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.

Multiple steady state co-rotating dust vortices in streaming plasma

“…They also observed the structural transition of the dust cloud into multiple vortices when the plasma density exceeds a critical value n i * . Further, in the series of recent experiments carried out by M. Choudhary et al (2018) [9,10], who has analyzed dust dynamics in a domain of higher aspect-ratio L z /L r ≈3 to 5, indeed observed the structural transition from a single to co-rotating dust vortices of equal size by changing the applied discharge power through a threshold value P * . Fur-ther, in some cases, they also observed variation in the number of co-rotating vortices at different cross-sections of the elongated 3D dust cloud in an inhomogeneous rfplasma.…”

“…And the existence of adjacent multiple counter-rotating vortices is intuitively explained by the shear nature of driving mechanisms [6,12]. However, the appearance of self-similar co-rotating dust vortices reported in recent experiments with a background plasma flow of monotonic shear is rather counter-intuitive since a sharp dust flow shear layer is essentially present between two spatially adjacent co-rotating vortices, usually unexpected in steady fluid flow equilibria [8,9,11]. In such cases, the physics of single-fluid multiple vortices (like the Taylor-Couette vortices [13,14]) may not be directly applicable to the volumetrically driven multiplefluid cases such as those found in dusty plasma and biophysical complex systems [5,8,10].…”

Influence of external magnetic field on dust acoustic waves in a capacitive RF discharge

“…[17][18][19] The study of DAWs has a more than 25-year-old history. [17,19,24] Such low frequency dust acoustic modes are either excited by inherent instabilities such as ion-streaming instability and dust-dust instability [25][26][27][28] or externally forced triggered instabilities. [17,19,24] Such low frequency dust acoustic modes are either excited by inherent instabilities such as ion-streaming instability and dust-dust instability [25][26][27][28] or externally forced triggered instabilities.…”

“…The results of the collective phenomena of dusty plasma are predicted theoretically and experimentally in the form of linear and non-linear waves such as longitudinal dust acoustic waves (DAWs) [2][3][4][5] dust acoustic transverse waves, [6][7][8] dust lattice waves, [9][10][11] dust acoustic solitary waves, [12,13] and dust acoustic shock waves [14][15][16] as well as dust vortices. [17][18][19] The study of DAWs has a more than 25-year-old history. [20] In low-temperature discharges (DC or RF), the DAWs are excited in the dust grain medium which is confined in a strong electric field (E) of the cathode sheath [5,13] or anodic plasma [3] or rf sheath [21][22][23] or diffused plasma.…”

“…[20] In low-temperature discharges (DC or RF), the DAWs are excited in the dust grain medium which is confined in a strong electric field (E) of the cathode sheath [5,13] or anodic plasma [3] or rf sheath [21][22][23] or diffused plasma. [17,19,24] Such low frequency dust acoustic modes are either excited by inherent instabilities such as ion-streaming instability and dust-dust instability [25][26][27][28] or externally forced triggered instabilities. [29,30] However, the ion-streaming instability which arises due to the streaming of ions relative to the stationary dust particles is considered as a main free energy source to excite DAWs in DC or RF discharge.…”