Particle in cell calculation of plasma force on a small grain in a non-uniform collisional sheath

TL;DR

This study uses particle-in-cell simulations to analyze the plasma force on small grains in a non-uniform, collisional sheath, highlighting the importance of non-Maxwellian ion distributions and collisional effects.

Contribution

It introduces a detailed PIC calculation method for plasma forces on grains in collisional sheaths, accounting for non-Maxwellian ion distributions and collisional drag enhancements.

Findings

Force agrees with electric field plus ion drag model

Ion distribution functions vary with collisionality

Collisional effects significantly enhance drag force

Abstract

The plasma force on grains of specified charge and height in a collisional DC plasma sheath are calculated using the multidimensional particle in cell code COPTIC. The background ion velocity distribution functions for the unperturbed sheath vary substantially with collisionality. The grain force is found to agree quite well with a combination of background electric field force plus ion drag force. However, the drag force must take account of the non-Maxwellian (and spatially varying) ion distribution function, and the collisional drag enhancement. It is shown how to translate the dimensionless results into practical equilibrium including other forces such as gravity.