Collisional Effects on Nonlinear Ion Drag Force for Small Grains

TL;DR

This paper investigates how ion-neutral collisions affect the nonlinear ion drag force on small grains in plasma, revealing complex dependencies on collisionality and ion velocity distribution.

Contribution

It presents self-consistent, nonlinear calculations of ion drag force including collisions, highlighting the importance of ion velocity distribution and shielding length in force estimation.

Findings

Finite collisionality can double the drag force.

Collisionality eventually reduces the drag force after initial enhancement.

Drift velocity distribution yields larger drag than shifted Maxwellian at high collisionality.

Abstract

The ion drag force arising from plasma flow past an embedded spherical grain is calculated self-consistently and non-linearly using particle in cell codes, accounting for ion-neutral collisions. Using ion velocity distribution appropriate for ion drift driven by a force field gives wake potential and force greatly different from a shifted Maxwellian distribution, regardless of collisionality. The low-collisionality forces are shown to be consistent with estimates based upon cross-sections for scattering in a Yukawa (shielded) grain field, but only if non-linear shielding length is used. Finite collisionality initially enhances the drag force, but only by up to a factor of 2. Larger collisionality eventually reduces the drag force. In the collisional regime, the drift distribution gives larger drag than the shift distribution even at velocities where their collisionless drags are equal. Comprehensive practical analytic formulas for force that fit the calculations are provided.