Ion collection by a conducting sphere in a magnetized or drifting collisional plasma

Abstract

Ion collection by dust grains and probes in plasmas with a neutral background is of interest in the study of both space and terrestrial plasmas, where charge-exchange collisions can play an important role in ion collection. Further, background drifts or magnetic fields can significantly affect the ion collection by and the potential structure near such objects, and should therefore also be included. These effects, however, are difficult to include in a theoretical treatment, and thus this problem lends itself to a computational approach. To be able to tackle problems with a neutral background, the 3D3v hybrid particlein- cell code SCEPTIC3D has been upgraded to include charge-exchange collisions. This required the development of a new Monte Carlo based reinjection scheme. The new reinjection scheme and other upgrades are described in detail, and the collisionless operation of the reinjection scheme is validated against the old SCEPTIC3D reinjection scheme, while its collisional operation is validated through comparisons with the reinjection scheme in SCEPTIC (2D). The new reinjection scheme can easily be modified to allow the injection of an almost arbitrary distribution function at the domain boundary, enabling future studies of the sensitivity of ion collection to the injected velocity distribution. Studies of ion collection in magnetized or drifting plasmas using the upgraded code extend earlier stationary, unmagnetized results, which showed an enhancement of ion current at intermediate collisionality. It is found that this enhancement is gradually suppressed with increasing background neutral drift speed, and is entirely absent for speeds above the ion sound speed. Adding a magnetic field rather than a neutral drift appears to in fact increase the collisional ion current enhancement.