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Physicist Suying Jin, DOE / Princeton Plasma Physics Laboratory. Credit: Suying Jin
Scientists have found a new way to prevent magnetic bubbles plasma to interfere with fusion reactions – offering a potential way to improve the performance of fusion energy devices. And this comes from the management of radiofrequency (RF) waves to stabilize the magnetic bubbles, which can expand and create disturbances that can limit the performance of ITER, the international installation under construction in France to demonstrate the feasibility of energy. fusion.
Magnetic Islands
Researchers at the Princeton Plasma Physics Laboratory of the US Department of Energy (DOE) (PPPL) have developed the new control model for these magnetic bubbles, or islets. The new method modifies the standard technique of regular deposition of radio (RF) rays in plasma to stabilize islands – a technique that is ineffective when the width of an islet is small compared to the characteristic size of the region over which the RF ray settles. his power.
This region refers to the “damping length,” the area over which RF power would typically be deposited in the absence of any non-linear feedback. RF power efficiency can be drastically reduced when the region size is larger than the island width – a condition called “low damping” – because much of the power then leaks from the island.
Tokamaks, donut-shaped fusion facilities that can encounter such problems, are the most widely used devices by scientists around the world who seek to produce and control fusion reactions to provide a virtually inexhaustible supply of safe and clean energy to generate 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% of the visible universe – to generate the massive amounts of energy that drive the sun and the stars.
Overcome the problem
The new model predicts that deposition of rays in pulses rather than steady-state fluxes can overcome the leakage problem, said Suying Jin, a graduate student of the PPPL-based Princeton Plasma Physics program and lead author of ” an article describing the method in Plasma physics. “The pulsation can also provide increased stabilization in high damping cases for the same average power,” she says.
For this process to work, “the heartbeat must be performed at a rate that is neither too fast nor too slow,” she said. “This ideal point must be compatible with the speed at which the heat dissipates from the island by diffusion.”
The new model builds on the previous work of the co-authors and advisers of Jin Allan Reiman, Emeritus Researcher at PPPL, and Professor Nat Fisch, Program Director in Plasma Physics at Princeton University and Associate Director of Academic Affairs at PPPL. Their research provides the nonlinear framework for the study of RF power deposition to stabilize magnetic islands.
“The importance of Suying’s work,” said Reiman, “is that it dramatically expands the tools that can be implemented on what is now recognized as perhaps the key problem in economic meltdown using the tokamak approach. The Tokamaks are plagued by these unstable and natural islands, which lead to a sudden and disastrous loss of plasma. “
Adding from Fisch: “Suying’s work not only suggests new methodologies of control; its identification of these newly predicted effects may force us to re-evaluate past experimental findings in which these effects may have played an unrecognized role. His work is now motivating specific experiments that could clarify the mechanisms at play and indicate exactly how best to control these disastrous instabilities.
Original model
The original model of RF deposition was shown to increase temperature and drag current to the center of an island to keep it from growing. Nonlinear feedback then occurs between the power deposition and island temperature changes, resulting in greatly improved stabilization. The diffusion of heat from the plasma to the edge of the island governs these temperature changes.
However, in high damping regimes, where the damping length is less than the island size, this same nonlinear effect can create a problem called “shading” during steady-state deposition which results in failure of the block. current of the RF ray before it reaches the center of the island.
“We first looked at the pulsed RF patterns to solve the observation problem,” said Jin. “However, it turned out that in high damping regimes, the nonlinear feedback actually pulses to exacerbate the shading, and the radius falls even earlier. So we reversed the problem and found that the nonlinear effect can then cause pulses to reduce the power leakage out of the island in low damping scenarios.
These predicted trends naturally lend themselves to experimental verification, Jin said. “Such experiments,” she noted, “would aim to show that the pulsation increases the temperature of an islet until optimal plasma stabilization is achieved.”
Reference: “Pulsed RF Schemes for Tear Mode Stabilization” by S. Jin, NJ Fisch and AH Reiman, June 9, 2020, Plasma physics.
DOI: 10.1063 / 5.0007861
Funding for this research comes from the DOE Office of Science.
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