Effects of flows on edge localized mode stability in spherical tokamaks*

The CETOP project explores ELM-free regimes in spherical tokamaks (STs), focusing on wide and enhanced pedestal regimes. ELM stability in STs significantly differs from conventional tokamaks (CTs) due to larger resistivity and other effects. To develop ELM-free scenarios, we investigate factors influencing ELM behavior in STs to predict ELM stability. One such factor is rotational flows. In STs, toroidal rotation can destabilize ELM stability, especially for counter-current flows [N. Aiba et al. J. Plasma Fusion Res. Series 9 (2010) 74]. Our study evaluates flow impacts in NSTX/NSTX-U discharges using the extended MHD NIMROD code. The role of poloidal rotation in NSTX is unclear due to lack of direct measurements; hence, we use neoclassical XGC code predictions and scale rotation profiles to analyze poloidal and toroidal rotation effects. Findings show toroidal rotation destabilizes low-n modes driving type-I ELMs in NSTX/NSTX-U, regardless of flow direction. Our linear analysis uses the single-fluid model, but we also show that two-fluid and gyro-viscosity effects generally enhance mode stability, with primary conclusions on flow effects remaining consistent. We also present nonlinear simulations of selected NSTX discharges, discussing implications for other STs like MAST and future configurations like STAR. Finally, we compare predictions from NIMROD and M3D-C1 codes, highlighting key similarities and differences and their impact on ELM stability in STs.