Description
One major goal of nuclear fusion simulations is to study the properties of turbulent transport, which is the dominant energy loss mechanism in nuclear fusion reactors. Mitigating this transport channel has the potential to significantly improve the energy efficiency of future reactors. In recent decades, gyrokinetic codes such as GENE, GYRO, and ORB5 have become standard tools for studying turbulent transport in the plasma core, where density and temperature gradients as well as perturbations of equilibrium quantities are small. However, experimental evidence suggests that key confinement mechanisms are driven by plasma edge phenomena, where the standard gyrokinetic ordering reaches its limits. BSL6D, an electrostatic, fully kinetic ion code, aims to shed light on these phenomena, but is currently limited by the assumption of standard adiabatic electrons.
As a step toward the full drift-kinetic electron model, we have implemented an implicit electron species that can be regarded as the massless limit of drift-kinetic electrons. In this model, electrons are adiabatic on magnetic flux surfaces, but maintain a constant flux-surface averaged density. The critical challenge lies in determining the flux-surface averaged component of the electric potential so that both species evolve quasi-neutral, a problem that corresponds to suppressing Langmuir-like oscillations in the flux surface averaged ion density. To maintain numerical stability in the presence of large radial density variations, we have implemented specific interpolation error corrections in the field solver.
The two-species model enables the simulation of zonal flows within the kinetic ion species, allowing for the investigation of their impact on non-gyrokinetic turbulence. To verify the model, we first benchmark the zonal flow dynamics against established gyrokinetic simulations. After that, we analyze their influence on turbulent transport driven by slab ion Bernstein waves.