A new plasma escape mechanism could protect fusion vessels from excessive heat

The furious exhaust heat generated by the molten plasma in a commercial reactor may not be as harmful to the interior of a craft as previously thought, according to researchers at the US Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL), Oak Ridge. ITER National Laboratory and Organization (ITER).

“This discovery fundamentally changes the way we think about the way heat and particles move between two critical regions at the edge of the plasma during fusion,” said PPPL Senior 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 fusionfollowing on from previous publications on the subject.

To achieve fusion, temperatures inside the tokamak—the donut-shaped device that holds the plasma—must climb above 150 million degrees Celsius. That’s 10 times hotter than the center of the sun. Keeping something hot is challenging, even though the plasma is largely kept from internal surfaces by magnetic fields. These fields keep most of the plasma confined in a central region known as the core, forming a donut-shaped ring.

However, some particles and heat escape from the confined plasma and collide with the plasma-facing material. New findings from PPPL researchers suggest that particles escaping from the core plasma inside the tokamak collide with a larger area of ​​the tokamak than previously thought, greatly reducing the risk of damage.

Past research based on physics and experimental data from current tokamaks suggested that the exhaust heat would be concentrated in a very narrow strip along a section of the tokamak wall known as divertor plates. The divertor, which is designed to remove exhaust heat and particles from the burning plasma, is critical to the tokamak’s performance.

“If all that heat hits that narrow area, then that part of the divertor plate gets damaged very quickly,” said Chang, who works in PPPL’s ​​theory department. “It can mean frequent downtime. Even if you’re just replacing this part of the machine, it won’t be fast.”

The problem has not stopped the operation of existing tokamaks, which are not as powerful as those that will be needed for a commercial fusion reactor. However, in the past few decades, there has been considerable concern that a commercial-scale device would create a plasma so dense and hot that the scattering plates could be damaged. One proposed plan involved adding impurities to the edge of the plasma to radiate the energy of the escaping plasma, reducing the intensity of the heat hitting the divertor material, but Chang said that plan was still challenging.

Escape route simulation

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

The simulation showed how the plasma particles traveled over the surface of the magnetic field, which was supposed to be the boundary separating the confined plasma from the unconfined plasma, including the plasma in the divertor region. This magnetic field surface – generated by the external magnets – is called the final confining surface.

Several decades ago, Chang and his collaborators discovered that charged particles known as ions cross this barrier and collide with the scatter plates. They later discovered that these escaping ions caused the heat load to be concentrated in a very narrow area of ​​the scattering plates.

A few years ago, Chang and his collaborators discovered that plasma turbulence can allow negatively charged particles called electrons to cross the last confining surface and spread the thermal load on the divertor plates in ITER by a factor of 10. However, the simulation still assumed that the last containment surface was not disturbed by the plasma turbulence.

“In the new paper, we show that the last containment surface is strongly perturbed by plasma turbulence during fusion, even when there are no perturbations caused by external coils or sudden plasma instabilities,” Chang said. “A good last-bound surface doesn’t exist because of a crazy, turbulent magnetic surface disturbance called homoclinic entanglement.”

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

He explained: “This means that the divertor surface is even less likely to be damaged by exhaust heat combined with radiative electron cooling by injecting impurities into the divertor plasma. The research also shows that turbulent homoclinic entanglements can reduce the likelihood of sudden instabilities at the edge of the plasma as they weaken their driving force.”

“The last containment surface in the tokamak should not be trusted,” Chang said. “However, ironically, it can increase fusion performance by reducing the chance of steady-state damage to the divertor surface and by eliminating the transient burst of plasma energy onto the divertor surface from sudden edge plasma instabilities, two of the most performance-limiting issues.” in future commercial tokamak reactors.”