Fusion researchers have long used the Lawson criterion to define the conditions for ignition, but it does not say how to reach them efficiently. Physicists at the Princeton Plasma Physics Laboratory have now extended that rule with four additional factors, and their calculations point to a less energy-hungry route: heat the plasma first, then compress it. They published the work in Physical Review Letters.
The team describes the energy requirements as a mountain landscape. Many proposed reactors try to climb straight over by raising density first and then adding enormous heat. The new calculations suggest that going around the peak, by heating before compressing, avoids the highest energy demands. A key landmark is the Cordey saddle, a low passage between regions where plasma still needs external heating and regions where it sustains itself. In an ideal pure fuel, that saddle sits at Q = 5, meaning fusion output is five times the external heating; contaminants and strong magnetic fields can shift it higher.
The findings could inform the design of tokamaks and stellarators, the two leading magnetic confinement approaches. Because the study treats several physical processes together rather than separately, it offers a more realistic roadmap for future reactors and may help engineers avoid costly design mistakes.