Innovative Model can Boost the Performance of Fusion Energy Devices

An innovative method discovered by researchers avoids magnetic bubbles in plasma from disrupting fusion reactions. This latest approach offers a promising solution to enhance the performance of fusion energy devices.

The new method was developed by controlling radio frequency (RF) waves to stabilize the magnetic bubbles. These magnetic bubbles can extend and cause disruptions that can restrict the performance of ITER—the international facility being constructed in France to prove the viability of fusion power.

Magnetic Islands

The research team from the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) has designed this new model to manage these magnetic islands or bubbles.

The new technique alters the traditional method, in which RF rays are steadily deposited into the plasma to stabilize the magnetic islands. But the latter technique is not efficient, especially when an island has a small width when compared to the typical size of the area across which the RF rays deposit their power.

This area represents the “damping length,” that is, the region across which the power of the RF rays would be usually deposited even if nonlinear feedback is absent.

“Low-damping” is a condition when the size of the area is higher than the thickness of the island. During such a condition, the effectiveness of the RF power can be considerably decreased as much of the power tends to leak from the island.

Such issues can be experienced by tokamaks, which are doughnut-shaped fusion facilities. Tokamaks are also the most extensively used devices by researchers worldwide. The researchers look for ways to create and manage fusion reactions to offer an almost inexhaustible supply of clean and safe power to produce electricity.

Reactions like these integrate light elements in the form of plasma to produce large amounts of energy that fuels the stars and the Sun. Plasma is the state of matter made up of atomic nuclei and free electrons and constitutes 99% of the visible universe.

Overcoming the Problem

The latest model estimates that the leakage problem can be resolved by depositing the rays in pulses instead of depositing them in steady state streams, stated Suying Jin, a graduate student in the Princeton Program in Plasma Physics located at PPPL and the study’s lead author. The new technique has been described in the Physics of Plasmas journal.

Pulsing also can achieve increased stabilization in high-damping cases for the same average power.

Suying Jin, Study Lead Author and Graduate Student, Princeton Plasma Physics Laboratory

To make this procedure work, “the pulsing must be done at a rate that is neither too fast nor too slow,” Jin added. “This sweet spot should be consistent with the rate that heat dissipates from the island through diffusion.”

The novel model builds on previous studies performed by co-authors and advisors of Jin. These include Allan Reiman, a Distinguished Research Fellow at PPPL, and Professor Nat Fisch, the director of the Program in Plasma Physics at Princeton University and also an associate director for academic affairs at PPPL.

The team’s study offers the nonlinear framework for analyzing the deposition of RF power to stabilize the magnetic bubbles.

The significance of Suying’s work is that it expands considerably the tools that can be brought to bear on what is now recognized as perhaps the key problem confronting economical fusion using the tokamak approach. Tokamaks are plagued by these naturally arising and unstable islands, which lead to disastrous and sudden loss of the plasma.

Allan Reiman, Distinguished Research Fellow, Princeton Plasma Physics Laboratory

Professor Fisch added, “Suying’s work not only suggests new control methodologies; her identification of these newly predicted effects may force us to re-evaluate past experimental findings in which these effects might have played an unappreciated role. Her work now motivates specific experiments that could clarify the mechanisms at play and point to exactly how best to control these disastrous instabilities.”

Original Model

The original RF deposition model demonstrated that it not only increases the temperature but also drives current in the middle of an island to prevent it from growing. Following this, nonlinear feedback kicks in between the temperature variations of the island and the power deposition, enabling considerably enhanced stabilization. Controlling such temperature variations causes the heat to diffuse from the plasma at the island edge.

But in the case of high-damping regimes, in which the damping length is smaller when compared to the island size, the same kind of nonlinear effect can lead to an issue known as “shadowing” at the time of steady state deposition. When this shadowing problem occurs, the RF ray runs out of power even before it reaches the middle of the island.

We first looked into pulsed RF schemes to solve the shadowing problem. However, it turned out that in high-damping regimes nonlinear feedback actually causes pulsing to exacerbate shadowing, and the ray runs out of power even sooner. So we flipped the problem around and found that the nonlinear effect can then cause pulsing to reduce the power leaking out of the island in low-damping scenarios.

Suying Jin, Study Lead Author and Graduate Student, Princeton Plasma Physics Laboratory

Predicted trends like these naturally lend themselves to experimental corroboration, Jin added.

“Such experiments would aim to show that pulsing increases the temperature of an island until optimum plasma stabilization is reached,” Jin concluded.

The study was financially supported by the DOE Office of Science.

Journal Reference:

Jin, S., et al. (2020) Pulsed RF schemes for tearing mode stabilization. Physics of Plasmas. doi.org/10.1063/5.0007861.