A Novel Path to Fusion Ignition: Heat First, Then Add Fuel

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 was 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.

Around the Mountain, Not Over It

Delgado-Aparicio said the result is like finding the most efficient way through a mountain range. Imagine the energy a plasma needs to reach ignition laid out as terrain, where the point of ignition can be found behind the peak of a tall mountain. Many fusion efforts plan to climb straight to the summit by raising the plasma’s density first and then adding heat. The new map shows a better way to the peak: heating the plasma first, then raising its density once it’s hot.

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

In mathematical terms, the updated route to ignition goes through an area known as the Cordey saddle. It is the lowest point on a ridge that separates a plasma that still requires external heating from one that burns on its own. The team compared different routes using a quantity called Q, the ratio of fusion power produced to heating power supplied. In a clean “ideal” plasma of pure fuel, the saddle sits where fusion returns about five times the heating power put in. Adding real-world effects, such as impurities in the plasma and super high magnetic fields, moves the saddle and raises the Q required to reach it.

PPPL: Laying the Groundwork for a Fusion Pilot Plant

The research could change the way certain fusion systems, known as tokamaks and stellarators, are designed and built. While several approaches are currently being explored by research institutions and private companies to generate electricity for the power grid using energy from fusion, tokamaks are a leading candidate. These doughnut-shaped machines try to use magnetic fields to hold a plasma in place and add energy to the plasma so it is dense and hot enough to reach ignition. Finding the best route to ignition is just one of the ways that PPPL is closing the nation’s critical science and technology gaps essential to achieving commercial fusion.

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 team folded in the following four other factors:

1. Helium ash, or spent fuel that can build up and dilute the plasma
2. Light and heavy impurities in the plasma that come from the inner walls of the fusion system
3. Radiation losses given off by negatively charged particles as they travel, which is known as synchrotron radiation
4. Heat that flows out of the plasma, a situation that increases 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." The aim," said Ono, "is to give the field a shared, more realistic model for judging a fusion design before it is built." The old assumption that the Lawson criterion is a single fixed value, he said, leaves out effects that can change the answer sharply. “Fusion experiments cost a great deal of money, and you do not want to make mistakes you could have caught beforehand,” he said.

Why the Walls of a Fusion System Matter

More than a dozen next-generation fusion machines have chosen tungsten for their inner walls because the metal can withstand the extreme heat from the plasma, which will need to reach temperatures hotter than the core of the sun. The new research shows that tungsten at just one part in 10,000 inside the plasma can roughly double the pressure needed to reach ignition. That research only considers things in two dimensions. Extended to three dimensions, the pressure needed to reach ignition can climb past the point where the plasma is stable.

Interestingly, the same physics offers a safeguard. Fusion designers have long worried about thermal “runaway” instability, in which fusion heat drives more fusion, which drives more heat. The team found the losses that make ignition harder also push back against that cycle. Together, they can hold a burning plasma in a steady state on its own. (Unlike a fission power plant, a fusion system holds only seconds of fuel at a time, so losing control means the plasma cools and the burn stops rather than anything worse.)

The paper points to ways to make ignition easier to reach, too. Walls coated with liquid lithium, an approach PPPL has studied for years, 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.

The research is theoretical, meaning it is based on calculations rather than measurements because no experiment today reaches the temperatures where the Cordey pass sits. But the team plans digital experiments to test the heat-first route.

“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.

This work was supported by DOE, Office of Science, Office of Fusion Energy Sciences, under contract DE-AC02-09CH11466, including support for a 2015 Fusion Energy Sciences Early Career Award, a 2018 Diagnostic Innovation and Development award and the 2021 and 2025 Long-Pulse Tokamak Research Programs.