Dynamics of Magnetic Island and its Effect on Equilibrium in LHD Plasmas

Abstract

The dynamics of magnetic islands in a magnetohydrodynamic equilibrium configuration of the Large Helical Device (LHD) are intriguing due to significant variations in their behaviour. When an externally perturbed magnetic field is applied to the configuration, plasma flow effects play a crucial role in modifying the shape of the magnetic islands. The response of these islands depends critically on the interplay between plasma viscosity and poloidal flow. Under certain conditions, magnetic islands either heal or grow spontaneously. Additionally, intermediate states are observed under specific conditions. The identification of these intermediate states fundamentally challenges the traditional understanding of magnetic island dynamics, which primarily focuses on binary outcomes. These intermediate states result from a delicate balance between electromagnetic and viscous torques, revealing a sophisticated mechanism underlying magnetic island behaviour. Investigating the isotopic effects on magnetic island behaviour based on these findings, we discovered that the boundary between magnetic island growth and healing differs for hydrogen (H) and deuterium (D) plasmas. Specifically, magnetic islands in D-plasma require higher beta values for healing and exhibit a significantly broader region of intermediate states compared to H-plasma, emphasizing the important role of the isotope effects in magnetic island behaviour. Furthermore, during detachment transitions, the growth and healing tendencies of magnetic islands show a clear correlation with the transition process, providing valuable insights into the interplay between external perturbation fields and plasma equilibrium. These findings offer critical insights into the physics of magnetic islands in helical plasmas, establishing a foundation for future advances in nuclear fusion research.