In a cornerstone of fusion power, new research shows that heat from plasma fusion is spread more evenly in tokamak reactors, offering new ways to improve reactor efficiency and overall lifespan while reducing the potential for failure .
New findings by researchers at the US Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL), in collaboration with Oak Ridge National Laboratory and ITER, currently the world’s largest fusion experiment, reveal that when reactors on a commercial scale produce large amounts of very intense heat release during plasma fusion, may not be as potentially harmful to the interior of the reactor as previously thought.
The new research could enable new options for extending the operational life of fusion reactors and overturns previous perceptions of the movement of heat and particles between two critical regions at the edge of the plasma during the fusion process. The new research was led by PPPL’s managing principal investigator physicist Choongseok Chang.
Tokamaks are large toroidal (i.e. doughnut-shaped) devices that scientists use to produce controlled fusion reactions from hot plasma. While operating, temperatures within the tokamak can often exceed 150 million degrees Celsius to achieve fusion, mimicking processes that occur naturally on the Sun and exceeding these solar temperatures by a factor of ten.
Tokamaks require magnetic fields to confine the plasma in the core of the device, although a few particles and excess heat will escape and collide with the inner walls.
However, based on Chang and his team’s findings, these escaped particles are spread over a larger area than previous findings suggested, thus limiting the potential for serious damage.
In the past, it was assumed that the waste heat during fusion reactions would be more narrowly directed to so-called baffle plates. This part of the inner wall of the tokamak is crucial in helping to remove the burnt heat and particles from the hot plasma in the tokamak. However, concentrations along the length of the deflector plates can sometimes lead to failure, limiting the potential for commercial scale use.
In new simulations performed by Chang and his team, which include a computer code known as the X-Point Included Gyrokinetic Code (XGC), plasma particles essentially maintain a path across the surface of the magnetic field, breaching the boundary zone separating the confined plasma within tokamak of the unconfined plasma, which includes the plasma that arrives in the divertor region.
Over time, Chang’s research showed that ions appeared to cross the boundary, focusing the heat load on a very focused area of the deflector plate, and that plasma turbulence resulted in negatively charged electrons crossing the boundary, greatly expanding the heat shock zone on the deflector plates. at ITER, the multinational fusion facility currently being assembled in France.
However, the recent study by Chang and the international team revealed that the last closed surface, previously thought to be stable, is disrupted by plasma turbulence during fusion, leading to what the researchers describe as “homoclinic knots”.
Homoclinic entanglements were found to increase the width of the heat shock zone by up to 30 percent more than past estimates based solely on turbulence had shown. Chang and team say the wider heat distribution they found in their simulations makes the divertor surface much less likely to be damaged when combined with radiative cooling, which results from injecting impurities into the plasma of the diverter.
Although the final confining surface within the tokamak cannot be completely trusted, the new research does show that this instability can actually improve fusion performance and reduce the likelihood of damage to the divertor surface while in steady state .
The risk of sudden release of plasma energy is also reduced. These findings address two of the major performance-limiting issues facing fusion energy researchers with respect to the 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.
