Charging and Growth of Fractal Dust Grains

TL;DR

This paper models how fractal dust grains in plasma environments acquire charge and grow, revealing that electrostatic effects may facilitate coagulation despite like-charge repulsion, impacting dust evolution.

Contribution

It introduces a modified OML_LOS model for calculating charge and dipole moments of fractal dust aggregates, enabling improved coagulation modeling in plasma environments.

Findings

Charges on aggregates are often reduced enough to allow coagulation.

The model predicts significant coagulation despite like-charged grains.

Heuristic charging scheme can be integrated into coagulation models.

Abstract

The structure and evolution of aggregate grains formed within a plasma environment are dependent upon the charge acquired by the micron-sized dust grains during the coagulation process. The manner in which the charge is arranged on developing irregular structures can affect the fractal dimension of aggregates formed during collisions, which in turn influences the coagulation rate and size evolution of the dust within the plasma cloud. This paper presents preliminary models for the charge and size evolution of fractal aggregates immersed in a plasma environment calculated using a modification to the orbital-motion-limited (OML) theory. Primary electron and ion currents incident on points on the aggregate surface are determined using a line-of-sight (LOS) approximation: only those electron or ion trajectories which are not blocked by another grain within the aggregate contribute to the charging current. Using a self-consistent iterative approach, the equilibrium charge and dipole moment are calculated for the dust aggregate. The charges are then used to develop a heuristic charging scheme which can be implemented in coagulation models. While most coagulation theories assume that it is difficult for like-charged grains to coagulate, the OML_LOS approximation indicates that the electric potentials of aggregate structures are often reduced enough to allow significant coagulation to occur.