Modeling tokamak edge plasma with the continuum full-F code COGENT

COGENT is an Eulerian gyrokinetic code being developed for edge plasma modeling. The underlying numerical discretization employs a locally field-aligned coordinate system and a mapped multi-block grid technology to effectively handle strongly anisotropic turbulence in a diverter geometry. The gyrokinetic system, typically used for ion species, can be combined with lower-dimensional fluid equations, such as those governing fluid response of electrons or neutrals. To investigate ion-scale turbulence in X-point geometries, an electrostatic hybrid kinetic ion – fluid electron model that captures ITG and resistive modes was developed. Our recent efforts have focused on expanding this hybrid model to incorporate low-beta electromagnetic effects. The electromagnetic system accommodates high-frequency shear-Alfven waves, requiring explicit simulations to satisfy a strict Courant constraint for time stepping. To address this challenge in COGENT, we utilize an implicit-explicit time integration framework and a physics-based preconditioner that accurately captures the dynamics of linear Alfven waves, enabling us to advance over the stiff Alfven time scale. Additionally, we have integrated a fluid magnetohydrodynamics (MHD) module into the suite of COGENT tools. The module accounts for finite-beta effects and leverages the field-aligned grid technology available in the code.