Equipped with armor tiles, DIII-D takes “negative triangularity” plasma to new highs

April 26, 2023, 7:00AMNuclear News
A side-by-side comparison of a standard plasma configuration (at left) and the plasma created during the negative triangularity campaign at DIII-D, which was made possible by the installation of a temporary divertor region. (Image: General Atomics)

The DIII-D National Fusion Facility in San Diego, Calif., has completed a monthlong research campaign using a negative triangularity plasma configuration inside its fusion tokamak and produced initial data that “appear very encouraging,” according to an April 24 news release from General Atomics (GA), which operates the Office of Science user facility on behalf of the Department of Energy. Full experimental results on “the highest-powered negative triangularity experiments in the history of the U.S. fusion research program” are expected this summer, according to GA.

Researchers from 15 institutions around the world participated in the experiment to explore the potential benefits of negative triangularity for exhaust handling and particle confinement. If such benefits can be confirmed and applied to magnetic confinement fusion energy concepts, they could translate to improved efficiency and economics.

Shaping up: DIII-D heats plasma at temperatures in excess of 100 million degrees Celsius inside a vacuum vessel with a D-shaped cross section. Almost every experiment in DIII-D uses positive triangularity plasma confinement, which means the plasma inside—if seen on cross section—would form a similar D shape, seemingly molded to fit the interior of the tokamak.

The cross section of a plasma confined with negative triangularity, on the other hand, would present a reversed or mirror image of that D shape, with the rounded side of the D facing the center of the tokamak. According to GA, that plasma would be less likely to impact the tokamak’s inner walls, which could potentially offer benefits for the design of future fusion power plants.

Experimental design: To prevent the harsh conditions within the tokamak from damaging the inner walls under any plasma configuration, DIII-D must channel excess heat away from the edge of the plasma. Heat exhaust is guided by magnetic fields into a region known as the divertor, where it is dispersed, cooled, and vented.

DIII-D is the largest operating magnetic fusion research facility in the United States, and it is also, according to GA, “one of the most flexible tokamaks in the world, which allows it to run innovative experiments and research campaigns that cannot be conducted anywhere else.”

That flexibility permitted modifications to DIII-D’s interior prior to the recent negative triangularity experiment. DIII-D’s divertor is a permanently installed component positioned for a standard plasma configuration. Because the heat exhaust generated during a negative triangularity campaign would be outside of the installed divertor region, special armor tiles were installed in a temporary divertor region. Once that armor was in place, the campaign ran for four weeks in January and February 2023, producing 178 run hours, which represents 22 percent of the 800 run hours planned for DIII-D in 2023.

Teamwork: “The negative triangularity research team deserves a lot of credit for their planning and execution of this research campaign,” said Richard Buttery, director of the DIII-D National Fusion Facility. Participants joined the research campaign from eight universities, four national laboratories, GA, and several international institutions, including the Swiss Federal Institute of Technology Lausanne, Germany’s Max Planck Institute for Plasma Physics, and Japan’s National Institute for Fusion Science.

The team was led by Kathreen Thome, a GA scientist at DIII-D, and Carlos Paz-Soldan, associate professor of applied physics at Columbia University's School of Engineering. Paz-Soldan recently served as the co-lead for a collaboration between Columbia University and the Massachusetts Institute of Technology, facilitating a graduate-level fusion reactor design course in which students designed fusion reactors leveraging negative triangularity for electricity production. Max Austin, a DIII-D scientist affiliated with the University of Texas–Austin, served as the deputy research lead.

“In my entire time working at DIII-D, I have never seen the research team come together like this,” said Thome. “From our incredible engineers who made this campaign possible, to our world-class operations and science groups, this was an international team effort from top to bottom. It was truly special to be part of it. I know the entire team is very excited to dive into the results.”

“Our campaign consisted of several experiments each targeting an important part of tokamak plasma physics, and our team benefited from great expertise on all fronts,” said Paz-Soldan. “We were able to gather data to push the limits of the plasma’s behavior in this unique configuration, and to understand how all the pieces fit together into a successful fusion reactor operating regime."

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