The paradox startled scientists at the U.S Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) more than a dozen years ago. The more heat they beamed into a spherical tokamak, a magnetic facility designed to reproduce the fusion energy that powers the sun and stars, the less the central temperature increased.

“Normally, the more beam power you put in the higher the temperature gets,” said Stephen Jardin, head of the theory and computational science group that performed the calculations, and lead author of a proposed explanation published in Physical Review Letters. “So this was a big mystery: why does this happen?”

Through high-resolution computer simulations Jardin and colleagues discovered that heightened pressure at certain locations breaks up the nested magnetic surfaces formed by the magnetic fields that wrap around the tokamak to confine the plasma. The breakup flattened the temperature of the electrons inside the plasma and thereby kept the temperature in the center of the hot, charged gas from rising to fusion-relevant levels.

“So what we now think is that when raising the injected beam power you’re also increasing the plasma pressure and you get to a certain point where the pressure starts destroying the magnetic surfaces near the center of the tokamak,” Jardin said, “and that’s why the temperature stops going up.”