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Whistler waves appear to scatter runaway electrons in fusion plasmas

  • The first direct observation of whistler waves in a fusion plasma took place at the DIII-D tokamak, an experimental vessel designed to withstand conditions hotter than the sun. Credit: DIII-D National Fusion Facility, U.S. Dept. of Energy.

  • An ORNL-led team is the first to directly observe elusive whistler waves inside a highly energized magnetic field as runaway electrons zoom around a laboratory fusion plasma. Credit: Don Spong, Fred Jaeger and Cornwall Lau/Oak Ridge National Laboratory, U.S. Dept. of Energy.

  • The first direct observation of whistler waves in a fusion plasma took place at the DIII-D tokamak, an experimental vessel designed to withstand conditions hotter than the sun. Credit: DIII-D National Fusion Facility, U.S. Dept. of Energy.

  • An ORNL-led team is the first to directly observe elusive whistler waves inside a highly energized magnetic field as runaway electrons zoom around a laboratory fusion plasma. Credit: Don Spong, Fred Jaeger and Cornwall Lau/Oak Ridge National Laboratory, U.S. Dept. of Energy.

May 9, 2018 – When whistler waves are present in a fusion plasma, runaway electrons pay attention. 

A research team led by the Department of Energy’s Oak Ridge National Laboratory is the first to directly observe the elusive waves inside a highly energized magnetic field as electrons zoom around the plasma.

Using sophisticated technologies, the team made direct measurements of whistler waves produced when a laboratory plasma becomes unstable and generates runaway electrons.

“Observing whistler waves helps us better understand their underlying physical mechanisms, which could open an avenue to develop new techniques to control runaway electrons and keep them from potentially damaging fusion reactors,” said ORNL’s Don Spong who led the study published in Physical Review Letters.

For decades, scientists have been building the scientific basis for nuclear fusion as an energy source to generate electricity. Fusion happens when the nuclei of lighter atoms join under extraordinarily high temperatures to create a heavier nucleus, releasing energy. The hot, fused atoms form a plasma that is confined by high magnetic fields in an experimental vessel called a tokamak designed to withstand conditions hotter than the sun.

Instabilities can occur in such an extreme environment, causing a dramatic quenching, or cool down, of the plasma and producing runaway electrons that could veer off and burn holes in the tokamak’s interior wall. 

Runaway electrons also occur in nature. They are energized when lightning strikes or solar substorms disrupt the plasma environment of the Earth’s ionosphere, which is the atmosphere’s ionized upper layer. 

“We have known that runaway electrons drive whistler waves in the ionosphere during natural events, which led to theories that runaways would also drive similar electromagnetic waves in a tokamak plasma,” Spong said. 

Scientists have theorized that whistler waves regulate space weather and may help mitigate the damaging effects of energetic electrons on satellites orbiting the Earth, he said.

“As we learn more about the characteristics and excitation of whistler waves in tokamaks, we may be able to mimic similar behavior to protect plasma-facing components,” Spong added.

The team’s experiments took place at the DIII-D National Fusion Facility, a DOE user facility, which is operated by General Atomics in San Diego, California. The work was funded by the DOE Office of Science.

The study titled, “First Direct Observation of Runaway-Electron-Driven Whistler Waves in Tokamaks,” included coauthors Don Spong and Cornwall Lau of ORNL; Carlos Paz-Soldan with General Atomics; and William Heidbrink with the University of California, Irvine, and others. 

ORNL is managed by UT–Battelle for DOE’s Office of Science, the single largest supporter of basic research in the physical sciences in the United States. DOE’s Office of Science is working to address some of the most pressing challenges of our time. For more information, please visit http://energy.gov/science/.