NIMROD Modeling of Transient-Induced NTM

Extended MHD NIMROD simulations are used to study a neoclassical tearing mode (NTM) induced by an external magnetic perturbation pulse. Simulations use a kinetic reconstruction of a DIII-D ITER baseline scenario discharge and include experimental flow profiles inferred from Charge Exchange Recombination (CER) measurements. In the simulation, a n$=$1 external magnetic perturbation, containing a broad poloidal spectrum, is applied as a 1 ms pulse. The perturbation initially generates a slowly growing m/n$=$2/1 seed island. Following the pulse, high-n core modes are destabilized in a sequence. Initially the 6/5, 5/4, and 4/3 modes go unstable. The 6/5 and 5/4 modes saturate as the 4/3 mode grows to large amplitude, and the 3/2 mode is destabilized. As the 3/2 mode grows to a large amplitude, the 4/3 mode saturates while the 2/1 NTM seed island transitions to a phase of rapid growth and becomes dominant. An analysis of the radial induction equation will be presented to investigate how the nonlinear mode interactions drive the increased 2/1 growth.