GEM-X: an efficient global gyrokinetic simulation of the tokamak edge including the scrape-off layer (SOL)

Development is underway of GEM-X - a fast-time-to-solution gyrokinetic particle code for modeling the edge pedestal, including the SOL region. GEM-X uses the fully-nonlinear δf method, capable of resolving locally large δf/f with weight control and increased particle number. Allowing for modeling the blobby transport in the SOL. GEM-X has a full-f option, but then the code will have no advantage over the existing full-f codes. Equilibrium profiles, including the 2D axisymmetric electrostatic potential, are determined using SOLPS-ITER with the fluid drifts. The equilibrium profiles from SOLPS-ITER, along with a fix-gradient source allows for the more efficient nonlinear δf approach. A critical feature is the capability to solve the linear initial-value problem assuming the SOLPS-ITER solution as an equilibrium with Maxwellian plasma distribution functions, improved by using toroidal canonical angular momentum and energy as constants of motion. The magnetic drift drive term v_D · ∇f_0 can be dropped to avoid axisymmetric neoclassical evolution. This allows for the identification of unstable modes in the steep gradient and SOL regions. As important, linear calculations allow for detailed benchmarking, as is done routinely with the core gyrokinetic codes. It is impractical to benchmark full-f calculations with evolving profiles. GEM-X uses a drift-fluid electron model with closure using drift-kinetic electrons. We choose the drift-fluid electron with kinetic electron closure because it is helpful for a staged development path. Another advantage of a particle approach is that the finite-gyroradius (FLR) effects is straightforward to implement. We will show some initial results from the fluid electron model.