Parametric Dependence of Concave Profile in Magnetic Island and Verification In GENE

Abstract

Magnetic island in magnetic confinement fusion research refers to a magnetic configuration in the form of an island generated by current perturbations. It is commonly known that magnetic islands typically have flat profiles inside due to the characteristics of parallel transport along reconnected field lines. However, actual profiles ensured by parallel transport have been demonstrated to be functions of helical flux surfaces, allowing for profile solutions with convex or concave shapes rather than a flat form. In this paper, we first trace the derivation of gyrokinetics, the basis for simulations. We analyze the results of local and global simulations for the case of an island with a generally flat profile. Subsequently, we examine the simulation results for the case of an island with a concave profile. We then explore why islands with concave profiles exist using a simple Lagrangian model and introduce the key parameter 𝛼, which explains concaveness through effective potential. The analysis of gyrokinetic simulation results revealed a clear proportionality between the concaveness of the island profile and 𝛼. Contrary to the conventional belief that larger islands become flatter, the results showed a tendency for larger islands to exhibit increased concaveness.