Modeling ICRF mode conversion in VSim with a kinetics-only delta-f (KODF) method

We present a new delta-f PIC method, kinetics-only delta-f (KODF), for modeling the nonlinear phase space evolution of plasma species distribution functions. Conventional delta-f methods evolve computational markers along characteristic trajectories to model perturbations around a known equilibrium distribution, with a marker weight variable tracking deviations of the distribution function from equilibrium. In KODF, we generalize this concept to incorporate cold linear plasma waves into the known (quasi)analytic plasma behavior. Perturbations modeled by KODF methods are thus nonlinear, finite-temperature perturbations (modeled with PIC) atop an equilibrium and its associated cold linear waves (which can each be modeled analytically or numerically without PIC noise). We demonstrate an implementation of KODF in the VSim code. VSim's semi-implicit FDTD methods [Smithe, PoP 14, 056104 (2007)] are used to model the fluid behavior of cold plasma waves; source terms arising from these waves (e.g. from gradients of cold current/charge densities) concurrently drive and evolve responsive warm plasma effects in a PIC formulation. We explore the noise-reduction capabilities of the KODF algorithm and its ability to model waves of interest in RF heating scenarios (e.g. mode-converted IBWs).