TRIMEG-GKX: An electromagnetic gyrokinetic particle code with a piecewise field-aligned finite element method for micro- and macro-instability studies in tokamak core plasmas

• Adoption of object-oriented programming in gyrokinetic particle simulations, supporting scalable studies involving multiple species. • The implementation of a high-order piecewise field-aligned finite element method and a mixed particle-in-cell-particle-in-Fourier method within an electrostatic gyrokinetic particle simulation model, enabling efficient 3D and 2D simulations of tokamak plasmas. • A rigorous strong-form treatment of the gyrocenter equations of motion, enhancing accuracy and enabling filter- and buffer-free simulations. • A novel cache optimization strategy, delivering up to a tenfold speedup in specific simulation cases. • Support for unstructured meshes, facilitating whole-device simulations with complex geometries. The features of the TRIMEG-GKX code are described with emphasis on the exploration using novel/different schemes compared to other gyrokinetic codes, particularly the use of object-oriented programming, filter/buffer-free treatment, and a high-order piecewise field-aligned finite element method. The TRIMEG-GKX code solves the electromagnetic gyrokinetic equation using the particle-in-cell scheme, taking into account multi-species effects and shear Alfvén physics. The mixed-variable/pullback scheme has been implemented to enable electromagnetic studies. This code is parallelized using particle decomposition and domain cloning among computing nodes, replacing traditional domain decomposition techniques. The applications to study the micro- and macro-instabilities are demonstrated, including the energetic-particle-driven Alfvén eigenmode, ion temperature gradient mode, and kinetic ballooning mode. Good performance is achieved in both ad hoc and experimentally reconstructed equilibria, such as those of the ASDEX Upgrade (AUG), Tokamak á configuration variable (TCV), and the Joint European Torus (JET). Future studies of edge physics using the high-order C 1 finite element method for triangular meshes in the TRIMEG-C1 code will be built upon the same numerical methods.