Whistling While You Work: Fusion Scientists Find Inspiration in Earth’s Ionosphere
Electromagnetic whistler waves measured for the first time at DIII-D National Fusion Facility
San Diego, May 9, 2018 – The challenge of fusion energy is often compared to capturing – and holding – lightning in a bottle. The analogy is apt because lightning and a fusion energy plasma have a lot in common, including very high temperatures, massive electric charges and complex fluid dynamics.
Researchers at the DIII-D National Fusion Facility in San Diego recently found another characteristic shared between the two types of plasmas: an odd electromagnetic wave known as a whistler. If their theories are correct, the whistler observation could help better understand runaway electrons in tokamaks and could even help control these destructive particles.
For more than a century, mysterious electromagnetic waves that occur naturally in the Earth’s ionosphere – generally caused by lightning – have been detected over telephone lines, antennas and satellites. They were named “whistlers” because of their characteristic time-varying frequencies, which are unmistakable when the signals are converted into sound.
Theorists have for years predicted that whistlers could exist in a tokamak – a toroidal vacuum chamber in which plasmas are heated to 100 million degrees to cause fusion – but experimentalists were never able to directly observe the waves. Recently, however, a team at DIII-D was able to generate extremely diffuse plasmas with a low magnetic field that yielded the characteristic whistling of the electromagnetic oscillations. The researchers determined that the whistlers are driven by runaway electrons in the tokamak and that they play a role in regulating runaway generation and evolution.
“The whistler measurements were interesting from a fundamental science standpoint, because they improve our understanding of how runaway electrons behave in our experiments,” said Carlos Paz-Soldan, a DIII-D researcher and member of the team. “We are very motivated to see how we can excite these waves to improve runaway electron control in tokamak fusion reactors.”
The research was featured in a study recently published in Physical Review Letters.
Runaway electrons develop due to an unusual feature of plasmas – a collisional drag that decreases with increasing velocity. This allows energetic electrons that are in the presence of an electric field in a tokamak to freely accelerate to high energies. Runaway electrons in fusion reactors only reach a terminal velocity as they approach the speed of light, per Einstein’s theory of relativity.
Runaway electrons are a significant concern for future large tokamak devices, such as ITER, and must be mitigated due to their potential to cause significant wall damage. Multiple approaches for controlling runaways are being explored at DIII-D and other fusion facilities.
Runaway electron-driven whistler instabilities and the wave-induced scattering observed at DIII-D demonstrate that the evolution of runaway electrons may involve more than just classical collisional and radiative processes. This is an important mechanism to include in predictions of runaway generation. It also raises the possibility that externally driven waves in the whistler frequency range could be used to suppress and control runaways. The team intends to pursue this concept by installing a high-frequency helicon antenna at DIII-D to stimulate whistler waves. While much work remains to be done, the team thinks there is a possibility that exciting whistlers or similar waves in the plasma could prevent or control runaway electrons by bleeding energy from the particles.
The study titled, “First Direct Observation of Runaway-Electron-Driven Whistler Waves in Tokamaks,” included coauthors Don Spong and Cornwall Lau of Oak Ridge National Laboratory; General Atomics’ Paz-Soldan; and William Heidbrink with the University of California, Irvine, and others.
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