On the unconstrained expansion of a spherical plasma cloud turning collisionless: case of a cloud generated by a nanometre dust grain impact on an uncharged target in space

Nano and micrometre sized dust particles travelling through the heliosphere at several hundreds of km s-1 have been repeatedly detected by interplanetary spacecraft. When such fast moving dust particles hit a solid target in space, an expanding plasma cloud is formed through the vaporization and ionization of the dust particles itself and part of the target material at and near the impact point. Immediately after the impact the small and dense cloud is dominated by collisions and the expansion can be described by fluid equations. However, once the cloud has reached μm dimensions, the plasma may turn collisionless and a kinetic description is required to describe the subsequent expansion. In this paper we explore the late and possibly collisionless spherically symmetric unconstrained expansion of a single ionized ion-electron plasma using N-body simulations. Given the strong uncertainties concerning the early hydrodynamic expansion, we assume that at the time of the transition to the collisionless regime the cloud density and temperature are spatially uniform. We also neglect the role of the ambient plasma. This is a reasonable assumption as long as the cloud density is substantially higher than the ambient plasma density. In the case of clouds generated by fast interplanetary dust grains hitting a solid target, some 107 electrons and ions are liberated and the in vacuum approximation is acceptable up to meter order cloud dimensions. As such a cloud can be estimated to become collisionless when its radius has reached μm order dimensions, both the collisionless approximation and the in vacuum approximation are expected to hold during a long lasting phase as the cloud grows by a factor 106. With these assumptions, we find that the transition from the collisional to the collisionless regime could occur when the electron Debye length λD within the cloud is much smaller than the cloud radius R0, i.e. Λ ≡ λD/R0 ≪ 1. This implies a quasi-neutral expansion regime where the radial electron and ion density profiles are equal through most of the cloud except at the cloud-vacuum interface. The consequence of Λ being much smaller than unity implies that the electrostatic fields within a cloud generated by a dust impact on a neutral target is ˜100 times weaker than in the case of grains hitting a spacecraft, where the positive potential of the target is strong enough to strip-off all the electrons from the expanding cloud leading to a ‘Coulomb explosion’ like regime (e.g. Peano et al 2007 Phys. Plasmas \bf 14 056704).