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By shooting pellets of frozen hydrogen into fusion plasma in a tokamak, scientists at the DIII-D National Fusion Facility have been able to control instabilities in the magnetic field that holds plasma together.
By shooting pellets of frozen hydrogen into fusion plasma in a tokamak, scientists at the DIII-D National Fusion Facility have been able to control instabilities in the magnetic field that holds plasma together.
Composite image by the author; graphics courtesy General Atomics and Oak Ridge National Laboratory.

The Science

Scientists use large machines called tokamaks to research fusion power. These tokamaks use magnetic fields to control plasma used to produce a fusion reaction. “Magnetic islands” are unstable structures that form in these magnetic fields. These islands release energy from the plasma and stop the fusion reaction. Researchers at the DIII-D National Fusion Facility (a DOE Office of Science user facility) discovered that firing frozen pellets of deuterium (hydrogen with an extra neutron) deep into the plasma caused the magnetic islands to shrink. Computer simulations suggest that the pellets cause turbulence. This turbulence changes the amount of the electric current moving through the island, which drives its size.

The Impact

Fusion reactors currently use microwave beams to stabilize these magnetic islands. Calculations show that firing the pellets can shrink the islands enough that tokamaks can eliminate them with 70 percent less microwave power than they would use otherwise. This makes the fusion process more efficient and stable.

Summary

Fusion tokamaks operate by confining a plasma within powerful magnetic fields long enough that it can be heated to the extreme temperatures where fusion reactions can occur. Instabilities in the magnetic fields can allow energy to escape, stopping the reaction. One such instability is a phenomenon known as a magnetic island, which is a structure that tears holes in the magnetic field. In some cases, islands can be eliminated by driving a localized current inside them with microwave beams, but this requires a significant amount of energy. Researchers at DIII-D observed that firing frozen pellets of deuterium deep into the plasma caused magnetic islands to shrink. Computer simulations determined that the shrinkage was likely caused by increased turbulence in the plasma due to the injected pellets. The shrunken islands can then be completely eliminated with 70% less microwave power than what is normally required. Saving these resources could improve the net electricity output of a reactor. Thus, the approach may offer substantial benefits for future fusion reactors.

Contact

Laszlo Bardoczi
General Atomics
bardoczil@fusion.gat.com

Funding

This work is supported by the General Atomics Postgraduate Research Participation Program administered by Oak Ridge Associated Universities. This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Fusion Energy Sciences, using the DIII-D National Fusion Facility, a DOE Office of Science user facility.

Publications

Bardoczi et al., “Controlled NTM Healing via Fueling Pellets and its Impact on ECCD Requirements for Complete NTM Stabilization.” Nuclear Fusion. 59 (2019) [DOI: 10.1088/1741-4326/ab472d]