Magnetic Island Bifurcation Caused by Non-Axisymmetric Coil Perturbations in DIII-D Experiments

Here we study DIII-D experiments where controlled magnetic island bifurcation at the q=2 surface is caused by non-axisymmetric coil perturbations. Changing the phase of the perturbation coil current is used to rotate the island and enhance or suppress the 2/1 mode at key phases of the rotation. We analyzed several experiments where the bifurcation at the q=2 surface occurs at different island rotation frequencies. TRIP3D simulations are used to reconstruct the magnetic field topology at successive times during each rotation showing the change in magnetic topology with a time resolution of 10ms. For each magnetic configuration, histograms of the magnetic field line displacements are fitted to q-Gaussian distributions to statistically measure how magnetic field line diffusion changes with topology. We conjecture that, in the case of highly magnetized plasma, electron diffusion will be affected by changes in the field line of diffusion. The implications of plasma confinement and disruptions are discussed.