For more than 70 years, fusion energy researchers have used a particular equation to judge whether a plasma would stay hot and dense enough—for long enough—to reach a point where it could sustain itself without any more external power added. While that equation marks the finish line, also known as ignition, it says nothing about the best way to reach it. New research from the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) maps a path to ignition conditions using far less energy than any other path.

The original “ideal” equation, known as the Lawson criterion, was first developed in the 1950s. Now PPPL physicists Luis Delgado-Aparicio, Masayuki Ono and Jonathan Menard have reformulated that ideal criterion while adding four other conditions for reaching and holding a burning plasma to plan the best process for heating and powering the plasma within a fusion energy system. Their work is published in the journal Physical Review Letters.

Plasma is the fourth state of matter: a hot gas made of electrically charged particles. A burning plasma is one that has reached the point of ignition. It has enough heat from fusion reactions to keep the plasma burning without any more external heating. Reaching ignition efficiently is a central goal of fusion energy research.