Implicit BGK Collision Operator for Gyrokinetic Simulation with Moments-Conserving Maxwellian*

Tokamak confinement is highly influenced by heat and particle transport in the scrape-off layer (SOL). Experiments on ASDEX have reported a significant enhancement of perpendicular transport due to a transition between filamentary regimes as the effective collisionality increases in this region [D. Carralero et al 2017 Nucl. Fusion]. Therefore, it is compelling to study the underlying mechanism with gyrokinetic simulation using Gkeyll, an electromagnetic continuum full-f code. High collisionality leads to tiny explicit time step limits and long simulation times. To achieve this regime with reasonable computational cost, we formulate an implicit BGK collision operator, taking advantage of its simple form. The major challenge is the conservation of moments when projecting a Maxwellian onto the numerical basis functions. We match the moments of the projected Maxwellian exactly to the moments of the numerical particle distribution function at each step by solving a nonlinear system of equations with an iterative method. With this implicit BGK collision operator enabling simulation at high collisionality, we present some initial results for the SOL of ASDEX with realistic geometry and profiles of various plasma parameters close to the experiments.