Dating back nearly a half-century, the UW-Madison programs in plasma physics and fusion technology are among the oldest, broadest, largest and most productive programs of this kind in the nation. On campus, they include approximately 75 faculty and staff members, 60 graduate students and 30 undergraduate students whose education and research frequently cross departmental and college boundaries. And, nearly 350 PhD recipients (more than any other U.S. university) are making important contributions in industry, government, universities and laboratories around the world.
By generating and harnessing plasma, or highly heated ionized gas, in a variety of fusion experiments, these researchers hope to develop technologies capable of delivering a clean, virtually inexhaustible source of energy. They also study the basic properties of plasma, plasma science and astrophysical phenomena, and plasma-aided manufacturing techniques.
At UW-Madison, one key area of emphasis is on magnetic plasma confinement and magnetic fusion; with experts in several additional areas, the programs span three departments in two colleges. These programs—in the Departments of Engineering Physics and Electrical and Computer Engineering in the College of Engineering and the Department of Physics in the College of Letters and Science—receive about $12 million annually in Department of Energy research funding, primarily from the Office of Fusion Energy Sciences.
This past winter, researchers from two UW-Madison plasma fusion experiments received $10.7 million in funding from the U.S. Department of Energy Office of Fusion Energy Sciences. The Helically Symmetric eXperiment (HSX) drew $5.1 million, plus an additional $900,000, while two grants to the Pegasus Toroidal Experiment totaled $4.7 million.
Reshaping fusion research
HSX is one of only two stellarators operating in the United States and is the only machine of its shape in the world.
A stellarator is like a twisted doughnut that uses the physical shape of coils 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, the most prevalent fusion-research devices. These properties make stellarators the main alternative to tokamaks.
HSX’s special design alleviates particle leakage, a challenge found in other stellarators. Outside the machine’s 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, thereby improving particle confinement.
Electrical and Computer Engineering Professor David Anderson directs the 15 faculty members, scientists and students who make up the core of the HSX team. HSX research ranges from plasma transport using modulated heating experiments to magnetic field reconstruction and turbulence studies via probes. Currently, 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.
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 and study ways to optimize the technology.“ 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,” Hegna says.
Energy inspired by the sun
Among the most promising magnetic-confinement fusion devices for generating energy, a tokamak is shaped like a doughnut with a hole in the center. It uses powerful magnetic fields both to confine and drive a plasma—in the largest experiments, a collection of particles potentially hotter than the center of the sun—as it flows through the device. As the particles collide, they release energy through a nuclear fusion reaction. Understanding how to create, contain, sustain and harness that energy is a primary challenge of fusion-energy research.
Pegasus is a very-low-aspect-ratio tokamak, meaning its center hole is very small and its shape appears almost spherical. Built more than a dozen years ago as a prototype, the experiment now is valuable as a testbed for research that could apply to larger U.S. and international experiments, including ITER, the international thermonuclear experimental reactor under construction in France. “This facility prepares students to work directly on the large fusion facilities,” says Raymond Fonck, Steenbock Professor of Physical Science and Professor of Engineering Physics. “Here, students can do things that people care about all the way up the food chain.”
The new funding provides an opportunity for the experiment to achieve a higher level of technical performance that could align Pegasus research even more closely with large-scale tokamaks, says Fonck. It will support upgrades to the Pegasus power supplies, magnetic field and diagnostic capabilities. Additionally, it will enable Pegasus researchers to build on advances that will allow them study the physics of the device at higher current and higher temperatures. “It’s making that jump to the next level of activity so we uncover the physics that may show up in a fusion reactor scale,” says Fonck. “Once we get to that stage, we are at the position where we need to test those things at a larger facility.”