Radio Wave Breakthrough Helps Stabilize Fusion Reactions


From Popular Mechanics

Scientists from Princeton University and the Department of Energy's Princeton Plasma Physics Laboratory (PPPL) have used radio frequency waves and temperature to stabilize the white-hot and volatile plasma that swirls inside of fusion reactors like tokamaks and stellarators.

The radio waves disrupt the magnetic islands that form and disrupt the plasma flow, and temperature magnifies the stabilizing effect. As the saying goes, the disruptor of your disruptor is your friend.

What’s the problem this technology is trying to solve? Well, tokamak fusion reactors involve a twisting donut of elemental plasma that’s as hot as the sun. As the plasma travels through the reaction chamber, it’s directed and contained by an extremely strong magnetic field, meaning the plasma is basically “free floating” without touching any part of the structure. In the super-heated plasma stream, nuclear fusion takes place—ideally.

But there’s a dangerous side effect of that magnetic field. As the plasma travels, the magnetic field forms odd, “bubble-like structures” called magnetic islands.

“These may be produced from helical current perturbations in the plasma,” Oak Ridge National Laboratory published in a report from—wait for it—1974. “Physically the islands are a result of having a harmonic component of the radial component of the perturbed magnetic field in resonance with the unperturbed rational surface field lines, in the presence of shear.”

Photo credit: Oak Ridge National Laboratory
Photo credit: Oak Ridge National Laboratory

As soon as an island disrupts the flow of plasma, the tokamak can lose temperature that takes days or months to recover. The plasma can also strike and damage the structure of the tokamak itself, and repairing them is extremely difficult. The work of fusion physicists has included “island control” for decades, and the longest sustained plasma streams often end with an island complication.

So 46 years after the Oak Ridge report, magnetic islands are still one of the major obstacles in the development of tokamaks. Stellarators, invented before tokamaks but confined to their shadow for decades, are back in the public eye because of their claim to do a better job controlling the donut of plasma. Instead of the smooth exterior of a tokamak, stellarators are encrusted with hundreds of strange cylinders and rectangular compartments for magnets.

The new research is the latest development in a stability solution that dates back to the 1980s. Researchers realized then that blasting the plasma with radio waves went a long way toward stabilizing the stream and reducing the factors that led to disruptions from islands. In 2019, a new generation of researchers identified a way to improve just how stabilizing those radio waves could be, boosting them using tiny temperature fluctuations called perturbations.

By “depositing” accumulating radio frequency waves within the magnetic islands, scientists can suspend the islands and prevent them from growing. And in the new paper, researchers suggest that by carefully controlling and varying the temperature of the plasma, they can magnify the controlling effect of radio waves.

This nonlinear treatment of the temperature introduces new variables, too. The scientists say they’re excited about the possibilities for new control schemes and other subsequent research.

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