A significant advance in numerical fusion energy sciences by University of Wisconsin-Madison researchers provides a new tool for solving pressing challenges for ITER, the international fusion experiment under construction in France.
Heinke Frerichs
As the largest fusion experiment ever built, the ITER reactor aims to demonstrate a fusion energy output that is 10 times as large as the energy required to heat its plasma, an ultra-hot ionized gas. Ultimately, researchers hope to harness fusion, the process that powers the stars, to develop a virtually unlimited, environmentally friendly energy source. The United States is a main shareholder on this device, which is considered an essential experiment within the recently adopted new national strategy to a fusion pilot plant in the 2040s.
The research team, led by Heinke Frerichs, an associate scientist in the Department of Nuclear Engineering and Engineering Physics, and Engineering Physics Professor Oliver Schmitz, developed a computational modeling approach that, for the first time, allows them to model specific conditions for ITER, including how 3D plasma boundaries will affect the fusion system. The researchers detailed their advance in a paper in the journal Physical Review Letters.
“This is a major step forward in our modeling and predictive capabilities for plasma regimes that are relevant for ITER,” says Frerichs, a member of Schmitz’s Plasma Edge Physics with 3D Boundaries research group. “For example, this tool will be highly useful for understanding the interaction between the plasma and material surfaces.”
The ITER Tokamak. Image: US ITER.
As a tokamak fusion device, the ITER reactor is shaped like a doughnut and will use powerful magnetic fields to confine the plasma. But even with this magnetic confinement, heat and particle loads will bombard the plasma-facing reactor components. And, left unmitigated, these power fluxes can melt or heavily erode material surfaces, reducing material lifetimes and degrading the fusion device’s performance.
To control transient power fluxes in tokamak devices such as ITER, researchers apply external 3D magnetic control fields to the plasma. However, this 3D control field breaks the toroidal symmetry of the tokamak’s magnetic confinement system, inducing a 3D plasma edge topology and turning it into a 3D system. Applying the control fields causes the nicely smooth surface of the doughnut-shaped plasma to become wiggly. This stabilizes transient particle and heat flux ejections, which would reduce the lifetime of the plasma facing components, but at the same time requires researchers to investigate this new state as fully 3D system. Modeling a 3D system is computationally much more challenging.
“When a fusion device is toroidally symmetric, researchers can apply two-dimensional models to characterize or make predictions,” Frerichs says. “But we can’t apply those two-dimensional models anymore when we’re dealing with a 3D system. So, for ITER, that means we can’t really predict what will happen when we apply the 3D magnetic control fields.”
To address this challenge, Frerichs and his collaborators developed a model that allows researchers to analyze how ITER will perform as a 3D system. They drew on computing resources from the Center for High Throughput Computing at UW-Madison in addition to a computing cluster at ITER. “Going from 2D models to 3D models adds a lot of computational demand,” Frerichs says.
Frerichs says this new modeling capability could enable researchers to develop an integrated solution for optimizing the control of steady state and transient power fluxes in the ITER reactor. “We need to bring these power fluxes to a sustainable level for ITER,” he says. “There’s more work to do to solve this problem, but this advance puts us on a good path forward.”
To learn more about this research, read this ITER Newsline article.
This work was supported by grants from the U.S. Department of Energy under grant DE-SC0020428 and DE-SC0013911 as well as from research funds provided by the UW-Madison Department of Nuclear Engineering and Engineering Physics and the College of Engineering.