Gyrokinetic modeling of neoclassical tearing modes using the electromagnetic X-point Gyrokinetic Code (XGC)

Neoclassical tearing modes in tokamak plasmas are an active area of research due to their impact on global plasma stability. They have been mainly studied with magnetohydrodynamic (MHD) codes due to the relatively smaller computational cost compared with kinetic models. The downside of fluid methods is the approximate treatment of kinetic effects such as collisional parallel or turbulent cross-field transport through simple fluid closures and turbulence models. In contrast, this work uses the electromagnetic total-f gyrokinetic particle-in-cell code XGC to model low toroidal mode number ("low-n") tearing modes using a recently installed hybrid spectral/finite-element field solver that provides higher accuracy and greater numerical stability for low-n modes than XGC's standard field solver. We present verification studies of low-n tearing mode physics using XGC's delta-f option against linear eigenvalue solvers and MHD codes. Using XGC's total-f algorithm, we study neoclassical tearing modes in a single first-principles kinetic model without relying on fluid closures or coupling to other codes.