Advance in Controlling "Runaway" Electrons in Tokamaks

April 6, 2020

A multi-institutional team of scientists working on the DIII-D tokamak device at General Atomics has discovered a way to mitigate the problem of “runaway” electrons that threatens the performance of future tokamak fusion power plants. These high energy electrons can cause potential damage to materials. They are not a significant threat to operation in current experiments but, if not controlled, are a significant issue for future fusion devices such as ITER and power plants.

General Atomics has issued the following press release:

San Diego, April 6, 2020 - Scientists at the DIII-D National Fusion Facility have made an important discovery that could significantly improve the reliability of future fusion power plants by helping prevent damaging phenomena known as “runaway” electrons (REs). The research, published in an article today in the journal Nuclear Fusion, discovered new mechanisms that dissipate REs through exploitation of resonances that occur between the high-energy electrons and the magnetic field configuration.

This improved understanding of RE behavior suggests a pathway toward controlling them in large fusion devices known as tokamaks, such as the ITER experiment under construction in France and the commercial power plants that will follow it.

“Runaway electron beams are a significant challenge for tokamak operation because of their potential to seriously damage the walls of the device,” said Andrey Lvovskiy, who led the multi-institutional team at DIII-D. “To control their impacts on ITER, we need better information about their behavior and effects.”

DIII-D, which General Atomics operates as a national user facility for the Department of Energy’s Office of Science, hosts researchers from more than 100 institutions across the globe, including 40 universities. The heart of the facility is a tokamak that uses powerful electromagnets to produce a doughnut-shaped magnetic field for confining a fusion plasma. Plasma is the “fourth state of matter” in which electrons are stripped from the nuclei of their atoms, leaving charged ions that are confined by the magnetic fields. (See Fusion Energy 101 explainer below for more detail on how fusion works.)

In DIII-D, plasma temperatures several times hotter than the Sun are routinely achieved. At such extremely high temperatures, hydrogen isotopes can fuse together and release energy.

However, certain conditions in a tokamak can cause a loss of plasma confinement, known as a disruption. These disruptions can separate an existing population of confined, energetic electrons, which are further accelerated to nearly the speed of light by the quenching plasma. These high-energy particles, known as “runaway” electrons, can cause significant damage to materials that they encounter.

In current tokamaks, the potential damage is rarely a threat to operation. The much larger size and current of future power-plant scale reactors, including ITER, means the damage potential there is greater. However, the complex physics of RE beams makes predicting the degree of impact difficult.

To improve understanding, the DIII-D team intentionally created and maintained a RE beam at low current and impurity content. Using an ultrafast gamma-ray detector, they were able to measure the gamma radiation from the RE beam to determine the energy, number, and spatial distribution of the REs. They were also able to measure the current distribution inside the RE beam, which is an important element in understanding the beam’s stability.

The results suggest that REs may excite kinetic instabilities in the plasma that both drain energy from the runaway electron population and act to redirect some electrons so that they are lost from the runaway beam before reaching very high energy. This points toward a method to control REs and mitigate potential damage to the tokamak walls, though further work on detection and exploitation of RE-driven instabilities is necessary. The team is continuing its research in this area, as RE-driven kinetic instabilities have been an active area of study over the past few years at DIII-D.

Another novel result from these studies is the measurement of the current distribution inside the RE beam. Theoretical models predict that the RE beam can have a “peaked” current profile, that is, its current is concentrated in the center of the beam. Such a profile can destabilize the beam and cause magnetohydrodynamic (MHD) instabilities (those driven by plasma current and pressure), which worsen the confinement of REs.

The experiments on DIII-D confirmed these theoretical models, observing both the predicted peaking of the RE current profile and associated impact. Scientists further showed that, by increasing the RE current, they could deliberately produce MHD instabilities and destabilize the beam. It is still unclear whether this effect will be advantageous or a problem in future fusion devices operating at high current, as an uncontrolled instability could cause both significant loss of REs and potential damage to the chamber walls. Thus, further work on mitigation and control techniques is necessary.

“These discoveries are very important for our understanding of the physics of runaway electrons,” said David Hill, Director of DIII-D. “The research is one more example of how collaboration between research institutions is essential for the progress of fusion science.”

The work, part of DOE’s Frontier Science Campaign that funds research relevant to both fusion and general plasma science, involved researchers from Oak Ridge Associated Universities, General Atomics, the University of Milano-Bicocca, the University of California San Diego, and Oak Ridge National Laboratory.

For more information contact: Zabrina Johal - 858-455-4004 - zabrina.johal@ga.com