The physicists Raffi Nazikian and Qiming Hu with numbers from the research behind them. Photo credit: Elle Starkman / PPPL Office of Communications
Imagine an airplane that can only reach an altitude or two after takeoff. This limitation is similar to the plight of scientists trying to avoid instabilities that limit the path to cleaner, safer, and abundant fusion energy in donut-shaped tokamak facilities. Researcher at the US Department of Energy’s Princeton Plasma Physics Laboratory (DOE) (PPPL) and General Atomics (GA) have now released a groundbreaking explanation of this tokamak limitation and how it can be overcome.
Toroidal or donut-shaped tokamaks are prone to strong heat and particle bursts, so-called Edge Localized Modes (ELMs). These ELMs can damage the reactor walls and must be controlled to develop reliable fusion performance. Fortunately, scientists have learned to tame these ELMs by creating spiraling magnetic fields on the surface of the plasma that drives fusion reactions. However, the taming of ELMs requires very specific conditions that limit the operational flexibility of tokamak reactors.
In a fusion reactor, the hydrogen is heated until it becomes a cloud-like ionized plasma. Image credit: ITER
ELM suppression
Now researchers from PPPL and 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 contains key predictions to optimize the effectiveness of ELM suppression in ITER, the massive international fusion device under construction in southern France to demonstrate the feasibility of fusion force.
Fusion, the force that powers the sun and stars, combines elements of light in the form of plasma – the hot, charged state of matter made up of free electrons and atomic nuclei and making up 99 percent of the visible universe – to create massive amounts of energy. Tokamaks are the most common devices used by scientists looking to recreate Fusion as a renewable, carbon-free source of virtually unlimited energy to produce electricity.
PPPL physicists Qiming Hu and Raffi Nazikian are the lead authors of a paper describing the model in Physical Examination Letters. They find that under normal conditions, the corrugated magnetic field can only suppress ELMs for very precise values of the plasma current that creates the magnetic fields that limit the plasma. This creates a problem because tokamak reactors must be operated over a wide range of plasma flow in order to study and optimize the conditions required to generate fusion power.
Modify magnetic waves
The authors show how, by modifying the structure of the helical magnetic waves applied to the plasma, ELMs should be eliminated over a greater area of plasma flow with improved generation of fusion power. Hu said he believes the results could provide ITER with the broad operational flexibility needed to demonstrate the feasibility of fusion energy. “This model could have a significant impact on the suppression of ELMs in ITER,” he said.
In fact, “we predicted exactly when we could achieve ELM suppression over larger areas of the plasma flow,” said Nazikian, who oversees PPPL research on tokamaks. “By trying to understand some of the strange results we saw with DIII-D, we figured out the key physics that control the range of ELM suppression that can be achieved with these helically corrugated magnetic fields. We then went back and found a method that could routinely generate wider operating windows for ELM suppression in DIII-D and ITER. ”
Improved tokamak operation
The results open the door to improved tokamak operations. “This work describes a way to expand the operational space to control edge instability in tokamaks by modifying the structure of the waves,” said Carlos Paz-Soldan, GA researcher and co-author of the paper. “We look forward to testing these predictions with our improved field coils, which are planned for DIII-D in a few years.”
Returning to the airplane analogy, “If you could only fly at one or two different altitudes, travel would be very limited,” said PPPL physicist Brian Grierson, co-author of the paper. “By setting the restriction, the aircraft could fly over a wide range of altitudes to optimize its trajectory and accomplish its mission.” In the same way, an approach is presented in the present work which aims to expand the ability of fusion reactors to work free of ELMs, which can damage the plants and hinder the development of tokamaks for fusion energy.
Reference: “Wide operating window for suppressing the edge-localized mode through resonant magnetic disturbances in the DIII-D tokamak” by QM Hu, R. Nazikian, BA Grierson, NC Logan, DM Orlow, C. Paz-Soldan and Q. Yu, 23. July 2020, Physical Examination Letters.
DOI: 10.1103 / PhysRevLett.125.045001
Support for this research comes from the DOE Office of Science. The team includes researchers at the University of California at San Diego and at the Max Planck Institute for Plasma Physics. Part of the data analysis was carried out with the integrated OMFIT modeling framework developed by GA scientists.
