One of the major challenges for researchers working on the development of fusion power is preventing what are called runaway electron which are particles that are generated during fusion. They can disrupt fusion experiments by boring holes in the walls of tokamaks. Scientists led by researchers at the U.S. Department of Energy's (DOE) Princeton Plasma Physics Laboratory (PPPL) have developed a novel diagnostic with wide-ranging capabilities to detect the appearance and the linear and exponential growth phase of high-energy runaway electrons. This may allow researchers to determine how to prevent electron damage.
Luis Delgado-Aparicio is a physicist at PPPL. He led the experiments that detected the early runaway electrons on the Madison Symmetric Torus (MST) at the University of Wisconsin-Madison. He said, "We need to see these electrons at their initial energy rather than when they are fully grown and moving at near the speed of light. The next step is to optimize ways to stop them before the runaway electron population can grow into an avalanche.” Delgado-Aparicio is the lead author of a first paper that details the findings of the research team in the Review of Scientific Instruments.
Fusion reactions generate huge amounts of energy by combining light elements in the form of a plasma which is the hot, charges state of matter composed of free electrons and atomic nuclei that makes up ninety nine percent of the visible universe. Researchers all over the world are trying to produce and control nuclear fusion on Earth to generate a virtually inexhaustible supply of safe and clean power for generating electricity.
PPPL collaborated with the University of Wisconsin to install a multi-energy pinhole camera on MST which served as a testbed 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). It is unique in its ability to record not only the properties of the plasma in time and space but in its energy distribution as well.
That ability allows researchers to characterize both the evolution of the superhot plasma as well as the birth of runaway electrons which begins at low energy. Delgado-Aparicio said, “If we understand the energy content I can tell you what is the density and temperature of the background plasma as well as the amount of runaway electrons. So by adding this new energy variable we can find out several quantities of the plasma and use it as a diagnostic.”
Use of the novel camera moves fusion technology forward. Carey Forest is a physicist and a University of Wisconsin professor who oversees the MST. He describes it as “a very robust machine that can produce runaway electrons that don't endanger its operation.”
Forest said, “Luis's ability to diagnose not only the birth location and initial linear growth phase of the electrons as they are accelerated, and then to follow how they are transported from the outside in, is fascinating. Comparing his diagnosis to modeling will be the next step and of course a better understanding may lead to new mitigation techniques in the future.”
Delgado-Aparicio is already looking ahead. He said, “I want to take all the expertise that we have developed on MST and apply it to a large tokamak.” Two post-doctoral researchers who Delgado-Aparicio oversees can build on the MST finding but will do so at the Tungsten (W) Environment in Steady-state Tokamak (WEST) operated by the French Alternative Energies and Atomic Energy Commission (CEA) in Cadarache, France.
Delgado-Aparicio said, “What I want to do with my post-docs is to use cameras for a lot of different things including particle transport, confinement, radio-frequency heating and also this new twist, the diagnosis and study of runaway electrons. We basically would like to figure out how to give the 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 also co-authored the paper about this research. This group included seven physicists from the PPPL and eight from the University of Wisconsin. Three researchers from the University of Tokyo, Kyushi University and the National Institutes for Quantum and Radiological Science and Technology in Japan participated as did five members of Dectris, a Swiss manufacturer of detectors and one physicist from Edgewood College in Madison, Wisconsin.