In the world of fusion energy, scientists and engineers study the fourth state of matter known as plasma in an effort to design and build a new type of power plant. Relying on the heat produced by two small atoms smashing together, a network of such facilities would help create a novel source of stable electricity and help ensure America's energy independence. And while scientists in this endeavor are devoting their attention to complex machinery and temperatures hotter than the surface of the sun, they are also trying to determine the best designs for such a power plant by focusing on geometry.
In fusion systems, shape matters. The earliest device designed by Lyman Spitzer Jr., the founder of the U.S. Department of Energy's Princeton Plasma Physics Laboratory (PPPL), was shaped like a figure eight. A later system, known as the tokamak, was developed in the 1960s and shaped like a doughnut, in an effort to keep the plasma confined by creating a central electrical current that formed vital confining magnetic fields. Other fusion devices were shaped like straight lines or twisty crullers.
Additionally, some fusion systems look like cored apples. Known as spherical tokamaks, they resemble doughnut-like tokamaks that have been compressed, making the hole down the center far narrower than before. Scientists have found that spherical tokamaks have properties that could confine plasma energy more efficiently than conventional tokamaks. These properties could help generate a plasma with the necessary temperature and density for a sufficient amount of time to create a fusion that heats itself, like a mini star on Earth.
Unlike doughnut-shaped conventional tokamaks, spherical tokamaks are shaped like cored apples. That shape gives spherical tokamaks special properties that could aid the design of a fusion power plant. (Photo and illustration credit: Kyle Palmer and Michael Livingston / PPPL Communications Department)
Spherical tokamaks can also confine a relatively large plasma pressure for a given magnetic field strength. That ratio of plasma pressure to magnetic pressure is known as beta; achieving a high beta is a long-standing goal because it reflects efficient use of the confining magnetic field. This matters for designing fusion power plants since producing strong magnetic fields can be expensive. A high beta means a plant can reach the plasma pressures it needs with less magnetic field, easing one of the costliest engineering demands.
Designed to be the most powerful spherical tokamak in the world
PPPL will study the capabilities of spherical tokamaks using its primary fusion experiment: the National Spherical Torus Experiment-Upgrade (NSTX-U). The largest spherical tokamak in the United States and designed to be the most powerful spherical tokamak in the world, NSTX-U boasts a list of impressive statistics.
- Its central magnet bundle will use up to 4 million amps of electrical current to produce 1 tesla of magnetic field strength. In comparison, a typical lightning bolt has only 30,000 amps, and 1 tesla is 20,000 times more powerful than the magnetic field at Earth's surface.
- It will use cutting-edge artificial intelligence (AI) systems to improve its performance.
- It will allow scientists to study the large amounts of heat that will flow within fusion power plants.
With these capabilities, NSTX-U aims to access the highest plasma stored energy of any spherical tokamak to date.
NSTX-U will also have state-of-the-art measurement systems, or diagnostics, that capture key features of the plasma, such as temperature and density, providing critical insights to advance fusion energy science. It will be an international user facility - public and private institutions and companies will use NSTX-U as a test bed to verify how new materials and components perform when exposed to plasma and to establish trusted AI tools for real-time data analysis and control.
The National Spherical Torus Experiment-Upgrade is a spherical tokamak, a fusion device shaped more like a cored apple than the doughnut-like shape of conventional tokamaks. It's the largest such device in the U.S., and designed to be the most powerful in the world.
Spherical tokamaks have special properties that boost fusion operations
Some scientists are excited about spherical tokamaks because they have a combination of characteristics that could make the concept a good option for future fusion power plants. Those characteristics fall into three broad categories.
Advantage 1 - Small size
Spherical tokamaks are attractive concepts because they are relatively compact and therefore require less building material for their construction. "If you have to build something like a regular tokamak that has a much larger diameter, you have to use a lot more steel, concrete and copper, among other things," said Jack Berkery, deputy director for NSTX-U research. "So a spherical tokamak's relatively small size means it's theoretically cheaper to construct."
Advantage 2 - Better performance due to enhanced magnetic pressure
Unlike stars, which confine plasma using gravity, fusion systems on Earth must hold plasma using magnetic fields. Spherical tokamaks confine plasma remarkably well for the relatively modest magnetic fields they use.
The reason for this good confinement is complex. The spiraling magnetic field lines in a spherical tokamak thread throughout the device, but they end up wrapping around the central magnet bundle more than they do around the outer edge. This configuration allows the field lines to expose more of their convex, curved shape to the plasma. For technical physics reasons, these convex magnetic field lines help stabilize the plasma, calming large, unwanted motion, or instabilities, that can dissipate the plasma energy.
"Think of water in a bathtub," said Stefan Gerhardt, PPPL senior managing research physicist. "If you're taking a bath and moving around a lot, you can create a big wave that can grow and make the water unstable."
By stabilizing the plasma and enabling it to retain more of its heat, the convex magnetic field line surfaces simultaneously increase the plasma's pressure. That's because high temperatures generate high pressures. Spherical tokamaks, therefore, can put plasma under greater amounts of pressure and foster more fusion reactions by reducing heat loss, all without adding more power to the magnets.
Advantage 3 - Spinning to reduce turbulence
But large instabilities aren't the only type of unwanted plasma motion that can interfere with fusion reactions. The plasma also has smaller perturbations that can become persistent wiggles. "Think of them like smaller ripples on the surface of water," Gerhardt said. Scientists seek to prevent these wiggles because, like the larger instabilities, they can move heat out of the plasma, which reduces both temperature and pressure and makes fusion reactions less likely.
Spherical tokamaks are somewhat shielded from this effect because their plasma can rotate relatively quickly. As the plasma rotates - pushed by beams of neutral atoms that scientists use to help heat the plasma - different parts of the plasma move at different speeds. That means that one part of the ripple will move more quickly than another part, elongating it and causing it to stretch until it has broken apart. Once broken, they can no longer move particles and heat out of the plasma as easily.
"Spherical tokamaks have an advantage because scientists can make the plasma within them spin more easily than the plasma within regular tokamaks, and that's because of the plasma's smaller size," said Steven Cowley, PPPL director. "In addition, because the magnetic field lines on the outside of the device are at an angle, you get a better, more effective shearing effect. That effect, which is what breaks up the ripples, is simply stronger in spherical tokamaks."
The vacuum vessel and central magnet bundle of PPPL's NSTX-U are pictured above. The device will help scientists determine the best shape for future fusion power plants. (Photo credit: Michael Livingston / PPPL Communications Department)
NSTX-U will help planning for future fusion power plants
Once NSTX-U is operational, scientists will use it to determine whether the confinement time - the amount of time that heat is contained in the plasma before it is lost - continues to increase in spherical tokamaks as the devices approach power plant conditions. "We want to study whether this property continues to improve as we go toward a reactor or if it kind of flattens out," Berkery said, "because that projection will make a big difference for a future device. You want to have a high density, a high temperature and a high confinement time - that's the triple product that fusion researchers talk about."
In doing so, they will try to determine which type of tokamak shape improves performance best. If a compact shape produces the best performance, exactly how compact should it be? "If we're going to build spherical tokamak power plants, then understanding the right aspect ratio to make more cost-effective fusion is critical," Gerhardt said.
NSTX-U will be central to answering this significant question.
