Unique university experiment empowers the future of fusion

Fusion research at the University of Wisconsin–Madison is bridging industry knowledge gaps to make limitless clean energy a reality. 

Scientists have long been investigating ways to harness the energy produced by fusion reactions—the same type of reactions that power the sun and other stars. While adding fusion to the energy mix was once considered a far-off dream, today there are a multitude of private companies vying to design and deploy the first fusion pilot plants by 2035. 

With this new wave of interest and investment in the private sector, university experiments are more crucial than ever. There is a strong and growing need for technology innovation, workforce development and improvements in the physics understanding necessary to support the development of economically viable fusion energy systems. 

The UW–Madison Helically Symmetric eXperiment (HSX) is uniquely positioned to meet those needs. Leveraging its flexibility and distinct magnetic field topology—and supported by a recent renewal of funding from the U.S. Department of Energy—HSX is advancing fusion science and technology through diverse and innovative research initiatives. 

Bringing HSX up to temp

HSX is a stellarator, a device that uses 3D magnetic fields to confine high-temperature plasma. Established through a collaboration with the Max Planck Institute for Plasma Physics in Germany, it’s the only stellarator in the world with quasi-helical symmetry. 

“The data we get from HSX is the only of its kind that you can get in the world, and the things we do with HSX can very significantly affect the decisions of future private companies in terms of where they will allocate their resources,” says Michael Richardson, a PhD student in the HSX lab.

Early experimental results proved that HSX’s quasi-helical symmetry is exceptionally effective at reducing neoclassical transport, a historical limitation of stellarators. Studies revealed that overcoming turbulent transport is the next frontier for improving confinement in stellarators.

Associate Professor Benedikt Geiger has led HSX since 2022 alongside Assistant Professor Adelle Wright, who joined the team in 2024. Geiger leads the experimental plasma physics side, looking at heat and particle transport, diagnostic developments, and plasma wall interaction. Wright focuses on theory exploration and simulation, particularly in the field of magnetohydrodynamics. 

HSX is also staffed by four instrumentation engineers and technologists who play a critical role in keeping the experiment running smoothly and continuously improving the facility. In recent years, they upgraded the system’s magnetic coils for greater flexibility, allowing the system to run a diverse range of experiments.

Currently, the team is installing a new gyrotron heating system which will improve plasma performance, reaching temperatures up to 60 million degrees Fahrenheit. HSX was also recently provided a neutral beam injector from the Max Planck Institute, a device that injects neutral particles of gas into the plasma, enabling even hotter, denser plasmas. 

Data that drives change

University experiments like HSX have the flexibility to investigate novel and high-risk concepts, allowing researchers to address critical gaps in the plasma physics understanding needed to support fusion power plant design and operation. Those knowledge gaps include areas such as plasma wall interactions, fusion materials research, divertor solutions, and turbulence theory. 

“If you really want to move forward with fusion power plants, then you have to investigate these technologies,” says Geiger. 

The impact of HSX research on private fusion companies can be seen clearly in the recent work of UW–Madison spinoff Type One Energy, whose co-founders include Chris Hegna, Professor in the Department of Nuclear Engineering and Engineering Physics and Electrical and Computer Engineering Professor Emeritus David Anderson, who served for decades as the director of HSX before Geiger stepped into the role. The physics basis for a practical fusion pilot power plant published by Type One Energy in March 2025 builds in part upon HSX research.

Training the fusion workforce

Universities not only support future fusion power plants through research, they also train the skilled workforce needed to design, build and operate those plants. 

Located in the heart of the engineering campus, HSX is readily accessible to students. The lab is a dynamic, hands-on training ground, providing meaningful and creative research opportunities that equip the next generation of the fusion workforce to drive the field forward. 

“Students who work on HSX get to experience the entire experimental lifecycle,” says Wright. “They develop their hypothesis, build the tools they need to take measurements, operate the machine, and analyze the data.”

That experience isn’t exclusive to graduate students, either. 

“We have strong engagement from undergraduate students, and we see that as our mission—to train undergraduates,” says Geiger. 

Undergraduate student Nick Merrell, a junior in the Engineering Physics program, works in the HSX lab doing a research project that involves designing a spectrometer to analyze light emitted by the plasma and identify impurities. 

“Ideally, we would only have hydrogen and helium in the plasma for fusion reactions, but since we’re a plasma confinement lab, we’re looking at how intentionally putting in different impurities might impact energy loss and how we can control it,” says Merrell. He notes that the project trained him in a “fun mix” of skills from designing mechanical parts in CAD to wiring sensors to understanding the theory and physics involved. 

Francisco Chavarria, also a junior studying engineering physics, is working on a computational project analyzing numerical errors in a stellarator physics code used to model the plasma in the device. Last summer, he also worked on an experimental project using an electron gun to map the field lines of HSX and confirm the magnetic topology. 

