Dust vortices in a direct current glow discharge plasma: a delicate balance between ion drag and Coulomb force

Dust vortices with a void at the centre are reported in this paper. The role of the spatial variation of the plasma potential in the rotation of dust particles is studied in a parallel plate glow discharge plasma. Probe measurements reveal the existence of a local potential minimum in the region of formation of the dust vortex. The minimum in the potential well attracts positively charged ions, while it repels the negatively charged dust particles. Dust rotation is caused by the interplay of the two oppositely directed ion drag and Coulomb forces. The balance between these two forces is found to play a major role in the radial confinement of the dust particles above the cathode surface. Evolution of the dust vortex is studied by increasing the discharge current from 15 to 20 mA. The local minimum of the potential profile is found to coincide with the location of the dust vortex for both values of discharge currents. Additionally, it is found that the size of the dust vortex as well as the void at the centre increases with the discharge current.