Control of particle noise in core-edge coupled total-f gyrokinetic simulations*

The first-principles based Whole Device Model Application (WDMApp) project is an Exascale Computing Project (ECP) with the objective to develop a high-fidelity model of magnetically confined fusion plasmas by coupling gyrokinetic codes that are optimized for different regions of the plasma, the core and the edge region. The total-f particle-in-cell code XGC will perform the simulation of the edge region including the high-confinement-mode pedestal, the magnetic separatrix, and the scrape-off layer.

Coupling with a core code involves the coupling of the electromagnetic field equations over an overlap and the exchange of the particle distribution function in surrounding narrow buffer regions [J. Dominski et al. Phys. Plasmas 28 022301 (2021)]. The distribution functions are transferred with the help of a particle resampling technique. However, this approach is sensitive to particle noise. To investigate this issue without involving an extra complication due to grid interpolation, we couple an XGC core simulation to an XGC edge simulation and study different ways of controlling the noise in the distribution function exchange. Moreover, we discuss the importance of the noise control to enable coupled simulations over longer transport time scales necessary to evolve radial pressure profiles.