“The fusion field is constantly evolving,” says Chavarria. “I like the fact that I could spend years in this field and still not understand everything. This is the pursuit of science. There are always more questions to answer.”

Merrell and Chavarria each work closely with staff scientist mentors. HSX currently has four scientists that together advise nine graduate students and fourteen undergraduates. The mentorship, hands-on opportunities, and direct access to the experiment that UW–Madison students experience at HSX prepares them to contribute meaningfully to the future of fusion science and technology.

“There are a growing number of HSX alumni that now hold leadership positions at fusion institutes and companies, underscoring the success and impact of our program,” says Geiger. 

HSX alumni are making their mark at companies like Type One Energy, Thea Energy, and Helion Energy, as well as research laboratories including the Princeton Plasma Physics Laboratory, Oak Ridge National Laboratory, and the Wendelstein 7-X experiment in Germany. 

Stellar simulations

The accessibility of the HSX stellarator program is not only beneficial for experimental plasma research, but for computational research as well.

“From a theory and simulation perspective, it’s really beneficial to have a close connection with the experiment,” says Wright, explaining how access to an experiment is crucial for validating models, theories, and simulation codes, as well as understanding how to make predictions and hypotheses testable.

One recent focus for Wright’s team is macroscopic dynamics in HSX, known as magnetohydrodynamics (MHD). Kassia Schraufnagel, a PhD student co-advised by Geiger and Wright, uses multiple diagnostics to monitor magnetic and temperature fluctuations in the plasma. 

When multiple fluctuations occur simultaneously, the plasma is potentially experiencing MHD instabilities. By operating HSX with different plasma geometries, the team detects and characterizes various MHD modes. The goal of their research is to better understand and control the impact of global instabilities, like MHD, in future quasi-helically symmetric fusion devices.

The group has also produced the first simulations of turbulent transport in the entire HSX device using a state-of-the-art gyrokinetic code called GENE-3D. The unique geometry of HSX provides an excellent target for codes like GENE-3D. These types of activities facilitate international collaboration including efforts to benchmark gyrokinetic codes against experimental data.

In addition to advancing the physics basis for stellarators as a fusion energy concept, simulation and theory are also useful for informing design priorities for fusion pilot plants and validating those designs once they are created. There are many possible stellarator configurations and few have been built, so researchers rely on simulations to study the physics properties of different configurations. 

“Achieving fusion is really difficult,” says Wright. “It requires a wide range of expertise so, out of necessity, we should be working together.” 

Fusing new partnerships

Understanding this need to work together, the HSX lab prioritizes collaboration, both internally and externally. 

HSX lab members span three departments at UW–Madison including Nuclear Engineering & Engineering Physics, Electrical and Computer Engineering, and Physics, enabling interdisciplinary collaboration between graduate students, faculty, and staff with varied research interests.  

Lab members are allocated time on the machine to run their experiments and diagnostics. These run days are followed by group meetings where lab members come together to discuss the data, reflect on hypotheses, and contemplate the implications of their findings. 

“I’m one of the newer students in the lab, but I’ve been working closely with some of the older graduate students to learn how to run the machine and switch between different configurations,” says Schraufnagel. “It’s nice to see how supportive everyone is and how we exchange information.”

The broader fusion environment at UW–Madison is rich and highly collaborative as well. 

“If you want to be an experimental plasma physicist, this is a really great place to be,” says Richardson, noting that UW–Madison has multiple plasma physics experiments, including HSX, the Pegasus-III Experiment, Wisconsin HTS Axisymmetric Mirror (WHAM), the Madison Symmetric Torus (MST), the Big Red Ball (BRB), and several other small scale experiments housed in the Engineering Research Building. 

Beyond university-level partnerships, HSX attracts scientists from around the world for collaborations, engaging in a wide range of partnerships with public institutions and private fusion companies.

“The public-private partnership space is changing extremely quickly,” says Wright. “I’ve seen real growth in the interaction of fusion companies with universities.” Wright notes that there are increasingly more effective mechanisms to facilitate those partnerships, including high-demand programs like INFUSE and ARPA-E.

In September 2025, Geiger and Alexis Wolfmeister, the co-PI on the project, were awarded an INFUSE grant in collaboration with Type One Energy to develop a cost-effective, compact divertor and edge spectroscopy solution for fusion pilot plants. The technology will be tested on HSX before being integrated into Type One Energy’s Infinity One plant.

The fusion industry is rapidly evolving to advance technologies that promise a brighter, cleaner, more sustainable future. HSX scientists at the UW–Madison are invested in closing the knowledge gaps and training the next generation to make that promise a reality.

---