Title: Experimental observation of self excited co--rotating multiple vortices in a dusty plasma with inhomogeneous plasma background,
Abstract: We report an experimental observation of multiple co--rotating vortices in a
extended dust column in the background of non--uniform diffused plasma.
Inductively coupled RF discharge is initiated in the background of argon gas in
the source region which later found to diffuse in the main experimental
chamber. A secondary DC glow discharge plasma is produced to introduce the dust
particles into the plasma. These micron sized poly-disperse dust particles get
charged in the plasma environment and transported by the ambipolar electric
field of the diffused plasma and found to confine in the potential well, where
the resultant electric field of the diffused plasma (ambipolar E--field) and
glass wall charging (sheath E--field) hold the micron sized particles against
the gravity. Multiple co--rotating (anti--clockwise) dust vortices are observed
in the dust cloud for a particular discharge condition. The transition from
multiple to single dust vortex is observed when input RF power is lowered.
Occurrence of these vortices are explained on the basis of the charge gradient
of dust particles which is orthogonal to the ion drag force. The charge
gradient is a consequence of the plasma inhomogeneity along the dust cloud
length. The detailed nature and the reason for multiple vortices are still
under investigation through further experiments, however, preliminary
qualitative understanding is discussed based on characteristic scale length of
dust vortex. There is a characteristic size of the vortex in the dusty plasma
so that multiple vortices is possible to form in the extended dusty plasma with
inhomogeneous plasma background. The experimental results on the vortex motion
of particles are compared with a theoretical model and found some agreement.