Topological bifurcation of magnetic islands in NSTX-U

Topological bifurcations of magnetic islands are observed in two M3D-C¹ linear resistive magnetohydrodynamic calculations of model NSTX-U equilibria during the application of small 3D magnetic perturbation fields. In comparison to a DIII-D ELM suppression case, reconstructed from shot 147170 at time slice 3745 ms, we propose that a stronger kink response in the NSTX-U plasma may be the primary cause of the bifurcations. Island evolution and bifurcations in both the DIII-D and the two NSTX-U cases with the increase of perturbation current are analyzed. Further, a hypothesis is proposed that such magnetic island topology bears an intrinsic property to confine charged particles and cause their density inside the islands to rise, which is examined by implementing a random kick algorithm in the magnetic field line integration code TRIP3D-GPU to approximate electron collisional transport. Although full consideration of all the forces between electrons is missing, as it requires 3D large scale kinetic simulation in toroidal geometry, the positive result of this approximate transport simulation does provide a first step confirmation towards fully validating the hypothesis.