This still image from a new simulation shows how plasma from the pedestal region is connected to the divertor plasma region via the so-called final confinement surface. The long and thin lobes fluctuate in time and space. Simulation credit: Seung-Hoe Ku/Princeton Plasma Physics Laboratory on DOE’s Summit computer at Oak Ridge National Laboratory; Visualization credit: Dave Pugmire and Jong Youl Choi / Oak Ridge National Laboratory
The exhaust heat from commercial-scale fusion reactors may be less harmful than previously thought.
New research shows this plasma Fusion heat spreads more evenly in tokamak reactors, indicating less risk of damage to critical components, improving reactor life and efficiency.
According to researchers at the Princeton Plasma Physics Laboratory (PPPL), Oak Ridge National Laboratory, and the U.S. Department of Energy (DOE) ITER Organization (ITER), the intense exhaust heat produced by plasma fusion in a commercial reactor could scale may not be as damaging to the reactor interior as previously thought.
“This discovery fundamentally changes the way we think about how heat and particles travel between two critically important regions at the edge of a plasma during fusion,” says PPPL Managing Principal Research Physicist Choongseok Chang, who led the team of researchers behind the discovery. A new article describing their work was recently published in the journal Nuclear fusionin response to previous publications on this subject.
To achieve fusion, temperatures in a tokamak – the doughnut-shaped device that holds the plasma – must rise above 150 million degrees Celsius. That is ten times hotter than the center of the sun. Containing something that hot is a challenge, even though the plasma is largely kept away from the interior surfaces using magnetic fields. These fields keep most of the plasma confined in a central region known as the core, forming a doughnut-shaped ring. However, some particles and heat escape from the trapped plasma and strike the material facing the plasma. New findings from PPPL researchers suggest that particles escaping from the nuclear plasma in a tokamak collide with a larger area of the tokamak than once thought, significantly reducing the risk of damage.
Previous research based on physics and experimental data from contemporary tokamaks suggested that the exhaust heat would concentrate on a very narrow band along a section of the tokamak wall known as the divertor plates. The divertor is intended to remove exhaust heat and particles from the burning plasma and is critical to the performance of a tokamak.
The experimental ITER tokamak will have a divertor that runs in a ring around the bottom of the tokamak chamber. In the image above, the divertor is highlighted in yellow. Credit: ITER Organization
“If all this heat hits this narrow area, this part of the divertor plate will be damaged very quickly,” says Chang, who works in the PPPL theory department. “It can mean regular periods of downtime. Even if you just replace this part of the machine, it won’t happen quickly.”
The problem has not stopped the operation of existing tokamaks, which are not as powerful as those that will be needed for a commercial-scale fusion reactor. However, in recent decades there has been significant concern that a commercial-scale device would create plasmas so dense and hot that the divertor plates could be damaged. One proposed plan involved adding impurities to the edge of the plasma to radiate away the energy of the escaping plasma, reducing the intensity of the heat hitting the divertor material, but Chang said this plan is still a challenge used to be.
Simulating the escape route
Chang decided to investigate how the particles escaped and where the particles would end up on a device like ITER, the multinational fusion facility being built in France. To do this, his group created a plasma simulation using a computer code known as X-Point Included Gyrokinetic Code (XGC). This code is one of many developed and maintained by PPPL and used for fusion plasma research.
The simulation showed how plasma particles traveled through the magnetic field surface, which was intended as the boundary separating the confined plasma from the non-confined plasma, including the plasma in the divertor region. This magnetic field surface – generated by external magnets – is called the final confinement surface. A few decades ago, Chang and his colleagues discovered that charged particles, known as ions, crossed this barrier and hit the divertor plates. They later discovered that these escaping ions caused the heat load to be concentrated on a very narrow area of the divertor plates.
A few years ago, Chang and his colleagues discovered that plasma turbulence can allow negatively charged particles called electrons to pass through the final confinement surface and increase the heat load on the divertor plates in ITER by ten times. However, the simulation still assumed that the final confinement surface was not disturbed by the plasma turbulence.
“In the new paper, we show that the final confinement surface is strongly disturbed by the plasma turbulence during fusion, even in the absence of disturbances caused by external coils or abrupt plasma instabilities,” Chang said. “A good final confinement surface does not exist because of the crazy, turbulent magnetic surface disturbances called homoclinic tangles.”
Chang said the simulation showed that electrons connect the edge of the main plasma to the divertor plasmas. The path of the electrons, as they follow the path of these homoclinic tangles, widens the heat impact zone 30% more than the previous estimate of its width, based on turbulence alone. “This means that it is even less likely that the divertor surface will be damaged by the exhaust heat in combination with the radiative cooling of the electrons due to injection of impurities into the divertor plasma. The research also shows that the turbulent homoclinic tangles can reduce the chance of abrupt instabilities at the edge of the plasma because they weaken their driving force.”
“The final confinement surface in a tokamak should not be trusted,” Chang said. “But ironically, it can improve fusion performance by reducing the chance of damage to the divertor surface during steady-state operation and eliminating the transient burst of plasma energy to the divertor surface due to the sudden plasma instabilities, which are two of the most performance-limiting problems . in future commercial tokamak reactors.”
Reference: “The role of turbulent separatrix tangle in improving the integrated pedestal and heat exhaust problem for stationary tokamak fusion reactors” by CS Chang, S. Ku, R. Hager, J. Choi, D. Pugmire, S . Klasky, A. Loarte and RA Pitts, April 16, 2024, Nuclear fusion.
DOI: 10.1088/1741-4326/ad3b1e
This research received funding from the DOE’s Fusion Energy Sciences and Advanced Scientific Computing Research to the SciDAC Partnership Center for High-fidelity Boundary Plasma Simulation under contract DE-AC02-09CH11466.