Geiger lands DOE Early Career Award to investigate impurity transport in stellarators

In the quest to harness fusion as a viable energy source, the scientific community needs a better understanding of how impurities travel inside magnetically confined plasmas, the hot ionized gases of hydrogen isotopes that fuel fusion reactions.

When particles of this ultrahot plasma eventually bump into the walls of a fusion device’s vacuum chamber, the wall material is eroded and small amounts of highly charged impurities penetrate into the plasma.

“These heavy impurities dilute the fuel for fusion and emit intense line radiation that cools down the plasma,” says Benedikt Geiger, an assistant professor of nuclear engineering and engineering physics at the University of Wisconsin-Madison. “That’s a problem because we need the plasma to be extremely hot for fusion reactions to occur, so the accumulation of impurities in the plasma core hampers the feasibility of fusion energy.”

In a fusion device, instabilities in the plasma lead to turbulence, which can drive impurities into or out of the plasma core.

With support from a U.S. Department of Energy Early Career Award, Geiger will investigate how turbulence affects impurity transport in stellarators, which are fusion devices that use three-dimensional magnetic fields to confine the plasma inside a vacuum chamber.

“Turbulence is a very complex phenomenon and it’s not very well understood,” Geiger says. “In addition to providing new insights into turbulence, this research project will help us learn how to control impurities to avoid their accumulation.”

For this project, Geiger will conduct experiments on the Helically Symmetric eXperiment, or HSX, which is housed in the UW-Madison Department of Electrical and Computer Engineering, and at the massive Wendelstein 7-X stellarator experiment in Greifswald, Germany.

“These are the only two optimized stellarators in the world, and it’s great to have the opportunity to work with both of them,” Geiger says. “Also, the two devices have different 3D magnetic field structures, so comparing the two stellarators will allow us to study the transport from different perspectives.”

His experiments will involve injecting trace particles—the impurities—into the plasma using a laser ablation technique. Geiger will place a thinly coated glass plate inside the fusion device’s vacuum chamber and then shoot a laser onto the back side of the plate. The laser causes the coating to evaporate and distribute a carefully controlled amount of particles into the plasma edge.

Those impurities irradiate in the plasma, allowing Geiger to measure their radiation with a spectrometer and observe how they move from the plasma’s edge to its core. Knowing how long that journey takes and how the particles behave will enable Geiger to glean important information about the transport processes.

In his lab, Geiger and his collaborators have designed a new spectrometer system that will help them study impurity transport. Funding from the early career award now will enable him to build the system and install it to take measurements for this research.

“I’m very excited by this opportunity to actually implement our spectrometer,” Geiger says. “It will be quite interesting because the turbulence is expected to be very different between HSX and Wendelstein 7-X. But that’s only an expectation. Now, we might have a handle on measuring this and finding out.”