New plasma escape mechanism could protect fusion tanks from excessive heat

Furious exhaust heat generated by molten plasma in a commercial-scale reactor may not be as damaging to the vessel’s innards as once thought, according to researchers at the Princeton Plasma Physics Laboratory (PPPL). ) of the U.S. Department of Energy (DOE), Oak Ridge. ITER National Laboratory and Organization (ITER).

“This discovery fundamentally changes the way we think about how heat and particles move between two critically important regions at the edge of a plasma during fusion,” said Choongseok Chang, senior research physicist at PPPL, who led the team of researchers behind the discovery. A new article detailing their work was recently published in the journal Nuclear fusionfollowing previous publications on the subject.

To achieve fusion, temperatures inside a tokamak – the donut-shaped device that contains the plasma – must exceed 150 million degrees Celsius. That’s 10 times hotter than the center of the sun. Containing something this hot is a challenge, even though the plasma is largely kept away from internal surfaces by magnetic fields. These fields keep most of the plasma confined to a central region called the core, forming a donut-shaped ring.

Some particles and heat, however, escape from the confined plasma and strike the material facing the plasma. New findings by PPPL researchers suggest that particles escaping from the central plasma inside a tokamak collide with a larger area of ​​the tokamak than previously thought, significantly reducing the risk of damage .

Previous research based on physics and experimental data from current tokamaks suggests that the exhaust heat would be concentrated in a very narrow band along a portion of the tokamak wall known as the bypass plates. Dedicated to removing exhaust heat and particles from burning plasma, the diverter is essential to a tokamak’s performance.

“If all this heat reaches this narrow area, then this part of the bypass plate will be damaged very quickly,” said Chang, who works in PPPL’s ​​theoretical department. “This could lead to frequent downtime. Even if you just replace this part of the machine, it won’t be quick.”

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, many concerns have been raised that a commercial-scale device could create plasmas so dense and hot that the diverter plates could be damaged. One proposed plan involved adding impurities to the edge of the plasma to radiate energy from the escaping plasma, thereby reducing the intensity of the heat hitting the diverter material, but Chang said that plan remained a challenge .

Simulate the escape route

Chang decided to study how the particles escaped and where they would land on a device like ITER, the multinational fusion facility being assembled in France. To do this, his group created a plasma simulation using computer code known as X-Point Included Gyrokinetic Code (XGC). This code is one of several codes developed and maintained by PPPL that are used for fusion plasma research.

The simulation showed how plasma particles moved across the surface of the magnetic field, which was assumed to be the boundary separating confined plasma from unconfined plasma, including plasma in the diverter region. This magnetic field surface, generated by external magnets, is called the final confinement surface.

About 20 years ago, Chang and his colleagues discovered that charged particles, called ions, passed through this barrier and hit the diverting plates. They later discovered that these escaping ions caused the heat load to concentrate on a very narrow area of ​​the deflection plates.

A few years ago, Chang and his colleagues discovered that plasma turbulence could allow negatively charged particles, called electrons, to pass through the final confinement surface and expand the thermal charge on the diverting plates by 10 times. ‘ITER. However, the simulation still assumed that the last confinement surface was not disturbed by plasma turbulence.

“In the new paper, we show that the final confinement surface is strongly perturbed by 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 due to wild, turbulent disturbances of the magnetic surface called homoclinic entanglements.”

In fact, Chang said the simulation showed that the electrons connected the edge of the main plasma to the diverting plasmas. The path of the electrons as they follow the path of these homoclinic entanglements expands the thermal strike zone by 30% more than the previous estimate of the width based on turbulence alone.

He explained: “This means that it is even less likely that the diverter surface will be damaged by exhaust heat when combined with the radiative cooling of electrons by injection of impurities into the diverter plasma. The research also shows that turbulent homoclinic entanglements can reduce the likelihood of abrupt instabilities at the plasma edge because they weaken their driving force.

“The last containment surface of a tokamak should not be trusted,” Chang said. “But ironically, it could improve fusion performance by reducing the chance of damage to the divertor surface during stable operation and eliminating the transient burst of energy from the plasma to the divertor surface due to abrupt plasma instabilities, which are two of the most limiting problems in terms of performance in future commercial tokamak reactors.