Scientists have found a new way to prevent pesky magnetic bubbles plasma from disrupting fusion reactions – offers a potential avenue for improving the performance of fusion energy devices. And it comes from the management of radio frequency (RF) waves to stabilizing the magnetic bubbles, which can expand and cause disturbances that can limit the performance of ITER, the international facility under construction in France, to demonstrate the feasibility of fusion force.
Researcher at the US Department of Energy’s Princeton Plasma Physics Laboratory (DOE) (PPPL) have developed the new model for controlling these magnetic bubbles or islands. The novel process modifies the standard technique of steady deposition of radio waves (RF) in the plasma to stabilize the islands – a technique that proves to be inefficient when the width of an island compared to the characteristic size of the area over which the RF is located – Deposit rays, its power is little.
This range denotes the “attenuation length”, the range over which the RF power would typically be separated without non-linear feedback. The effectiveness of the RF power can be greatly reduced if the size of the region is larger than the width of the island – a condition known as “low attenuation” – as much of the power is then exiting the island.
Tokamaks, donut-shaped fusion devices where such problems can arise, are the most common devices used by scientists around the world who wish to create and control fusion reactions to ensure a virtually inexhaustible supply of safe and clean electricity for generating electricity. Such reactions combine light elements in the form of plasma – the state of matter made up of free electrons and atomic nuclei that make up 99 percent of the visible universe – to create the massive amounts of energy that power the sun and stars.
Overcome the problem
The new model predicts that separating the rays into pulses instead of stationary currents can overcome the leakage problem, said Suying Jin, a PhD student in the Princeton Plasma Physics Program at PPPL and lead author of a paper describing the method in Physics of plasmas. “By pulsing, increased stabilization can be achieved even with strong damping with the same average power,” she said.
For this process to work, “the pulsing must be at a rate that is neither too fast nor too slow,” she said. “This sweet spot should coincide with the speed with which heat is removed from the island by diffusion.”
The new model builds on previous work by Jin’s co-authors and advisors Allan Reiman, a Distinguished Research Fellow at PPPL, and Professor Nat Fisch, director of the Plasma Physics Program Princeton University and Assistant Director of Academic Affairs at PPPL. Her research provides the nonlinear framework for studying the RF power separation used to stabilize magnetic islands.
“The importance of Suying’s work,” Reiman said, “is that it greatly expands the tools that can be applied to what is now recognized as perhaps the main problem of economic merger using the tokamak approach.” Tokamaks are plagued by these naturally formed and unstable islands, which result in catastrophic and sudden loss of plasma. “
Fisch added, “Suying’s work doesn’t just suggest new methods of control; Your identification of these newly predicted effects could force us to reevaluate previous experimental results in which these effects may have played an unrecognized role. Their work now motivates specific experiments that could clarify the mechanisms and show exactly how these catastrophic instabilities can best be controlled. “
The original model of RF deposition showed that it increases the temperature and drives the current in the center of an island to prevent it from growing. Nonlinear feedback then occurs between the power separation and changes in the island’s temperature, which allows for much improved stabilization. These temperature changes are determined by the diffusion of heat from the plasma at the edge of the island.
However, in high attenuation regimes where the attenuation length is less than the island size, this nonlinear effect can create a problem called “shadowing” during stationary deposition, which causes the RF beam to lose performance goes out before reaching the center of the island.
“We looked at pulsed RF schemes first to solve the shadow problem,” said Jin. “However, it turned out that nonlinear feedback in regimes with high attenuation actually leads to the pulsation increasing the shadowing and the beam running out of current even earlier. So we turned the problem around and found that the nonlinear effect can then lead to pulses to reduce the power exiting the island in low attenuation scenarios. “
These predicted trends lend themselves, of course, to experimental verification, Jin said. “Such experiments,” she noted, “are designed to show that pulsing increases the temperature of an island until optimal plasma stabilization is achieved.”
Reference: “Pulsed RF Schemes to Stabilize Tear Mode” by S. Jin, NJ Fisch and AH Reiman, June 9, 2020, Physics of plasmas.
DOI: 10.1063 / 5.0007861
Funding for this research is provided by the DOE Office of Science.