Energy inspired by the sun
A University of Wisconsin-Madison tokamak experiment has received $4.7 million from the U.S. Department of Energy (DOE) to expand research of fusion-energy processes that mimic those in 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.
The UW-Madison experiment 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.
The DOE grant for Pegasus includes $4.2 million over three years from the Office of Fusion Energy Sciences and nearly $500,000 in American Recovery and Reinvestment Act (stimulus) funding.
These grants provide 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 Raymond Fonck, Steenbock Professor of Physical Science and Professor of Engineering Physics at UW-Madison. The funding 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.”
To that end, Fonck is collaborating closely with researchers at the Princeton Plasma Physics Laboratory and is chair of the Spherical Torus Coordinating Committee, a national group that developed a five-year roadmap to synchronize research at small and large U.S. experiments. “We made clear the logic of the program and why everybody’s doing what they’re doing,” says Fonck.
In the United States, fusion researchers are weighing the benefits of using tokamak devices such as Pegasus as the basis for building an experimental reactor for a fusion nuclear science program.
That effort—likely a decade away, says Fonck—ultimately is among the reasons Pegasus exists. “A potential candidate for this fusion nuclear science capability is the spherical tokamak because of its compact geometry,” he says.
Because of its low aspect ratio, Pegasus also is well-suited to study plasma startup techniques that address limitations on magnetic field strength and could scale up to the fusion level. Because of its importance both to the campus Pegasus program and the larger national program, this is the focus of Pegasus’ present research efforts.
In 2009, Pegasus researchers demonstrated a technique that, without using a solenoid magnet in the center, enables them to start the experiment and create a stable plasma by injecting current through a small plasma torch. “To start a plasma in this low aspect ratio, we can’t use the conventional technique,” says Fonck. “The way all tokamaks work is with magnetic induction. We don’t have enough magnetic induction.”
The plasma torch method addresses limits on magnetic field capacity in low-aspect-ratio tokamaks. “We’re gaining more knowledge on that, but to really take it to where you can predict what will happen if you do this elsewhere, we need to go to higher power and higher performance,” says Fonck. “We need to get higher current in the tokamak from this technique.”
In a related effort, he and his students will build on the startup technique and improvements to Pegasus’ magnetic field coils to explore the stability properties of plasmas at high pressure. Because of their shape, spherical tokamaks can achieve high-pressure plasmas with relatively low magnetic fields, a ratio known as beta. The higher the beta, the more efficient the future reactor. Pegasus’ new magnetic field coils mimic those of the large-scale international experiments and essentially enable the researchers to hold a plasma in one place while they heat it. “That’s mainly for the high beta push, but it also helps our startup efforts,” says Fonck.
Pegasus also is poised to address plasma instabilities related to so-called edge-localized modes that could seriously damage the containment systems in ITER and other large experiments. “It’s kind of an explosive ejection of an outermost layer of the plasma, almost like an onion that blows off its outer skin,” says Fonck.
Currently, he says, there’s an international imperative to understand this instability, which only manifests in high-performance fusion-grade tokamaks. However, because Pegasus operates with a very low magnetic field and high current, Fonck and his students also can incite an instability closely related to the edge-localized mode instability in the experiment.
Not only can Pegasus researchers create and see elements of the edge-localized mode instabilities, but because of the experiment’s unique low-temperature conditions, they also can observe, in real time, the conditions that give rise to them. “No one else can do that at the level of detail that we can do it,” says Fonck. “We can make some real contributions for the next few years.”
UW-Madison’s contributions to this international field also extend beyond plasma physics research, he says. “This facility prepares students to work directly on the large fusion facilities,” says Fonck. “Here, students can do things that people care about all the way up the food chain.”