New technique merging sound and math could help prevent plasma disruptions in fusion facilities

Monday, May 13, 2019
by Raphael Rosen, Communications and Public Outreach, Princeton Plasma Physics Laboratory

Scientists have created a novel method for measuring the stability of a soup of ultra-hot and electrically charged atomic particles, or plasma, in fusion facilities called “tokamaks.” Involving an innovative use of a mathematical tool, the method might lead to a technique for stabilizing plasma and making fusion reactions more efficient. The technique, which has been demonstrated on tokamaks in the United States, China, and South Korea, could in theory help stabilize the plasma in ITER, a multinational fusion facility being built in France to demonstrate the practicality of fusion power.

Fusion, which powers the sun and stars, combines the atomic nuclei in plasma to release massive amounts of energy. Scientists around the world are seeking to replicate fusion on Earth for a virtually inexhaustible supply of power to generate electricity.

The new technique was created by an international research team led by scientists from the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL). Physicists first send magnetic pulses into the plasma and use sensors to detect how the plasma responds, much like the way a bat uses sonar to gather information about its surroundings. The new part of the technique is the use of a mathematical tool known as a “transfer function” to predict what will happen to the plasma in a variety of conditions.

Specifically, the tool lets physicists determine how close the plasma is to becoming unstable and what the distortion of the unstable plasma would look like. The function could help predict when a particular eigenmode, a type of plasma vibration, might trigger a severe disruption in the plasma. The disruptions, analogous to the enormous bursts of plasma periodically emitted by the sun, could halt fusion reactions and damage the tokamak.

“The idea is that by using this method, scientists could plot the plasma’s stability every hundred milliseconds or so and determine quickly whether it is becoming unstable,” said PPPL physicist Zhirui Wang, lead author of a paper reporting the findings in Nuclear Fusion. “You would be able to tell which particular mode would become unstable first and then take action to prevent the disruption.”

The transfer function technique has been shown to work on the DIII-D National Fusion Facility in San Diego, the Experimental Advanced Superconducting Tokamak (EAST) in China, and the Korean Superconducting Tokamak Advanced Research (KSTAR) facility in South Korea. Therefore, Wang and others have concluded that the function produces accurate and reliable results on a range of machines built to different specifications.

Wang is optimistic that the findings could help physicists advance the development of fusion energy. “We found that this relatively simple magnetic pulse method for detecting stability really works. That’s the first step,” he said. “Next, we want to make the method fast enough to make measurements in real time to steer the plasma away from unstable operation. We believe it’s possible!”

Supporting this research were the DOE’s Office of Science (Fusion Energy Sciences) and China’s National Natural Science Foundation. Members of the research team included scientists from General Atomics, Columbia University, the Institute of Plasma Physics in China, and KSTAR.

PPPL, on Princeton University's Forrestal Campus in Plainsboro, N.J., is devoted to creating new knowledge about the physics of plasmas — ultra-hot, charged gases — and to developing practical solutions for the creation of fusion energy. The Laboratory is managed by the University for the U.S. Department of Energy’s Office of Science, which is the largest single supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, please visit