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Steffi Diem
October 2, 2020

Focus on new faculty: Stephanie Diem, advancing fusion science through plasma physics experiments

Written By: Adam Malecek

An undergraduate research experience as a freshman at UW-Madison ignited Stephanie Diem’s passion for fusion research and engineering.

Starting college, Diem considered a wide variety of majors before deciding on nuclear engineering, in part due to the excellent career outlook for nuclear engineers. “At the time, I had no idea what engineers really did,” Diem says.

To help pay for school, she sought out student hourly positions in the Department of Nuclear Engineering and Engineering Physics. The first job opening she came across was with Professor Ray Fonck, who took the time to explain his fusion research and gave her a tour of his lab and the Pegasus toroidal experiment.

“He hired me on the spot,” Diem says. “That was the day I learned about fusion energy and really became interested in engineering. I love that with fusion research you get to study the complex physical state of plasma, and then we can apply that knowledge to potentially realize fusion as a virtually unlimited, environmentally friendly energy source to help address climate change.”

Now, Diem has returned to her alma mater as an assistant professor of engineering physics. And she is working alongside Fonck, helping guide the Pegasus research program that first introduced her to fusion as an undergraduate.

Fusion, the process that fuels our sun and the stars, occurs when hydrogen nuclei fuse together in a plasma, an ultra-hot ionized gas, and as a result produce an enormous amount of energy. To be able to create this process on Earth, researchers use magnetic fields to confine the extremely hot plasma within fusion devices.

Diem’s research interests are in the area of experimental plasma physics for developing fusion energy with an emphasis on using experimental data to validate numerical models. Her research focuses on using microwaves to efficiently heat and drive current in magnetically confined plasma.

“It’s kind of like your microwave oven at home, except the microwave systems I work with are 1,000 times more powerful,” Diem says. “When the charged particles in the plasma are confined by a magnetic field, they have a certain frequency that they rotate at around the magnetic field. And so I can choose a radio frequency wave that’s in tune with the particles’ rotation frequency. If those frequencies match, I can actually kick a lot of energy into the plasma, providing heat and driving current.”

Driving current in the plasma induces the magnetic field that helps improve the plasma’s confinement. Researchers can also use the current to make the plasma more stable.

At UW-Madison, Diem works on the Pegasus-III experiment, which is currently being constructed and will play a significant role the national fusion research program supported by the U.S. Department of Energy’s Office of Fusion Energy Sciences.

The DOE recently awarded the Pegasus research program $7.9 million in new funding to reconfigure the decommissioned Pegasus toroidal experiment—converting it to a much more powerful facility that will provide a dedicated U.S. platform to study innovative techniques for starting a plasma, and for heating and driving current in these plasmas.

“Our research with the new Pegasus-III experiment aims to help reduce the cost and complexity of future fusion reactors,” she says. “It’s really great to be able to do this research at UW-Madison because we have the flexibility and creativity to develop new techniques and test them on a university-scale experiment before implementing them on larger devices.”

Diem graduated from UW-Madison in 2003 with a bachelor’s degree in nuclear engineering. She went on to earn her master’s and PhD degrees in plasma physics from Princeton University, where she conducted in-depth physics studies and experimental measurements of the microwave heating process on the National Spherical Tokamak Experiment at the Princeton Plasma Physics Laboratory for her graduate thesis.

After her PhD, Diem landed a position as a research and development staff scientist in the Fusion Energy Division at Oak Ridge National Laboratory. In her role at ORNL, Diem led and contributed to experimental research on a variety of fusion experiments around the world, including spending two years on long-term assignment at the DIII-D National Fusion Facility at General Atomics in San Diego, California. She joined the UW-Madison faculty in February 2020.

For Diem, UW-Madison’s excellence in fusion science and unique opportunities in experimental plasma physics were a big draw. “I really love UW-Madison for fusion energy and plasma science and technology because there are three departments on campus—nuclear engineering and engineering physics, electrical and computer engineering, and physics—that all have experiments and theory efforts dedicated to this field,” she says. “That’s really unique. UW-Madison stands out as having one of the broadest university research and graduate education programs in plasma and fusion research in the country.”

She’s also excited to become a colleague of some of the very professors who inspired her as an undergraduate. Diem says she appreciates the opportunities to collaborate with other experts in the Department of Nuclear Engineering and Engineering Physics who specialize in theory and computational plasma physics.

In addition to her research, Diem is passionate about sharing science with the public. She participated in the Alan Alda Science Communication Bootcamp at Stony Brook University in 2017, where she learned how to engage the public while communicating scientific topics.

That experience prompted her to adopt a much more interactive approach for her science outreach events. Rather than deliver prepared talks, Diem now prioritizes simple experiments that help members of the public learn scientific concepts by engaging in hands-on activities.

“When people experiment on their own and make discoveries, just watching the joy on their faces is amazing. I love it,” Diem says. “I’ve actually flown to outreach events where I checked two large suitcases filled with stuff for people to experiment with.”

Diem says she’s excited to bring a similar approach to teaching students in her courses. “I really prefer to interact with students instead of just talking at them,” she says. “I feel that gives a better learning experience if the students are part of the process rather than just an observer.”