DIII-D tokamak used to test spacecraft heat shield materials

November 16, 2021, 3:00PMNuclear News
A set of graphite rods was exposed to hot plasma in the DIII-D tokamak. Researchers measured the ablation behavior under extreme heat and particle flow to simulate conditions experienced by spacecraft heat shields during atmospheric entry. (Image: General Atomics)

As a spacecraft on a research mission hurtles at up to 100,000 miles per hour toward the surface of a gas giant like Jupiter, the atmospheric gases surrounding the spacecraft turn to plasma, and spacecraft temperatures increase to more than 10,000 °F.

Spacecraft destined for a high-temperature atmospheric entry use a heat shield designed to burn up at a controlled rate to keep excess heat from harming the scientific payload inside the craft. Ensuring that a heat shield can perform the intended function is a challenge, however, because those extreme conditions are difficult to replicate on Earth.

Researchers looking for an alternative testing environment have turned to the DIII-D National Fusion Facility at General Atomics in San Diego, Calif., where carbon-based materials used for heat shields are exposed to the plasma inside the tokamak. The researchers explained their work in a news release issued as they prepared to present their research during the 63rd Annual Meeting of the APS Division of Plasma Physics, held November 8–12 in Pittsburgh, Pa.

DIII-D has what it takes: Past heat shield testing approaches have used lasers, plasma jets, and hypervelocity projectiles, but no single method could simulate the heating conditions of a high-speed atmospheric entry.

While most experiments at DIII-D are intended to explore the physics basis for fusion energy, the Divertor Materials Evaluation System (DiMES) is designed to test materials for future reactors. DiMES can expose test samples to various plasma conditions and launch pellets of test material through the plasma.

“Certain regions of the plasma in DIII-D closely approximate the conditions created when heat shields impact planetary atmospheres at extreme velocities,” said Dmitri Orlov, of the University of California–San Diego, who led the multi-institutional team. “Our intent with these experiments was to leverage both these conditions and DIII-D’s rich suite of diagnostic instruments to develop a more accurate model of heat shield behavior.”

Orlov and Eva Kostadinova, of Auburn University, working with a team of scientists and undergraduate and graduate students, used DiMES to study carbon samples under extreme conditions and refine predictive models for heat shield behavior. The experiments were conducted under the Frontier Sciences program, which is funded by the Department of Energy to provide access to DIII-D and other DOE-funded facilities to the broader physics community.

To scale: The team was able to gather and extrapolate a range of data on the samples. “DIII-D features relatively long plasma discharges with well-controlled stable conditions at the edge, where the heat flux and the flow speed are similar to those experienced during atmospheric entries,” Kostadinova said. “This allowed us to simulate some of the most extreme conditions heat shields have experienced, such as the entry of the Galileo probe to Jupiter’s atmosphere, without the need to launch our test samples at high velocities.”

By applying scaling techniques, the researchers extrapolated the results to larger projectiles and longer exposures, which allowed for comparison with experimental data from previous space flight missions and other on-ground testing facilities. The results could help researchers plan exploration missions to the solar system’s gaseous giants and hyperbolic re-entries into Earth’s atmosphere.

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