Dynamic Phenomena in Complex Plasmas

Transcription

1 The University of Sydney Dynamic Phenomena in Complex Plasmas N.F. Cramer, S.V. Vladimirov, A.A. Samarian and B.W. James School of Physics, University of Sydney, Australia

2 Dusty Plasmas at the University of Sydney N. Cramer, S. Vladimirov, S. Maiorov (Theoretical Physics): Theory of Laboratory and Astrophysical Dusty Plasmas B. James, A. Samarian, F. Cheung, W. Tsang (Applied and Plasma Physics): Dusty Plasma Experiments M. Wardle (Research Centre for Theoretical Astrophysics): Charged Dust in Interstellar Clouds Collaborators: N. Prior, O. Vaulina, O. Ishihara, V. Tsytovich, F. Verheest, J. Sakai, M. Hellberg.

3 Dynamic Phenomena: Self-excited motions Oscillations Waves Vortex motions Rotation of Fine Dust Clusters in Axial Magnetic Field Dust Grains as Diagnostic Tool for Sheath Measurement in RF-Discharge Plasma Laser Excited Oscillations in Vertically Aligned Structures

4 Dynamics of Single Particle Potential energy of a dust grain with variable (solid lines) and constant charge (dashed lines) in the plasma sheath

5 Charging dynamics of the macroparticle of m g =10 5 m p with and without an ion flow. The time step is τ= s. Total simulation time is t 0 =190 and τ =6.5 x 10-8 s. The asymptotic charge is (a) Z =842 in the absence of the ion flow, M 2 =0 (b) Z =1067 for M 2 =0.6 and (c) Z =1146 for M 2 =2.4

6 Contour plots of the ion density, for three values of the speed of the ion flow (one is subsonic with M 2 =0.6, and two supersonic, with M 2 =1.2 and M 2 =2.4). A strong ion focus is formed at the distance of a fraction of the electron Debye length behind the dust grain

7 Dynamics of Few Particles Instabilities of Dust Particle Arrangements (Presentation of S Vladimirov et al.) Rotation of Dust Coulomb Clusters in Axial Magnetic Field (Presentation of Cheung et al.) Laser Excited Oscillations in Vertically Aligned Structures (Poster of Prior et al.)

8 AD Laser Driven Oscillations of Few Particles Structures (Poster of Prior et al.)

9 Instability of a string of 3 particles (paper of Vladimirov) AD

10 AD Rotational motion

11 Dynamics of Many Interacting Particles Various kinds of dust grain self-excited motion have been observed : Vertical oscillations in mono-layer dust structure, Complex wave motions in multi-layer structures, Vortex motion caused by an introduction of an additional electrode, Rotation and oscillation in non symmetrical electrode configurations.

12 Vibrational Modes of dust grain arrays Vibrations in simple versions of lattices of dust grains embedded in the sheath region near a horizontal electrode. Understanding the modes provide useful diagnostics and aid in analysing critical phenomena and phase transitions in such systems Horizontal vibrations of dust grains within one layer lead to acoustic-type modes. Vertical vibrations of dust grains in the layer lead to optical-mode-like dispersive waves (Vladimirov, Shevchenko, and Cramer, )

13 Vibrations of a one-dimensional horizontal chain of grains of equal masses M and constant charge Q

14 Vertical Oscillation For a mono-layer structure, the dust particles begin to oscillate spontaneously in the vertical direction when the pressure is decreased below a critical value. 30 mtorr and 100 W 30 mtorr and 35 W 30 mtorr and 15 W The amplitude of the oscillation is several millimetres and the frequency is greater than 10Hz. When the rf input power is decreased, the amplitude increases. For pressures below 35mTorr, the amplitude increases dramatically. This increase is greater for lower rf powers.

15 Vertical Oscillation Amp (mm) 1 0,8 0,6 0,4 Carbon Particles (2.1±0.1µm in diameter) P=20 W P=35 W P=50 W P=65 W P=80 W 0,2 P=100 W Pressure (mtorr) P=120 W

16 Second, consider two vertically ordered one-dimensional horizontal chains of grains with constant charges ion flow negatively charged electrode

17 The effect of the wake behind each grain in the Mach cone (Vladimirov and Nambu, 1995; Vladimirov and Ishihara ) > ion > flow > to the > electrode > 15 ρ λ D 0 M= Z/λ D -25 e l e c t r o d e

18 Modes of vibrations There are two modes of oscillations: ω 2 1 = γ 0 M 4Q2 1 + r 0 3 Mr 0 λ D exp r 0 λ D sin 2 kr 0 2 ω 2 2 = γ 0 +γ 1 γ 2 M 4Q2 3 Mr 1 + r 0 0 λ D exp r 0 λ D sin 2 kr 0 2

19 Modes of a chain of rod-like particles

20 Longitudinal compressive waves in 3-D structure (side) 1sec 2sec 3sec d p = 6.13 µm, P= 60 W, p= 30 mtorr We observed that density waves which travel downwards with a wavelength l=3mm and a period T=4x10-2 s, were generated by decreasing the input power or pressure, and by increasing the number of dust particles in the structure

21 Heartbeat Oscillation 0 & Surface Waves Wavelength λ=6mm Velocity v=1.5ms Peaks 1sec 2sec 0.02sec 0.04sec 0.06sec 0.08sec

22 Surface Waves AD Wavelength λ=6mm Velocity v=1.5ms -1

23 AD Void surface waves

24 AD Rotational Motion

25 Illustration of Dust Vortex (Top view) Grounded electrode Grounded electrode Pin electrode Pin Electrode Dust Vortex Dust Vortex Pin electrode Powered electrode Side View Top View

26 AD Vortex Motion