Effect of Controlled Magnetic Island Bifurcation on Electron Diffusion

TL;DR

This study investigates how controlled bifurcations of magnetic islands in tokamak plasmas affect electron diffusion pathways, revealing that topological changes can significantly alter confinement and transport behaviors.

Contribution

It demonstrates the impact of magnetic island bifurcation on electron diffusion using field line tracing simulations, providing new insights into plasma confinement mechanisms.

Findings

Bifurcation modifies electron diffusion regimes.

Diffusion varies with island mode and launch location.

Results suggest bifurcation influences particle confinement.

Abstract

Magnetic islands strongly influence cross-field electron transport in magnetized plasmas. In particular, bifurcations of the island topology modify the number and location of O-points, X-points, and separatrix boundaries, thereby altering diffusion pathways. In recent DIII-D experiments, external magnetic perturbations were used to rotate and periodically bifurcate the island on the q = 2 surface, causing a switchback between a q = 2/1-dominated structure and a narrower q = 4/2-dominated structure. To investigate how this topological change affects electron transport, we employ the field line tracing code TRIP3D with an implemented collisional operator. Thermal, tracer electrons launched from O-points, X-points, and outside separatrix boundaries reveal distinct diffusion regimes, including classical, subdiffusive, and superdiffusive behavior, depending on both the dominant island mode and launch location. These results suggest that island bifurcation can alter electron diffusion across rational surfaces, with direct implications for particle confinement. While the present work emphasizes diffusion as a general framework, the findings provide insight into the conditions under which electron trapping into an island or stochastization of the island's separatrix can enable additional mechanisms, such as the generation of energetic electrons.