In a historic moment for fusion energy, new research shows that the heat of plasma fusion is spread more evenly in tokamak reactors, suggesting new ways to improve reactor efficiency and overall lifespan while reducing the potential for damage.
New findings from researchers with 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 when commercial-scale reactors produce large amounts of very intense heat discharge during plasma fusion, it may not be as potentially harmful to the reactor interior as previously believed.
The new research may allow new opportunities to increase the operational lifetime of fusion reactors and overturns previous perceptions about 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 chief managing physicist, Choongseok Chang.
Tokamaks are large toroidal (ie doughnut-shaped) devices that scientists use to produce controlled fusion reactions from hot plasma. During operation, temperatures inside a tokamak can often exceed 150 million degrees Celsius to achieve fusion, mimicking the processes that occur naturally on the Sun and exceeding those solar temperatures by about ten times.
Tokamaks require magnetic fields to confine the plasma within the core of the device, although some particles and excess heat will escape and collide with the inner walls.
However, based on Chang and his team’s findings, these escaping particles are spread over a larger area than previous findings had suggested, thus limiting the potential for serious damage.
In the past, it was accepted that the exhaust heat during fusion reactions would be more narrowly focused on what are called divertor plates. This part of the inner wall of the tokamak is essential to help remove the discharge heat and particles from the hot plasma inside the tokamak. However, concentrations along the deflector plates can sometimes result in damage, which limits the potential for commercial-scale use.
In the new simulations performed by Chang and his team involving a computer code known as the X-point Gyrokinetic Code (XGC), the plasma particles essentially follow a path along the surface of the magnetic field, interrupting the boundary zone that separates the confined plasma inside the tokamak from the unconfined plasma, which includes the plasma arriving in the divertor region.
Over time, Chang’s research had shown that ions appeared to cross the boundary, concentrating the heat charge in a highly focused region of the divertor plate, and that plasma turbulence led to negatively charged electrons crossing the boundary, which greatly expands the heat stroke area in the divertor. license plate at ITER, the multinational fusion facility currently being assembled in France.
However, Chang and the international team’s latest study found that the last surface of the insulation, which was previously believed to be stable, is disturbed by plasma turbulence during melting, resulting in what the researchers describe as “homoclinic entanglements”.
Homoclinic entanglements were found to increase the width of the heat shock zone by up to 30 percent more than previous estimates had shown based on turbulence alone. Chang and team say that the wider distribution of heat they found to occur in their simulations makes it much less likely that the divertor’s surface will be damaged when paired with the radiative cooling that results from injecting impurities into the divertor’s plasma.
Although the final sealing surface inside a tokamak cannot be completely trusted, new research nevertheless shows that this instability can improve fusion performance and decrease the chance of damage to the divertor surface during steady-state operation.
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 regarding the future commercial use of tokamak reactors.
Micah Hanks is the editor-in-chief and co-founder of The Debrief. He can be contacted by email at micah@thedebrief.org. Follow his work at micahhanks.com and in X: @MicahHanks.