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Particle’s Dynamics in Dusty Plasma with Gradients of Dust Charges Institute for High Energy Densities, Russian Academy of Sciences, Moscow, Russia, O.

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Presentation on theme: "Particle’s Dynamics in Dusty Plasma with Gradients of Dust Charges Institute for High Energy Densities, Russian Academy of Sciences, Moscow, Russia, O."— Presentation transcript:

1 Particle’s Dynamics in Dusty Plasma with Gradients of Dust Charges Institute for High Energy Densities, Russian Academy of Sciences, Moscow, Russia, O. S. Vaulina, O. F. Petrov, V. E. Fortov School of Physics, University of Sydney, NSW 2006, Australia, A. A. Samarian, B.W.James2  Dust Vortices in Gas Discharge Plasma  Stochastic Dust Motion  Self-exited Dust Motion in Rf- Discharge

2 Typical conditions of experiments in gas discharge plasma Parameters of gas discharge plasma: Temperature of ions and electrons: T i << T e ~1-7eV Gas pressure: Р ~ Тorr Plasma concentration: ~ см -3 Neutral’s concentration : ~ см -3 rf- discharge dc- discharge Laboratory Dusty Plasma – weakly ionized gas with micron-sized dust particles (macroparticles)

3 Typical conditions of experiments Parameters of dust particles: Radius: a p ~ 1-10  м Charge: Z p ~ Concentration: n p ~ см –3 Kinetic TEMPERATURE: Т p ~ eV («Abnormal dust heating») Crystal Fluid Dust Vortices Oscillations of separate particles Laboratory Dusty Plasma – weakly ionized gas with micron-sized dust particles (macroparticles)

4 For typical conditions of ground-based experiments in gas discharge plasma Non-electrostatic forces F non - gravity force m p g, ion drag force F i  ( ) m p g, thermothoretic force F т < 0.1 m p g, F ext = e Z р ( ,y) E( ,y), Instability of the system with the dust charge gradients orthogonal to the non-electrostatic force  =  Z( l )/  l – due to inhomogeneity of plasma surrounding the dust cloud n e(i),T e(i) ),V e(i)  Electron Temperature Gradients (  T e /  r)  / ~ (1-100)% см -1  Variations in regular ion’s velocity (  V i /  r)  / ~ (1- 50)% см -1  Gradients of plasma densities (  (n i - n e )/  r)  / ~ (1- 50)% см -1 Equations of motions for particles with Z р ( ,y) in an electric field E ext of cylindrically symmetric trap:

5 Conditions for occurrence of dust instabilities Disperse Instability when the frictional force does not damp the dust oscillations (regular vibrations or random dust fluctuations similar to the Brownian motion) for Z >>  Z fr 2 <  c 2 <  o  / fr   = rot V    (y) F non y(  ) / {m p Z o fr },  o - shift parameter,  c – resonance frequency Dissipative Instability when a restoring force is absent (dust vortexes)  c 4 <  o  fr  Conditions of Occurrence

6 Dust vortices under ground-based conditions in dc- discharge argon, Р ~ Тоrr, iron particles ( a р ~ 3.5  м) 1. Direction of dust rotation is in accordance to theoretical estimation of dust charge gradients 2. Small variations of dust charge < 1-5% см -1 need for formation of these dust rotations in field of gravity in rf - discharge argon, Р ~ Тorr, ( a р ~ 1.4  м) Formation of combined dust oscillations due to variation of plasma parameters

7  o ~ c -1 2  o   F i /m p Z p fr, F i  0.3 m p g,  /Z p ~ % cм -1 Experiment Numerical Simulation «void» Scheme of gas discharge camera Argon P = Pa W= W T e = 1-3eV n i ~ 10 9 см -3 a р = 1.7  м Dust vortices in microgravity conditions (International Space Station, PKE - Nefedov)

8 Random fluctuation of dust charge Two basic reasons:  random nature of currents charging dust particles  stochastic dust motions in spatially inhomogeneous plasma (in presence of dust charge gradients) Random fluctuations of dust charges   fluctuation of interparticles potential ~ Z p (t) 2 ;  fluctuation of electric force ~ Z p (t)E in external electric field Е It leads to stochastic motions of dust particles additionally to their thermal Brownian motions

9 Influence of discrete charging currents on kinetic dust temperature Additional kinetic energy:  f T = e 2  Z p 2 E 2 /( fr m p )  Z p =   1/2 – amplitude and  с =1/ - time of correlations for charge fluctuations in plasma  In gas discharge plasma kinetic energy of macroparticles with radius а р > 10  м can reach  f T ~ 1 eV  f T 0.02 Тоrr  f T, эВ, c -1

10 Stochastic dust oscillations near the electrode of rf- discharge Т р ~ эВ Taking into account of spatial inhomogeneity of bulk plasma in region of stochastic dust motions  Dust charge gradients  y can lead to formation of stochastic dust motions with big kinetic energy   y =dZ p /dy Dependence of oscillation amplitude on pressure for particles : 1– 1  м; 2 – 2.1  м. Additional kinetic energy: Influence of spatial variation of dust charge on kinetic dust temperature

11 CONCLUSIONS The small dust charge gradients due to inhomogeneity of plasma surrounding macroparticles  can lead to the dust vortex formation, and  can influence on the stochastic dust motions in plasma of gas discharges.

12 Experimental Setup for Vertical Vortex Motion Dust vortex in discharge plasma (superposition of 4 frames) Melamine formaldehyde –2.67 μm (Side view)

13 Experimental Setup for Horizontal Vortex Motion Grounded electrode Dust Vortex Powered electrode Grounded electrode Dust Vortex Pin electrode Grounded electrode Pin electrode Dust Vortex Grounded electrode Pin electrode Dust Vortex Side View Top View Video Images of Dust Vortices in Plasma Discharge

14  -Dependency on Pressure Dependency of the rotation frequency  on pressure for vertical (a) and horizontal (b) vortices w с =   /2= F    /{2m p Z o fr }

15 Self-excited oscillation in extreme region

16 Equation of Motion Particle Driving Pad ParticleDispenser Top Ground Electrode Gas Inlet RF Supply 15MHz AC Power DC Power Side Observation Window Top Observation Window where Top View Where is the resonant frequency And is the electric field gradient

17 Radial potential distribution  / ~  (divE)

18 Dependences of critical amplitude and charge gradient

19 Summary The overview of experimental and theoretical investigations of charged gradient induced instabilities were presented. We attribute the observed instabilities to inhomogenaties in the plasma, and show that greater instability of dust structures can be explained by larger space charge gradient. The authors have clearly been developing and promoting this idea for the few years and are making some progress on the experimental and theoretical side.

20 Thanks Everybody !


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