Here Comes the Sun

This story appeared in the Fall 2020 Letters & Science magazine.

Unlike the solids, liquids and gasses that predominate physical life on earth, the majority of our universe exists in a fourth phase: plasma. Plasmas are ionized gasses—a mix of charged atoms and free electrons—that have intriguing electrical and magnetic properties. The field of plasma astrophysics asks questions about plasmas throughout the universe, helping researchers to understand the formation and destruction of stars, star systems and galaxies. 

Closer to home, our sun is also predominantly made up of plasma, and investigating its properties helps researchers to study the energy-producing nuclear fusion reactions that power the sun, as well as phenomena like solar winds that cause auroras and can affect satellite communications on earth.

This intersection of fusion research and astrophysics is where Professor of Physics Cary Forest has spent his entire career. Since joining the faculty in 1997, Forest has pushed the boundaries of plasma physics research at UW–Madison, continuing a tradition that started here in the early 1960s. As director of the Wisconsin Plasma Physics Laboratory, Forest helped to design and build the Big Red Ball, a three-meter–wide hollow sphere that allows scientists to study phe­nomena associated with magnetized plasmas, including aspects of the solar wind. The Big Red Ball and the Madison Symmetric Torus (MST), another plasma device, are now a U.S. Department of Energy (DOE) National User Facility, housed in Sterling Hall on the UW–Madison campus but available to researchers from across the country (thanks in part to a $12.5 million DOE grant to develop an integrated facility that would expand the frontier of astrophysical plasma research).

In April 2020, the DOE’s Advanced Research Projects Agency–Energy (ARPA–E) announced that Forest and colleagues had been awarded a $5 million fusion energy grant to explore the creation of an efficient plasma fusion device. Their work could advance the push for clean nuclear energy, without the radioactive byproducts, and it holds significant promise for creating efficient medical isotopes.

Fusion: the sun's sustainable energy source

In solar fusion reactions, a molten plasma made up of hydrogen ions and electrons spins at rapid speeds, allowing the ions to collide and fuse their nuclei. The result is helium atoms and high-energy neutrons. The sun is a continuous fusion reactor; the energy it produces from fusion sustains the high temperatures needed for fusion to happen, and gravity helps keep the ions in close proximity so that they can bombard each other. 

Essentially the same process happens in laboratory and other fusion reactors, just with slightly different isotopes of hydrogen—and much, much less efficiently. A large amount of energy is needed to heat the plasma to tempera­tures high enough for it to undergo fusion and, once those temperatures are achieved, it is difficult to contain the plasma long-term to generate a sustained fusion reaction. 

If researchers like Forest can efficiently harness the fusion process of our sun, it could yield enormous benefits. For example, those high-energy neutron products from fusion are already being used to produce medical isotopes, which are used in procedures such as PET scans or some cancer treatments, and improving the efficiency with which neutrons are produced would significantly lower treatment costs. The neutrons could also be used to produce steam, which could be converted into electricity—electricity that is produced without the release of greenhouse gasses and with fewer radioactive products than the fission reactions used by current nuclear power plants. 

“Our idea initially—funded by a UW2020 grant—was to build a neutron source which could go several orders of magnitude beyond current medical isotope production efficiencies but also provide a key first step in the direc­tion of advancing fusion energy, which could potentially be a clean power source in the future,” says Forest. 

The DOE grant will help Forest and colleagues take that step: determining whether they can harness the power of high-temperature superconducting magnets to create a more efficient plasma fusion device. With the funding, Forest and his team will design and build the Wisconsin HTS Axisymmetric Mirror, or WHAM—which will serve as a prototype for the next generation of fusion reactors.

Back to mirror machines, but with upgrade

Fusion research began in earnest in the 1960s, when scientists developed mirror machines. These cylindrical devices have strong magnetic fields on either end that act like mirrors, reflecting the charged plasma particles inward and retaining them and their heat in the machine. American researchers, including a UW team operating the Phaedrus Tandem mirror, halted mirror research three decades ago, mainly due to an inability to contain the plasma. 

Instead, U.S. research focused on donut-shaped devices called tokamaks and variants such as MST, which confined plasma much better than mirrors. Other countries, however, such as Russia and Japan, continued mirror research and made great advances.

“We’re producing fusion reactions all the time at MST, but it is a pulsed machine, as opposed to a continuous one like the mirror machines were. Pulsed operation adds complications to power plants: Just like for solar, if the heat comes and goes from a [theoretical] pulsed fusion plant, you need to store energy for when the reactor is off,” Forest explains. “Another advantage of the mirror machines is that their linear systems are much easier to build, take apart, and put back together than the toroidal machine.”

WHAM will essentially restart U.S. research on mirrors, but with significant technological upgrades and lessons learned from Japanese and Russian colleagues.

“We hope to go well beyond what was done in the mirror program because we have access to very-high-field superconducting magnets like those being built by our partners for toroidal plasmas. These magnets and heating systems simply weren’t available 20 years ago,” Forest says. “It’s a new look at an older concept using new technology.”

With the DOE funding, the UW–Madison team will build the device at the Physical Sciences Laboratory (near Stoughton) and begin operating it, which involves burning plasma to more than 100 million degrees Celsius, confining it magnetically with superconducting magnets and, eventually, using it for neutron production.

A key difference between WHAM and a power plant that relies on fusion (rather than fission, like current nuclear plants do) is the types of hydrogen isotopes they will be using in the reactor. Natural isotopes of hydrogen contain a single proton in their nucleus, but anywhere from zero to two neutrons. Protium (zero neutrons) and deuterium (one neutron) are stable isotopes, but tritium, or hydrogen with two neutrons, is radioactive.

“We’ll be making neutrons using the deuterium–deuterium reaction, not the deuterium–tritium reaction that you’d have in an actual fusion power plant,” Forest says. “We’re able to study things without being full-on nuclear.”

Still, Forest expects this prototype device will provide invaluable input into scaled-up devices. And, it should produce neutrons much more efficiently than current methods, helping to reduce the cost of medical isotope production, and offer the promise of energy production that is safer for the planet.

The DOE-funded work is part of a collaboration led by UW–Madison scientists that includes researchers at the Massachusetts Institute of Technology, who are developing a blanket to wrap the device for improved performance, and Commonwealth Fusion Systems, a company spun out of MIT that will produce and supply the magnets.