and you use far less energy.” The researchers describe this route using a region called the Cordey saddle. It represents the lowest point on a boundary separating plasma that still needs external heating from plasma capable of sustaining its own burn. In an ideal plasma containing only fusion fuel, the model places this point at a fusion gain, or Q, of about 5. That means the plasma produces roughly five times as much fusion power as the external heating power supplied. However, impurities, radiation losses and other real-world effects can shift the point and increase the required gain. The team incorporated four factors into its model: helium ash left behind by fusion reactions, light and heavy impurities entering from reactor walls, synchrotron radiation losses and heat escaping from the plasma. “When you leave these effects out, you say the design will work fine,” said PPPL physicist Masayuki Ono. “When you put them in, the picture changes, and it becomes quite important.” Tungsten changes ignition requirements The tungsten finding could be particularly important for future fusion reactors. More than a dozen next-generation fusion machines have selected tungsten for their inner walls because the metal can withstand extremely high temperatures. But if just one tungsten atom for every 10,000 particles enters the plasma, the team’s two-dimensional calculations show that the pressure required for ignition can roughly double. When the effects are extended to three dimensions, the required pressure could rise beyond the point at which the plasma remains stable. The researchers said approaches such as coating reactor walls with liquid lithium could help prevent tungsten from entering the plasma while also improving heat retention. Spin-polarized fusion fuel, in which fuel nuclei are aligned before fusion, could also increase the fusion reaction rate. Interestingly, the same energy losses that make ignition more difficult could help stabilize a burning plasma. The model indicates that these losses can counter thermal runaway, in which increasing fusion heat drives increasingly rapid fusion reactions. The findings currently remain theoretical. No existing experiment reaches the conditions around the Cordey saddle needed to experimentally test the proposed heat-first route. The researchers plan to conduct digital experiments to further investigate the approach. “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 PPPL physicist Jonathan Menard. The study was published in Physical Review Letters. Get the latest in engineering, tech, space & science - delivered daily to your inbox.With over a decade-long career in journalism, Neetika Walter has worked with The Economic Times, ANI, and Hindustan Times, covering politics, business, technology, and the clean energy sector. Passionate about contemporary culture, books, poetry, and storytelling, she brings depth and insight to her writing. When she isn’t chasing stories, she’s likely lost in a book or enjoying the company of her dogs.