In the plasma of a fusion device, a strong ion temperature gradient slows the growth of magnetic islands

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 the plasma in a “bottle.” However, magnetic islands, a type of instability in plasma, can destroy the confining magnetic field if they become large enough.

Researchers at the DIII-D National Fusion Facility found that unlike the flattened electron temperature profile, the ion temperature profile showed an abrupt change from island to island. The study is published in the journal Physical Examination Letters.

Magnetic islands are plasma instabilities that can grow to the point of 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 favoring island growth, but ion temperatures have never been measured within an island.

Recent work on the tokamak of the national DIII-D fusion facility made it possible to carry out the first measurement of this parameter. The results indicate that the ion temperature exhibits a steep gradient at the center of the magnetic islands.

To understand this gradient, an international team of researchers carried out a series of simulations. The abrupt change in ion temperature was explained by scaled-down simulations of the drift-kinetic neoclassical tearing mode (NTM). The researchers found that within an island, ions form “drifting island” structures that move away from the island, restoring the ions’ temperature. These results will be used in a physical model of island destabilization and will help constrain the Drift-Kinetic magnetic island appearance model, which will provide key information for the design of ITER and fusion power plants.

This magnetic island research provides insight into how islands affect plasma stability. Researchers expect to see islands large enough to cause a loss of confinement in tokomak devices such as ITER, the international experiment currently under construction in France, and future fusion plants. Researchers therefore need accurate modeling and prediction of the destabilization caused by the islands.

This understanding will help researchers develop operating conditions for fusion devices that avoid islands. This will also help improve plasma modeling and design of future devices.