Coarse-graining phase space in δf particle-in-cell simulations

A numerical scheme for periodically coarse-graining the distribution in the phase space for delta f particle-in-cell (PIC) simulation is presented. delta f is periodically deposited on a five-dimensional phase-space grid, then reevaluated at the particle position using interpolation. Any discontinuity of delta f in time arising from this coarse-graining procedure is reduced by resetting only a small fraction of the particle weight given by the interpolated value. An estimate of the numerical diffusion due to this smoothing procedure is provided in the limit of large particle number. Using three-dimensional toroidal ion-temperature-gradient driven turbulence as an example, the numerical scheme is demonstrated to effectively suppress the long-term increase of the particle weights, while keeping the turbulent flux unchanged. (C) 2007 American Institute of Physics.