A Plasma Escape Plan Solved a Monumental Fusion Roadblock
Scientists discovered a wider heat strike zone in tokamak reactors, enhancing heat management and bringing fusion energy closer to reality.
- To create reliable, commercial nuclear fusion on Earth, scientists need to heat up plasma inside tokamak reactors to 150 million degrees Celsius—a temperature roughly 10 times that of the Sun’s core.
- Scientist rely on one piece of technology, called the divertor, to remove heat and particles from the reactor while also avoiding core plasma contamination. But withstanding transient bursts of plasma, especially directed in a narrow region, is an immense engineering challenge.
- A new study from various nuclear fusion teams discovered that the strike zone from escaping electrons beyond the last confinement surface is actually 30 percent larger than previously believed, which could be a big boost to fusion’s future commercial ambitions.
Nuclear fusion is the ultimate end goal for humanity’s energy ambitions. The merger of light nuclei in the Sun’s core is the engine of the universe, and recreating that equation of pressure and immense heat on Earth would change life forever—but, as you’d likely expect, bottling a star isn’t easy.
For one, the Sun is hot and massive in ways the Earth very much isn’t. While scientists can’t really do much about the mass part of the equation, they can increase temperatures beyond the Sun’s 15-million-degree-Celsius core temperature to induce fusion. However, to induce the necessary quantum tunneling magic trick and actually fuse deuterium and tritium—two isotopes of hydrogen and the preferred fusion fuel—we need to crank the thermostat up to ten times that of the Sun’s core. That means the hearts of the toroidal fusion reactors known as tokamaks are by far the hottest places in the Solar System.
Arguably, the biggest hurdle between now and our endless energy future is how to handle that heat. For decades, scientists believed that heat exhaust from the magnetically contained, superheated plasma could focus on a very narrow band of a divertor plate—the mechanism in a tokamak responsible for removing heat and particles from the reactor. That focus could cause this crucial piece of a reactor to break down quickly and complicate the possible commercialization of fusion.
But a new study—conducted by researchers at the Princeton Plasma Physics Laboratory (PPPL), the Oak Ridge National Laboratory, and the International Thermonuclear Experimental Reactor (ITER) Organization—created plasma simulations using PPL-developed software named X-Point Included Gyrokinetic Code.
These simulations showed how plasma traveled across the magnetic boundary (known as the separatrix) meant to separate confined plasma in the reactor’s core from unconfined plasma in the region of the divertor. Instead of the heat being focused on a narrow band, the researchers discovered something else.
“In the new paper, we show that the last confinement surface is strongly disturbed by the plasma turbulence during fusion, even when there are no disturbances caused by external coils or abrupt plasma instabilities,” Choongseok Chang, co-author of a new paper that was recently published in the journal Nuclear Fusion, said in a press statement. “A good last confinement surface does not exist due to the crazy, turbulent magnetic surface disturbance called homoclinic tangles.”
These “homoclinic tangles” are complex networks of curves that form a path along which electrons flow from the contained main plasma to divertor plasma. This has the happy side effect of widening the heat strike zone by more than 30 percent when compared to what was previously believed—great news for engineers trying to design a system capable of withstanding temperatures in excess of 150 million degrees Celsius.
“This means it is even less likely that the divertor surface will be damaged by the exhaust heat when combined with the radiative cooling of the electrons by impurity injection in the divertor plasma,” Chang said.
“Ironically, it may raise fusion performance by lowering the chance for divertor surface damage in steady-state operation and eliminating the transient burst of plasma energy to divertor surface from the abrupt edge plasma instabilities, which are two among the most performance-limiting concerns in future commercial tokamak reactors.”
The divertor is only one piece of the immensely complicated fusion puzzle. But if scientists and engineers can design one capable of withstanding a tokamak’s immense temperatures while also minimizing contamination of core plasma, then humanity will be one big step closer to bottling the power of the Sun.