Effect of Controlled Magnetic Island Bifurcation on Electron Diffusion

TL;DR

This work links magnetic island topology to cross-field electron diffusion by combining DIII-D experiments that drive a periodic $q=2$ island bifurcation to a narrower $q=4/2$ structure with TRIP3D-based tracer simulations incorporating a collisional operator. By launching 10,000 tracer electrons from O-points, X-points, and outside separatrices, the study demonstrates a topology-dependent transition from subdiffusive trapping around O-points in the wider island to superdiffusive transport as new X-points emerge in the bifurcated structure, while edge stochasticity diminishes due to narrower islands. The analysis uses vacuum-field reconstructions, diffusion histograms with multiple fitting models, and Chirikov/SURFMN metrics to quantify stochasticity, linking island width and X-point geometry to confinement and potential energetic-electron mechanisms. Practically, the results suggest that controlled island bifurcation can modulate electron diffusion and may inform disruption-mitigation strategies that leverage magnetic topology with minimal diagnostic requirements.

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.

Figures (12)

• Figure 1: Plasma parameters (a) $q_{95}$ and (b) line averaged electron density for shots with magnetic island rotation during time interval 2000-3500 ms. Equilibrium reconstruction for shot 196099 at time slice 2440 ms.
• Figure 2: Diagram of coil geometry on DIII-D used for controlled formation and rotation of magnetic islands.
• Figure 3: (a) Coupling between the rotating $n=1$ I-coil field (red dashed line) and fixed-phase perturbations from the intrinsic error field (dashed green line) and C-coil (dashed orange line). Radial profiles of the $n=1$ magnetic field spectrum ($B_{m,1}$) at 2800 ms and 2850 ms show how the total field at rational surfaces results from constructive or destructive superposition of the I-coil, C-coil, and intrinsic contributions. The $q=2/1$ surface (blue shaded region) is particularly sensitive to this phase relationship. (b) Time traces of I-coil amplitude, I-coil phase, and C-coil phase illustrate how the rotating I-coil periodically aligns constructively or destructively with the fixed C-coil and error fields, driving transitions in the island structure.
• Figure 4: I-coil phase and Hard X-Ray (fplastic) for 10 Hz (a) and 5 Hz (b) rotational frequencies. Blue shaded regions indicate peaks in fplastic. Green shaded region shows consistent phase at which peaks occur in the I-coil phase.
• Figure 5: I-Coil current (a), X-ray scintillator detection of EEs (b), width of $q=2/1$ island (c), and vacuum island overlap width (d) for shot 196099 during the time interval 2000-3000 ms. Blue dashed lines represent RMP-induced island bifurcation, which occurs every 40-50 ms of the 10 Hz island rotation period and corresponds to peaks in X-ray data. Red lines show simulated times chosen (2400 ms and 2440 ms).