Investigation of Probe-Induced Dust Voids in a Weakly Magnetized Dusty Plasma

Abstract

Dust voids, or regions free of dust grains surrounded by dusty plasmas, have been observed in a wide array of experimental configurations, from microgravity systems, to 3-dimensional dust clouds, to 2-dimensional monolayers. Broadly, these dust voids have fallen into one of two modes: voids driven by a repulsive electrostatic force on the dust grains, opposed by an inward ion drag force (ion-collecting voids); and voids driven by an outward ion drag force, maintained by an inward electrostatic force (ion-streaming voids). By configuring an rf plasma system so that the main ionization volume falls within the interior of a 2-dimensional void in a dust monolayer, a system has been created in which dust voids can display both modes, as well as a transitionary mode between the two. A modification of the Avinash model of ion-collecting dust voids is applied to the ion-streaming modes and is used to estimate the ion drift velocities in both modes. The ions are found to be both subthermal and subsonic. The voids are also placed under the influence of an externally applied magnetic field. By comparing with the non-magnetized results, the effect of the magnetic field on the electric charge of the dust grains is calculated. The voids are also observed to deform significantly under the influence of the magnetic field, and this behavior is explained via the geometry of the dust confinement potential of the rf sheath, in conjunction with a complex arrangement of plasma drifts.