Physical Suying Jin, DOE / Princeton Plasma Physics Laboratory. Credit: Suying Jin
Scientists have found a new way to prevent pesky magnetic bubbles plasma of interfering with fusion reactions – providing a possible way to improve the performance of fusion energy devices. And it comes from managing radio frequency (RF) waves to stabilize the magnetic bubbles, which can expand and create perturbations that can limit the performance of ITER, the international facility under construction in France, to demonstrate the possibility of fusion power. .
Magnetic islands
Researchers at the US Department of Energy (DOE) Princeton Plasma Physics Laboratory (PPPL) have developed the new model for controlling these magnetic bubbles, as islands. The new method changes the standard technique for slowly depositing radio (RF) rays into the plasma to stabilize the islands – a technique that proves inefficient when the width of an island is small compared to the characteristic size of the region where the RF beam deflects its force.
This region is called the ‘attenuation length’, the area where the RF power would typically be distributed in the absence of non-linear feedback. The effectiveness of RF power can be greatly reduced if the size of the region is greater than the width of the island – a condition called “low attenuation” – because much of the power then leaks from the island.
Tokamaks, donut-shaped fusion facilities that can experience such problems, are the most widely used devices by scientists around the world trying to produce and control fusion reactions to provide a virtually inexhaustible supply of safe and clean power to generate electricity. generate. Such reactions combine bright elements in the form of plasma – the state of matter composed of free electrons and atomic nuclei that make up 99 percent of the visible universe – to generate the massive amounts of energy that drive the sun and stars.
Overcome the problem
The new model predicts that depositing the rays in pulses instead of currents in a stable state can overcome the leakage problem, said Suying Jin, a graduate student in the Princeton program in plasma physics based on PPPL and lead author of a paper describing the method. in Physics of Plasmas. “Pulse can also achieve increased stabilization in cases of high attenuation for the same average force,” she said.
For this process to work, “the pulsation must be done at a rate that is neither too fast nor too slow,” she said. “This sweet spot should correspond to the rate at which heat dissipates from the island by diffusion.”
The new model makes use of past work by Jin’s co-authors and advisors Allan Reiman, a Distinguished Research Fellow at PPPL, and Professor Nat Fisch, Director of the Program in Plasma Physics at Princeton University and associate director of academic affairs at PPPL. Their research provides the non-linear framework for the study of RF power deposition to stabilize magnetic islands.
“The significance of Suying’s work,” said Reiman, “is that it extends the tool that can be portable, which is now recognized as perhaps the most important problem for economic fusion with the Tokamak approach. Tokamaks are plagued by these naturally occurring and unstable islands. , leading to catastrophic and sudden loss of plasma. “
Fisch added: “Suying’s work not only suggests new control methodologies; their identification of these newly predictive effects may force us to evaluate experimental findings from the past in which these effects played an unappreciated role. Her work now motivates specific experiments that can clarify the mechanisms at play and point out exactly how best administrative disastrous instabilities exist. ”
Original model
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. Non-linear feedback then traps in between the force deposition and changes in the temperature of the island, which makes the stabilization strongly possible. Controlling these temperature changes is the distribution of heat from the plasma at the edge of the island.
In high attenuation regimens, where the attenuation length is smaller than the size of the island, this same non-linear effect can create a problem called “shadows” during steady state deposition, causing the RF beam to emit the power runs before it reaches the center of the island.
“We first researched pulsed RF regulators to solve the shadow problem,” Jin said. “However, it turned out that with high-attenuation regimes, non-linear feedback actually causes pulsating shadows to increase, and that the ray is even earlier out of power. So we ran around the problem and found that the non-linear effect can then cause pulsation to reduce the force leaking from the island in less low attenuation scenarios. ”
These predicted trends naturally lend themselves to experimental verification, Jin said. “Such experiments,” she remarked, “would be intended to show that pulsation increases an island’s temperature until optimal plasma stabilization is achieved.”
Reference: “Pulsed RF Regulations for Tear Mode Stabilization” by S. Jin, NJ Fisch and AH Reiman, June 9, 2020 Physics of Plasmas.
DOI: 10.1063 / 5.0007861
Funding for this research comes from the DOE Office of Science.
