Toroidal (doughnut-shaped) tokamaks are prone to intense bursts of heat and particles, called edge localized modes (ELMs). These ELMs can damage the reactor walls and must be controlled to develop reliable fusion power. Scientists have learned to tame these ELMs by applying spiraling rippled magnetic fields to the surface of the plasma that fuels fusion reactions.
However, the taming of ELMs requires very specific conditions that limit the operational flexibility of tokamak reactors. Now, researchers at the US Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) and General Atomics (GA) have developed a model that, for the first time, accurately reproduces the conditions for ELM suppression in the DIII-D National Fusion Facility that GA operates for DOE.
The model predicts the conditions under which ELM suppression should extend over a wider range of operating conditions in the tokamak than previously thought possible. The work presents important predictions for how to optimize the effectiveness of ELM suppression in ITER, the massive international fusion device under construction in the south of France to demonstrate the feasibility of fusion power.
Fusion combines light elements in the form of plasma—the hot, charged state of matter composed of free electrons and atomic nuclei that makes up 99% of the visible universe—to generate massive amounts of energy. Tokamaks are the most widely used devices by scientists seeking to replicate fusion as a renewable, carbon-free source of virtually limitless energy for generating electricity.
PPPL physicists Qiming Hu and Raffi Nazikian are the lead authors of a paper describing the model in Physical Review Letters. They note that under normal conditions the rippled magnetic field can only suppress ELMs for very precise values of the plasma current that produces the magnetic fields that confine the plasma. This creates a problem because tokamak reactors must operate over a wide range of plasma current to explore and optimize the conditions required to generate fusion power.
The authors show how, by modifying the structure of the helical magnetic ripples applied to the plasma, ELMs should be eliminated over a wider range of plasma current with improved generation of fusion power. Hu said he believes the findings could provide ITER with the wide operational flexibility it will need to demonstrate the practicality of fusion energy.
What we have done is to accurately predict when we can achieve ELM suppression over wider ranges of the plasma current. By trying to understand some strange results we saw on DIII-D, we figured out the key physics that controls the range of ELM suppression that can be achieved using these helically rippled magnetic fields. We then went back and figured out a method that could produce wider operational windows of ELM suppression more routinely in DIII-D and ITER.—Raffi Nazikian
The findings open the door to enhanced tokamak operation.
This work describes a path to expand the operational space for controlling edge instability in tokamaks by modifying the structure of the ripples. We look forward to testing these predictions with our upgraded field coils that are planned for DIII-D in a few years’ time.—GA scientist Carlos Paz-Soldan, co-author
Support for this research comes from the DOE Office of Science. Collaborators include researchers at the University of California, San Diego, and the Max Planck Institute for Plasma Physics. Part of the data analysis was performed using the OMFIT integrated modeling framework developed by GA scientists.
Q. M. Hu, R. Nazikian, B. A. Grierson, N. C. Logan, D. M. Orlov, C. Paz-Soldan, and Q. Yu (2020) “Wide Operational Windows of Edge-Localized Mode Suppression by Resonant Magnetic Perturbations in the DIII-D Tokamak” Phys. Rev. Lett. 125, 045001 doi: 10.1103/PhysRevLett.125.045001