Intergrain forces in low Mach-number plasma wakes

TL;DR

This study uses particle-in-cell simulations to analyze plasma wake interactions of negatively charged grains at low Mach numbers, revealing how oscillations and forces depend on flow velocity, collisions, and grain size.

Contribution

It provides detailed 3D simulation results on intergrain forces and wake structures in low Mach-number plasma flows, highlighting the effects of damping and collisions.

Findings

Oscillatory wakes vanish below Mach number ~0.3 due to Landau damping.

Neutral collisions suppress potential oscillations but not the attractive peak.

Transverse forces are generally small for slow flows except for large grains in nonlinear regimes.

Abstract

Large-scale particle-in-cell calculations of the plasma wake interactions of two negatively charged grains smaller than the Debye length are carried out using the COPTIC code over a wide range of subsonic plasma flow velocities. In plasmas with temperature ratio $T_e/T_i=100$, it is found that a single grain's oscillatory wake disappears for flow Mach numbers ($M$) less than approximately 0.3, which is the parameter regime where Landau damping is expected to be strong. Neutral collisions suppress potential oscillations above $M=0.3$, but not the trailing attractive potential peak caused by ion focussing. The transverse (grain-aligning) force on a downstream particle in the wake of another is obtained rigorously from the code in three-dimensional simulations. It shows general agreement with the force that would be deduced from the single-grain wake potential gradient. Except for relatively large grains in the nonlinear collisional regime, the grain-aligning force is very small for slow flow.