One of the world’s largest spherical-torus fusion experiments, located on the University of Wisconsin-Madison campus, recently received a major financial boost from the U.S. Department of Energy’s Office of Fusion Energy Science.
The three-year, $2.7 million grant provides funds to continue experiments on the Pegasus Plasma Experiment and will enable faculty, staff and student researchers to conduct more advanced studies of plasma at higher pressures.
An ionized gas, plasma is the fuel that drives fusion energy production via a process similar to that which powers the sun, where pressures and temperatures are so high that atoms fuse and release massive amounts of energy. Fusion potentially could be an environmentally safe and virtually limitless source of energy. And the amount of fusion power produced in the laboratory has increased by a factor of 10 million over the past 25 years — a pace of improvement faster than that achieved in computer microprocessors. Research emphasis now is on developing more cost-effective fusion reactor concepts.
Housed in the Department of Engineering Physics, Pegasus is based on the tokamak concept, a large toroidal (donut-shaped) device encircled by magnetic coils that help stabilize and confine the plasma in the reactor’s vacuum, or “empty space.” However, the device differs from the typical donut-shape of a tokamak, and looks much more like a sphere with a hole punched through its middle, resulting in a confinement device called a spherical torus.
Worldwide, there is a push to explore the spherical torus as a fusion-reactor concept and a means of studying fusion-grade plasmas, says Raymond Fonck, UW-Madison professor of engineering physics and Pegasus director. “It’s smaller and therefore cheaper to develop,” he says.
Compared with the world’s largest spherical torus experiments — the National Spherical Torus Experiment at Princeton Plasma Physics Laboratory and the Mega Amp Spherical Tokamak in Culham, England — Pegasus is even closer to a sphere in shape. “Its mission is to explore what happens when you make this hole in the torus as small as possible,” says Fonck. “The smaller you make the hole, the more efficient the system is because the less magnetic field you need to keep the plasma stable.”
However, confining a 1-million-degree-Kelvin plasma with a magnetic field is something like holding a ball of Jello with rubber bands. Eventually, some of the Jello will pop out of the rubber bands, making it easier for the rest to follow. Similarly, in plasma, the phenomenon worsens as the pressure increases, says Fonck.
Studying the high-pressure-to-low-magnetic-field ratio, his group encountered some stronger-than-expected instabilities. “This device allows you to run at very low magnetic fields to stabilize the plasma,” he says. “But it’s so low, we actually excited another group of instabilities.” And at about that point, Pegasus researchers also encountered the limitations of their equipment — some of which was nearly 30 years old.
The renewed funding will support the group as it remodels the experiment and upgrades its power, control and diagnostic systems. More studies will wait until late spring, when Fonck expects Pegasus modifications will be complete. And as has been the case since the experiment’s inception more than five years ago, undergraduate and graduate students will continue their high level of involvement — and responsibility.
Currently, about 15 students are contributing both muscle and their scientific minds to the laboratory’s renovation. “We can get to the some of the same physics regimes and the same scientific regime as the big, state-of-the-art national lab experiments can get,” says Fonck. “The nice thing, though, is even though we’re the third largest in the world, we’re still small enough that it’s very attractive to students. So the students, in the end, learn most of everything you learn on the national labs’ system.”