The Role of Island Bifurcation on Deconfinement of Energetic Electrons

Recent Frontier Science DIII-D experiments demonstrated that energetic electrons (EEs) of energies >10 MeV can be trapped within magnetic islands in the core plasma. Synchrotron emission camera data showed that the electrons remain mostly trapped inside the island. However, during island rotation, periodic bursts of EEs were detected by X-ray scintillators, suggesting that these particles can become deconfined and hit the wall. Here we argue that changing the direction of the I-coil current (needed for the rotation) changes the contribution of the dominant wave mode creating an island on the q=2 surface. As a result, the q=2 island chain bifurcates between a structure with 2 O-points and 2 X-points and a structure with 4 O-points and 4 X-points. To verify the role of island bifurcation on the deconfinement of EEs, the 3D magnetic field topology is reconstructed using the field line tracing code TRIP3D. Electron diffusion across the bifurcating q=2 island is then modeled by implementing a collisional operator in TRIP3D. Tracer electrons launched at different starting locations relative to the island allow for comparison of electron diffusion across the different island structures resulting from the bifurcation.