Stars contain their plasma with the force of gravity, but here on earth, plasma in fusion tokamaks must be magnetically confined. That confinement is tenuous, because tokamaks are subject to edge localized modes (ELM)—intense bursts of heat and particles that must be controlled to prevent instabilities and damage to the fusion reactor.
Researchers at the Department of Energy’s Princeton Plasma Physics Laboratory (PPPL) and at General Atomics (GA) recently published a paper in Physical Review Letters explaining this tokamak restriction and a potential path to overcome it. They have developed a new model for ELM suppression in the DIII-D National Fusion Facility, which is operated by GA for the DOE. PPPL physicists Qiming Hu and Raffi Nazikian are the lead authors of the paper, which was announced on August 10 by PPPL.
ELM suppression: ELMs can be contained under certain conditions by applying spiraling, rippled magnetic fields to the surface of the plasma, but the conditions required for ELM suppression to work have limited the operational flexibility of tokamaks. By reproducing the conditions for ELM suppression in the DIII-D tokamak, the new model has revealed the potential for ELM suppression under an expanded range of operating conditions.
Under normal conditions, the rippled magnetic field can suppress ELMs only for very precise values of the plasma current that produces the magnetic fields that confine the plasma. According to PPPL, the researchers found that by modifying the structure of the helical magnetic ripples applied to the plasma, ELMs should be eliminated over a wider range of plasma current, improving the generation of fusion power.
ITER implications: Hu said he believes the findings could provide ITER with the wide operational flexibility it will need to demonstrate the practicality of fusion energy. “This model could have significant implications for suppressing ELMs in ITER,” he said.
Nazikian, who oversees PPPL research on tokamaks, said, “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."
At General Atomics: “This work describes a path to expand the operational space for controlling edge instability in tokamaks by modifying the structure of the ripples,” said Carlos Paz-Soldan, a GA scientist and a coauthor of the paper. “We look forward to testing these predictions with our upgraded field coils that are planned for DIII-D in a few years’ time.” The DIII-D, located in San Diego, Calif., is equipped with more than 80 diagnostic instruments to measure parameters throughout the plasma.