In a fusion energy milestone, new research shows that plasma fusion heat spreads more evenly in tokamak reactors, suggesting new ways of improving reactor efficiency and overall longevity while reducing the potential for damage.
The new findings by researchers with the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL), in cooperation with Oak Ridge National Laboratory and ITER, currently the world’s largest fusion experiment, reveal that when commercial-scale reactors produce large amounts of very intense heat exhaust during plasma fusion, it may not be as potentially damaging to the interior of the reactor as had been believed.
The new research could allow for new opportunities to enhance the operational lifespan of fusion reactors and upends previous perceptions about the movement of heat and particles between two critical regions at the edge of plasma during the fusion process. The new research was led by PPPL Managing Principal Research Physicist Choongseok Chang.
Tokamaks are large toroidal (i.e., donut-shaped) devices that scientists use to produce controlled fusion reactions from hot plasmas. While in operation, temperatures within a tokamak can often exceed 150 million degrees Celsius in order to achieve fusion, mimicking processes that occur naturally on the Sun and exceeding those solar temperatures by around ten times.
Tokamaks require magnetic fields to confine the plasmas within the core of the device, although a few particles and excess heat will escape and collide with the interior walls.
However, based on Chang and his team’s findings, these escaping particles are dispersed along a larger area than previous findings had suggested, thereby limiting the potential for serious damage.
In the past, it was accepted that exhaust heat during fusion reactions would be more narrowly focused on what are called divertor plates. This portion of the tokamak interior wall is crucial for helping to remove exhaust heat and particles from the hot plasmas within the tokamak. However, concentrations along the divertor plates could sometimes result in damage, which limits the potential for commercial-scale use.
In new simulations performed by Chang and his team that involved a computer code known as X-Point Included Gyrokinetic Code (XGC), plasma particles essentially maintain a path across the surface of the magnetic field, disrupting the boundary area separating the confined plasma within the tokamak from the unconfined plasma, which includes the plasma that arrives in the divertor region.
Over time, Chang’s research had shown that ions appeared to cross the boundary, focusing heat load on a very focused region of the divertor plate and that plasma turbulence led to negatively charged electrons crossing the boundary, which greatly expands the heat strike zone on the divertor plates in ITER, the multinational fusion facility currently under assembly in France.
However, Chang and the international team’s recent study revealed that the last confinement surface, which had previously been believed to be stable, is disturbed by plasma turbulence during fusion, resulting in what the researchers describe as “homoclinic tangles.”
Homoclinic tangles were found to increase the width of the heat strike zone by as much as 30 percent more than past estimates had shown based solely on turbulence. Chang and the team say that the broader distribution of heat that they have discovered occurring in their simulations makes it far less likely that the divertor surface will be damaged when paired with radiative cooling that results from impurity injection in the divertor plasma.
Although the final confinement surface within a tokamak can’t be entirely trusted, the new research nonetheless shows that this instability may actually enhance fusion performance and lower the chance of divertor surface damage while in steady-state operation.
The risk of sudden plasma energy releases is also reduced. These findings address two of the major performance-limiting issues fusion energy researchers have faced regarding future commercial use of tokamak reactors.
Micah Hanks is the Editor-in-Chief and Co-Founder of The Debrief. He can be reached by email at micah@thedebrief.org. Follow his work at micahhanks.com and on X: @MicahHanks.