GENE-X: A full-f gyrokinetic turbulence code based on the flux-coordinate independent approach

Understanding and predicting plasma turbulence in the scrape-off layer of a magnetic confinement fusion device is a key open problem in modern plasma physics. The transitional region between the core and scrape-off layer poses a difficult problem for turbulence simulations. The poloidal magnetic field vanishes at the X-point of a fusion device, which introduces a coordinate singularity in the commonly used field-aligned coordinates. In the present work, we present a full-f gyrokinetic code based on a locally field-aligned coordinate system that is flux-coordinate independent and free of singularities. The coordinate system, as well as the equations and numerical methods are described. In addition, careful numerical and physical verifications in closed magnetic flux surfaces are included.