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.

This paper reports on the dynamics of a 3-dimensional dusty plasma in a strong magnetic field. An electrostatic potential well created by a conducting or non-conducting ring in the rf discharge confines the charged dust particles. In the absence of the magnetic field, dust grains exhibit a thermal motion about their equilibrium position. As the magnetic field crosses a threshold value (B > 0.02 T), the edge particles start to rotate and form a vortex in the vertical plane. At the same time, the central region particles either exhibit thermal motion or E→×B→ motion in the horizontal plane. At B > 0.15 T, the central region dust grains start to rotate in the opposite direction resulting in a pair of counter-rotating vortices in the vertical plane. The characteristics of the vortex pair change with increasing the strength of the magnetic field (B ∼ 0.8 T). At B > 0.8 T, the dust grains exhibit very complex motion in the rotating torus. The angular frequency variation of rotating particles indicates a differential or sheared dust rotation in a vortex. The angular frequency increases with increasing the magnetic field from 0.05 T to 0.8 T. The ion drag force and dust charge gradient along with the E-field are considered as possible energy sources for driving the edge vortex flow and central region vortex motion, respectively. The directions of rotation also confirm the different energy sources responsible for the vortex motion.

In this paper, the collective dynamics of the large aspect ratio dusty plasma is studied over a wide range of discharge parameters. An inductively coupled diffused plasma, which creates an electrostatic trap to confine the negatively charged grains, is used to form a large volume (or large aspect ratio) dusty plasma at low pressure. For introducing the dust grains into the potential well, a unique technique using a secondary DC glow discharge plasma is employed. The dust dynamics is recorded in a 2-dimension (2D) plane at a given axial location. The dust fluid exhibits wave like behavior at low pressure (p<0.06 mbar) and high rf power (P>3 W). The mixed motion, waves and vortices, are observed at an intermediate gas pressure(p∼ 0.08 mbar) and low power (P<3 W). Above the threshold value of gas pressure (p>0.1 mbar), the clockwise and anti-clockwise co-rotating vortex series are observed on the edges of the dust cloud, whereas the particles in central region show the random motion. These vortices are only observed above a threshold width of the dust cloud. The streaming ions are considered the available free energy source to excite the waves in dust grain medium. The occurrence of the co-rotating vortices is understood on the basis of the charge gradient of dust particles which is orthogonal to the gravity. The charge gradient is a consequence of the plasma inhomogeneity from the central region to the outer edge of dust fluid. Since, a vortex has the characteristic size in the dissipative medium; therefore, a series of the co-rotating vortex on the both sides of dusty plasma is observed. The experimental results on the vortex formation and its multiplicity are compared to an available theoretical model and are found to be in close agreement.

The experimental observation of the self−excited dust acoustic waves (DAWs) and its propagation characteristics in the absence and presence of a floating cylindrical object are investigated. The experiments are carried out in a direct current (DC) glow discharge dusty plasma in the background of argon gas. Dust particles are found levitated at the interface of plasma and cathode sheath region. The DAWs are spontaneously excited in the dust medium and found to propagate in the direction of ion drift (along the gravity) above a threshold discharge current at lower pressure.The excitation of such low frequency wave is a result of the ion-dust streaming instability in the dust cloud. The characteristics of the propagating dust acoustic wave get modified in presence of a floating cylindrical object of radius larger than the dust Debye length. Instead of propagating in the vertical direction, the DAWs are found to propagate obliquely in presence of the floating object (kept either vertically or horizontally). In addition, horizontally aligned floating object gives rise to form a wave structure in the cone shaped dust cloud in the sheath region. The change in the propagation characteristics of DAWs are explained on the basis of modified potential (or electric field) distribution, which is a consequence of coupling of sheaths formed around the cylindrical object and the cathode.