As Tokamaks heat up, scientists must control unruly plasma

Plasmas inside tokamaks — the doughnut-shaped devices that can contain nuclear fusion reactions — can be unruly to handle.

Bursts of very hot plasma, similar to the solar flares that erupt from the surface of the Sun, come from the system operating at a high energy level. These jets can overheat the tokamak’s internal components. (A plasma is formed by heating gas until it is ionized. Nebulae, auroras, and lightning all contain glowing plasmas.)

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Looking for stability for fusion reactions, an international team of physics researchers has developed a sophisticated combination of magnetic controls to reduce these plasma spatters that can damage tokamaks. The published their work earlier this year in the magazine The physics of plasma.

Plasma instability

Inside a tokamak, deuterium, an isotope of hydrogen, circulates at a very high temperature and a low density. Tritium, another isotope of the same material, may also be there. Isotopes of an element have different numbers of neutrons in their nuclei from one another, but the same number of protons.

The core of a plasma in a tokamak can be 100 million degrees Celsius, says Egemen Kolemen, associate professor at Princeton University’s Department of Mechanical and Aerospace Engineering and co-author of the paper. Popular Mechanics. The wall temperature is between 400 and 1,260 degrees Celsius.

Deuterium travels around the doughnut-shaped torus. The torus is inside a heavily constructed outer container. Electromagnetic forces guide deuterium along its path. If instabilities occur in the plasma, such as edge-localized states, the ions can damage the inside of the torus.

Edge-localized modes “are bursts of energy” that emerge at the end of the plasma and hit the wall, Kolemen explains. “These bursts lead to huge increases in the heat load on the machine wall.”

In early fusion research, edge-localized states did not occur because the plasma was in a lower energy state. In recent years, fusion researchers have found that it is productive to operate tokamaks at a higher energy state, known as “H-state”, because it provides better performance in a small volume. But in the H state, deuterium becomes more difficult to control.

“We want this high state, but we don’t want this instability that leads to this burst of energy that increases the heat and maybe melts the wall,” Kolemen said.

Calming Edge localized modes

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Fusion researchers have tried a variety of ways to make edge-localized states settle down. These have included the addition of other gases, says Kolemen. The paper also explains that researchers have attempted to inject supersonic molecular beams using small periodic vertical equilibrium displacements and more.

For their part, Kolemen and his collaborators have used a controller that disrupts the plasma. “We’ve looked at using the 3D coils,” he says. “We have these … structures. They are like windows around the reactor on the torus. They create electric and magnetic fields that are not symmetrical with respect to the plasma cross-section. What you do with this 3D perturbation is you can change the stability of the ​​the edge. You can make the edge-localized states disappear. It’s amazing.”

Unfortunately, these disturbances make it more difficult to confine the plasma. This is where the dynamic controls come into play. The regulator adjusts the magnetic fields so that only 10 to 20 percent of the pressure is lost.

“We have plasma simulations that we use,” explains Kolemen. “We built an intelligent control system that looks at the plasma and adjusts the 3D disturbances and then sees what the plasma is doing. It looks at everything and adjusts the control. We’re basically trying to define the sweet spot and then adjust the plasma. Ultimately, it’s , you end up with the highest possible pressure of the plasma, but without any of the edge instabilities. We got much better results. We got rid of instabilities, but still keep the high confinement.”

The paper says the detector had a few false positives and false negatives during the experiments, so it should be replaced by one that doesn’t rely on calibration-specific tuning.

“I think fundamentally, I have faith that people will figure out some of these technical challenges,” says Greg Piefer, founder and CEO of SHINE Technologies. Popular Mechanics after reading the newspaper. “They’re difficult. They’re going to be expensive machines, at least with today’s technology. You’re going to have to compete with other energy sources. If you’re going to build a really complicated machine with lots of exotic materials, it’s going to take some time to get the cost down . You’re going to have to establish really good economies of scale. Really good supply chains.”

Piefer said he is optimistic about fusion’s future ability to power our power grid. “I think in the long term this is the way humans will produce energy. We’re at an inflection point right now. It’s exciting to be here right now and help usher in what we call the age of fusion.”