The University of Wisconsin-Madison stellarator, one of only two stellarators operating in the United States, has received a substantial grant from the U.S. Department of Energy, totaling $5.1 million over three years.
Now in its ninth year of generating plasma, the Helically Symmetric eXperiment, or HSX, is in its prime, says Electrical and Computer Engineering Professor David Anderson. HSX’s one-of-a-kind shape makes it a unique tool to explore key challenges to fusion energy, and members of the HSX team also are helping guide the future of stellarator research by leading a study to explore next steps for the field.
The most prevalent and well-developed fusion-research device is called a tokamak, which is shaped like a donut. When current is driven in the plasma the long way around the donut, it produces magnetic fields that contain plasma, an ionized form of gas.
A stellarator is like a twisted donut that uses the physical shape of coils, rather than current within the plasma, to generate magnetic fields. A stellerator’s three-dimensional magnetic fields theoretically can confine plasma indefinitely because they aren’t constrained by a transformer or the high-current instabilities that plague tokamaks. These properties mean stellarators are widely viewed as the main alternative to tokamaks for fusion reactors, Anderson says.
Despite the benefits of a stellarator, particles leak out faster than in a tokomak, which— unlike a stellarator—has a direction of symmetry in its magnetic fields. This is where HSX’s special design comes in: Outside the stainless steel vacuum vessel is a set of twisted copper coils that form a specially shaped magnetic “bottle” that restores a direction of symmetry to the magnetic field, improving confinement.
“HSX mimics the good confinement of a tokomak with the engineering advantages of a stellarator. It’s the only device in the world testing this property,” says Anderson.
The HSX team is composed of around 15 core faculty members, scientists and students. Research on the device ranges from plasma transport using modulated heating experiments to magnetic field reconstruction and turbulence studies via probes. Graduate students are constructing a beam line that is expected to double the device’s heating capacity and are collaborating with Oak Ridge National Laboratory to develop software codes for HSX. Additionally, the HSX team has a close research relationship with the TJ-II stellarator in Spain.
Chris Clark is an electrical and computer engineering PhD student working on an experiment to measure HSX’s impurity confinement properties. The plasma created in HSX is composed of a hydrogen isotope, and any other type of ion that gets into the plasma is considered an impurity that dilutes the fuel and radiates away energy. Impurities are hard to prevent; when hydrogen fuses, helium ions result. Additionally, ions from the wall of the vacuum vessel can also sneak into the plasma. “Impurity transport is something that has to be controlled, and all fusion devices have to consider this,” Clark says.
Clark is building a system to track and measure impurity radiation. He will place a vapor-deposed layer of aluminum on a glass slide and then vaporize the metal with a high-energy laser, causing the particles to fly into the plasma. Via an array of soft x-ray detectors, Clark will measure the amount of power radiated in the plasma—a level the impurity will increase— and reconstruct changes in that radiation to track how quick the impurity ions move through the plasma.
In addition to overseeing the wide variety of HSX projects, Anderson also is part of a team led by Engineering Physics Professor Chris Hegna to explore the future of stellarator research. The proposal, titled “Targeted optimization of quasi-symmetric stellarators,” has received an approximately $900,000, three-year grant from the U.S. Department of Energy.
The UW-Madison proposal is distinct from previous studies because it is not a design study. “The immediate goal isn’t to build a particular type of experiment,” Hegna says. “This is a paper study on ways to optimize stellarators beyond where we are now. Essentially, we’re enfranchised to do some deeper thinking about where we might go from here based on the knowledge we have at the moment.”
In the last few years, the plasma research community has held panels to explore the future of tokomak-alternative devices, and the UW-Madison proposal is a response to several issues raised in those reviews, including improvements in ion confinement and impurity transport.
Hegna and Anderson anticipate the study will become an international discussion. They plan to collaborate with colleagues at Germany’s Wendelstein 7-X stellarator, Japan’s Large Helical Device, Oak Ridge National Laboratory, and Princeton University. “We’re hoping to bring the pieces together and focus all of this expertise onto these particular issues,” says Hegna.
While the next-step proposal is separate from HSX, Hegna says the plasma physics community has learned a great deal from the device. “There are elements of HSX that are essential to something like this,” Anderson says. “Part of the new study is to look at questions that yet need to be answered and what a follow-on device would need.
“If certain questions are answered and the HSX concept is pursued, it could become part of a fusion reactor,” he says.