CONTACT: David Anderson, (608) 262-0172, dtanders@facstaff.wisc.edu; Chris Hegna, (608) 263-0810, hegna@engr.wisc.edu; Ray Fonck, (608) 263-7799, fonck@engr.wisc.edu
MADISON – Researchers with two University of Wisconsin-Madison plasma fusion experiments have 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 total $4.7 million.
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 like it in the nation.
By generating and harnessing plasma, or highly heated ionized gas, in a variety of fusion experiments, UW-Madison faculty, staff and students 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.
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. Collectively, 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.
On campus, the programs 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.
UNIQUE STELLARATOR CONTINUES TO RESHAPE FUSION RESEARCH
The UW-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 producing 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 doctoral 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.
ENERGY INSPIRED BY THE SUN
A UW-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.”
