Study proposes more efficient path to fusion ignition

Researchers at Princeton Plasma Physics Laboratory reformulated the criterion to ignite and sustain fusion plasma, and their calculations indicate a pathway to make the process more energy efficient while also demonstrating the impact of effects that can make ignition more difficult, such as small amounts of tungsten in the plasma.

The picture changes: The Lawson criterion, first developed in the 1950s, is an equation evaluating whether a fusion system will produce net energy. Now, PPPL physicists Luis Delgado-Aparicio, Masayuki Ono, and Jonathan Menard have published a Physical Review Letters paper reformulating the criterion and including additional conditions for a more complete theoretical view of plasma behavior.

According to PPPL, the strength of the paper is that "it puts many effects into one model at once. Earlier work treated them one at a time."

The model incorporates helium ash and tungsten impurities that can accumulate in the plasma, along with synchrotron radiation and heat losses that generally increase with temperature.

"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."

While the Lawson criterion is often expressed as a fixed value, accounting for these factors turns ignition into a multidimensional landscape to be traversed when planning the best process for heating and powering the plasma within a fusion energy system.

According to Delgado-Aparicio, the result is like finding the most efficient way through a mountain range. If ignition is the peak of a tall mountain, these calculations show an easier path to the top than climbing straight to the summit, which many fusion efforts currently plan to do.

"A lot of companies want to climb the mountain head-on and spend enormous energy to get there," he said. "Go around the peak instead. You reach the same place in a much smarter way, and you use far less energy."

A new route: Specifically, many fusion developers plan to raise the plasma's density first and then add heat, but PPPL's calculations indicate that it is advantageous to reverse this order of operations, heating the plasma first then raising its density once it's hot.

The aim, said Ono, is to give the field a shared, more realistic model for judging a fusion design before it is built.

"Fusion experiments cost a great deal of money, and you do not want to make mistakes you could have caught beforehand," he said.

The research could change the way tokamaks and stellarators are designed and built.

Notably, the calculations show that tungsten at just one part in 10,000 inside the plasma can roughly double the pressure needed to reach ignition.

The paper points to ways to make ignition easier to reach, too. Walls coated with liquid lithium can block tungsten from entering the plasma while improving heat retention, and spin-polarized fuel, in which the fuel nuclei are aligned before they fuse, raises the rate of fusion.

"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.