Simulations of NTM Seeding via MHD Transients*

Simulations demonstrating seeding of neoclassical tearing modes (NTMs) via MHD-transient-induced nonlinear multimode interactions are presented. NTMs are the leading physics cause of disruptions in tokamaks. Yet there are many open questions regarding their evolution, e.g., “How are NTMs generated from MHD activity?”, “How does a NTM lock and then trigger a disruption?”, etc. Many of these questions are inherently nonlinear, and simulations are needed to help address these issues in experimentally relevant equilibria. NIMROD simulations of NTMs are enabled by two recent code developments: the implementation of heuristic neoclassical stresses and the application of transient magnetic perturbations (MPs) at the boundary. NTMs are driven unstable by the bootstrap current, which arises due to collisional viscosity between passing and trapped electrons. This current is inherently kinetic. These simulations use heuristic closures that model the neoclassical electron and ion stresses. NTM growth requires a seed island, and NIMROD simulations apply a transient MP to generate this seed. The capability is demonstrated using kinetic-based reconstructions with flow of a DIII-D ITER Baseline Scenario discharge [R.J. La Haye, J.D. Callen, et al., Proceedings IAEA FEC 2020]. An applied transient MP seeds a 2/1 NTM that grows in two phases: a slow growth phase followed by a faster robust growth phase like that observed experimentally. Additionally, an evolving sequence of higher order core modes are excited. Power transfer analysis shows that nonlinear interactions between the core modes and the 2/1 helps drive the initial slow growth. Once the induced 2/1 magnetic island reaches a critical width, the NTM transitions to faster robust growth which is well described by the nonlinear modified Rutherford equation. This work highlights the role of nonlinear mode coupling in seeding NTMs.