In a Fusion Device Plasma, a Steep Ion Temperature Gradient Slows the Growth of Magnetic Islands

The first measurement of ion temperature in magnetic islands identified a steep gradient, providing insights for improving plasma confinement in tokamaks.

Contour plot of electron temperature (left) and radial profiles of electron temperature (center) and ion temperature (right) across an island, with ion temperature showing a gradient observed experimen-tally (blue) and reproduced in simulations (red).

The Science

Future fusion power plants will require good plasma confinement to sustain reactions and generate energy. One way to contain plasma for fusion reactions is to use a tokamak, a device that applies magnetic fields to “bottle” plasma. However, magnetic islands, a type of instability in the plasma, can destroy the confining magnetic field if they grow large enough. Scientists predict that islands grow as a result of the temperatures of electrons and ions in a plasma flattening out within the island. While researchers have observed this flattening for electron temperature, they have not measured island ion temperatures. Researchers at the DIII-D National Fusion Facility found that contrary to the flattened electron temperature profile, the ion temperature profile exhibited a steep change across islands.

The Impact

Fusion energy production requires good confinement of a stable plasma. This research on magnetic islands provides insights into how islands affect plasma stability. Researchers expect to see islands large enough to cause loss of confinement in tokomak devices such as ITER, the international experiment now under construction in France, and future fusion power plants. Therefore, researchers need accurate modeling and prediction of destabilization caused by islands. This understanding will help researchers develop fusion device operating conditions that avoid islands. It will also help improve plasma modeling and the design of future devices.

Summary

Magnetic islands are instabilities in plasma that can grow until they result in a loss of confinement, and this sudden release of energy can damage the inner wall of a tokamak. The electron temperature profile is known to flatten within islands, with this change supporting island growth, but ion temperature has never been measured in an island. Recent work on the tokamak at the DIII-D National Fusion Facility performed the first measurement of this parameter. The results indicate that ion temperature exhibits a steep gradient in the center of magnetic islands.

To understand this gradient, an international team of researchers performed a series of simulations. The steep ion temperature change was explained by reduced Drift-Kinetic Neoclassical Tearing Mode (NTM) simulations. The researchers found that within an island, ions form “drift island” structures that shift from the island, restoring ion temperature. These results will be used in a physical model of island destabilization and help constrain the Drift-Kinetic magnetic island onset model, which will provide key information for ITER and fusion power plant design.

Contact

Laszlo Bardoczi
University of California, Irvine
[email protected]

Funding

This work was supported by the Department of Energy (DOE) Office of Science, Office of Fusion Energy Sciences, using the DIII-D National Fusion Facility, a DOE Office of Science user facility. The simulations in this work performed at York Plasma Institute were carried out within the framework of the EUROfusion Consortium funded by the European Union via the Euratom Research and Training Programme.