Kinetic Study of Blob Dynamics in Realistic Geometry using the XGC1 Code

A large-amplitude field-aligned coherent structure (a blob) is seeded as an initial condition in the closed flux-surface region near the separatrix in the global gyrokinetic edge simulation code XGC1. XGC1 has realistic tokamak geometry including the scrape-off layer (SOL) and divertor x-point regions. In the simulations, a single blob is seeded with a uniform background density and temperature to simplify the physics and focus on the blob dynamics in the realistic magnetic geometry. Ions are gyrokinetic and electrons are drift-kinetic. We are especially interested in how kinetic electron dynamics compare with fluid theory and simulation. We investigate the scaling of radial blob velocity versus the amplitude of the perturbation and compare it with estimates from fluid theory. Additionally, we find that, as the blob evolves, the structure is not strictly field-aligned, and it appears to be crossing flux surfaces in a direction consistent with the ion magnetic drift. For large amplitude seeding, we see a strong spinning of the blob and we report the rotation frequency versus blob amplitude. The dipole field structure is seen for smaller perturbation strength but is washed out by the blob spin at a larger amplitude. The ExB drift still dominates the blob radial motion, since the averaged ExB drift agrees with the blob radial motion for both small and large amplitudes. We also investigate the radial blob motion and spinning versus collisionality. Finally, the theoretical description of the blob propagation for the current simulations will be presented.