In this work, we present plasma response simulations of magnetic island bifurcation in NSTX-U and DIII-D discharges. In tokamaks, magnetic islands can be either naturally occurring, such as tearing modes, locked modes, or externally driven, such as static magnetic islands produced by Resonant Magnetic Perturbations (RMPs) from field-errors and 3D control or Error Field Correction coils. These magnetic islands are known to have significant effects on the confinement of energy, particles and momentum. A particularly important class of MHD modes are rotating m,n = 2,1 islands, where m and n are the poloidal and toroidal mode numbers, that grow in width radially as their rotational frequency drops, and eventually lock. These have been identified as a primary mechanism involved in triggering dangerous, short time scale, drops in the stored thermal energy of the plasma followed by rapid decay of the toroidal plasma current, and the generation of highly energetic runaway electron (RE) beams.

First, linear two-fluid M3D-C1 simulations were performed for NSTX-U H-mode plasmas predicting a new class of internal magnetic island bifurcations. It is found that magnetic islands undergo a bifurcation sequence, starting from m,n islands on each resonant surface that are driven by the 3D RMP coil spectrum, and progressing to 2m,2n, and then 3m,3n, as the current in the NSTX-U RMP coil is increased. The resulting bifurcations create new pairs of x- and o-points that are encapsulated inside the original m,n island o- point.

Recently, magnetic island heteroclinic bifurcations were empirically observed in the DIII- D core tokamak plasma for the first time. We expand our understanding of heteroclinic bifurcations in NSTX-U and DIII-D using linear NIMROD simulations in order to determine how this process affects neoclassical tearing mode stability, growth rates, and locking that results in disruptions during discharges with various normalized poloidal pressure and aspect ratios.

* Work supported by US DOE under DE-FC02-04ER54698, DE-SC0021185, DE-SC0021284, and DE-FG02-05ER54809.