A major challenge for harvesting fusion energy on Earth

A key challenge for scientists working to produce the fusion energy on Earth that powers the sun and stars is to prevent so-called leaking electrons, particles triggered during disrupted fusion experiments. that can punch holes in tokamaks, the donut-shaped machines that house the experiments. Scientists led by researchers at the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) used a new diagnostic with extensive capabilities to detect the birth and linear and exponential growth phases of runaway electrons high-energy, which can help researchers determine how to prevent electron damage.

Initial energy

“We have to see these electrons at their initial energy rather than when they are fully developed and moving at a speed close to light,” said PPPL physicist Luis Delgado-Aparicio, who led the experiment that detected the first fugues on the Madison Symmetric Torus. (MST) at the University of Wisconsin-Madison. “The next step is to optimize the means to stop them before the leaking electron population can turn into an avalanche,” said Delgado-Aparicio, lead author of a first article that details the results of the Review of scientific instruments.

Fusion reactions produce large amounts of energy by combining light elements in the form of plasma, the hot, charged state of matter made up of free electrons and atomic nuclei that make up 99% of the visible universe. Scientists around the world seek to generate and control fusion on Earth for a virtually endless supply of safe and clean energy to generate electricity

PPPL worked with the University of Wisconsin to install the multi-energy pinhole camera on MST, which served as a test bed for the camera’s capabilities. The diagnostic upgrades and redesigns a camera that PPPL had previously installed on the now closed Alcator C-Mod tokamak at the Massachusetts Institute of Technology (MIT), and is unique in its ability to record not only the properties of plasma over time and space but also its energy distribution.

This feat allows researchers to characterize both the evolution of super hot plasma as well as the birth of leaking electrons, which start at low energy. “If we understand the energy content, I can tell you what the density and temperature of the background plasma is as well as how much electrons are escaping,” Delgado Aparicio said. “So by adding this new energy variable, we can discover multiple amounts of plasma and use it as a diagnostic.”

New camera

The use of the new camera advances the technology. “It has certainly been a great scientific collaboration,” said physicist Carey Forest, a professor at the University of Wisconsin who oversees the MST, which he describes as “a very robust machine that can produce electrons that don’t put in. danger its operation “.

As a result, Forest said, “Luis’s ability to not only diagnose the birthplace and initial linear growth phase of electrons when they are accelerated, and then track how they are transported from the outside to the inside. , is fascinating. Comparing its diagnosis to modeling will be the next step and, of course, a better understanding could lead to new mitigation techniques in the future. “

Delgado-Aparicio is already looking to the future. “I want to use all the expertise that we have developed on MST and apply it to a large tokamak,” he said. Two post-doctoral researchers supervised by Delgado-Aparicio can rely on the results of the MST, but at WEST, the Tungsten (W) Environment in Steady State Tokamak operated by the French Commission for Alternative Energies and Atomic Energy (CEA ) in Cadarache, France.

Range of uses

“What I want to do with my post docs is use cameras for a lot of different things including particle transport, containment, radiofrequency heating and also this new twist, diagnosis and study. electrons on the run, ”Delgado-Aparicio said. . “We would basically like to figure out how to give electrons a soft landing, and that could be a very safe way to deal with them.”

Two dozen researchers participated in the research with Delgado-Aparicio and co-authored the article on this work. There were seven physicists from PPPL and eight from the University of Wisconsin. They were accompanied by a total of three researchers from the University of Tokyo, Kyushi University and the National Institutes of Quantum and Radiological Science and Technology of Japan; five members of Dectris, a Swiss detector manufacturer; and a physicist from Edgewood College in Madison, Wisconsin.