Fusion with a Twist: Improving Stellarators

Newswise — Plasma many times hotter than the surface of the sun swirls inside of a large device. From the outside, the device looks like a metal ring surrounded by scaffolding and walkways. But inside, the device is creating the conditions needed to achieve fusion – the process that powers our sun and every star. Researchers supported by the Department of Energy’s (DOE) Office of Science are working to improve our understanding of the process of fusion with the goal of commercializing fusion energy. In particular, scientists at DOE’s Princeton Plasma Physics Laboratory (PPPL) have made a big step forward in advancing stellarators, one type of fusion device.

In the future, Fusion Energy Science, DOE Explains fusion could provide a clean source of energy that doesn’t produce greenhouse gases or long-lived radioactive waste. It works by heating light elements (like hydrogen) to form a plasma. Plasma is a hot, charged gas. As stars are made of plasma, it makes up 99 percent of the visible universe! To consistently produce energy from fusion, scientists must figure out how to harness plasma. The device needs to control the plasma so it can maintain a fusion reaction without adding more energy.

Fusion researchers are pursuing several different technologies, including magnetic confinement and inertial confinement. Two of the most common magnetic confinement configurations are tokamaks and stellarators. They both use very strong magnetic fields to confine the plasma and hold it in a donut shape.

One way that they differ is by the way the two produce those magnetic fields. Tokamaks have three large sets of magnetic field coils. One of them produces an electric current that runs through the center of the plasma. That electric current produces a magnetic field that boosts how well the plasma is confined. In contrast, stellarators have many magnet coils that loop around the outside of the plasma. They form twisting magnetic fields that wrap around the donut, without the need for a central current. 

Stellarators have some major advantages over tokamaks. They need less power to sustain the fusion reaction, the design is more flexible, and they are less likely to have disruptions in the plasma that damage the device’s walls. 

However, stellarators have one major issue – they can’t hold in the plasma’s heat as well as tokamaks. In particular, stellarators struggle to confine the most energetic particles in the plasma. Many of these are the particles that must be confined to sustain the fusion reaction. In addition, the more energetic particles are lost, the more likely they could damage the device’s walls. Because tokamaks’ symmetrical shape around an axis confines particles easily, they don’t have this problem. Scientists need to fix this fundamental issue before stellarators can be a viable design option.

Fortunately, researchers at PPPL have found a few ways to address this problem. They know that certain configurations of the magnetic field lead to the trapped particles acting in ways that help confinement.

Now, scientists need to know how to adjust the magnets to produce magnetic fields in the right shape. In theory, the best solution would be to simulate how every particle moves in every magnetic field. However, that would take near-infinite amounts of computing power and time. It’s just not practical.

Instead, PPPL researchers partnering with scientists from Auburn University, the Max Planck Institute for Plasma Physics in Germany, and the University of Wisconsin-Madison applied an alternative method that uses a lot less computing power. Rather than predicting how each particle moves, they developed an easy-to-compute proxy function that predicts how fast the particles move away from the magnetic fields. This number has a consistent relationship with how well the magnetic fields are confining the plasma. Using this proxy function, the team was able to develop a number of different possible plasma configurations that would lose fewer energetic particles.

Although other scientists had used this technique before, they had never applied it to this specific type of stellarator. The project used code that was developed at DOE’s Oak Ridge National Laboratory and PPPL. 

While these configurations aren’t designs for a specific device, they will help scientists move forward, knowing what paths to pursue. Using this method could help advance stellarator research. Eventually, it could enable stellarators to be a viable option for commercial fusion power.