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Introduction to Dusty Plasmas

Introduction to Dusty Plasmas
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Dr.AlexanderTyler,India,Teacher
Published Date:21-07-2017
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Introduction to Dusty Plasmas André Melzer Institute of Physics, Ernst-Moritz-Arndt-Universität Greifswald Germany Extended Lecture Notes see: www5.physik.uni-greifswald.deDusty (Complex, Colloidal) Plasmas Dynamics and transport in plasmas Coulomb crystallization of trapped particles Dusty Plasmas = M Miic cr ro os sc co op piic c p pa ar rt tiic clle es s iin n a a gaseous plasma environment Selwyn 1991 … Etching Deposition and… AstrophysicsDusty Plasmas in Astrophysics Comet Hale-Bopp Saturn ringsDusty Plasmas in Astrophysics Saturn rings 1981 2005Dusty Plasmas in the Atmosphere Noctilucent cloudsDusty Plasmas in Technology Plasma etchingDusty Plasmas in Technology Selwyn 1991 „Killer particle“Dusty Plasmas in the LaboratoryDusty Plasmas under MicrogravityContents of Lecture • Dust charging • Forces • Strongly coupled systems, particle-particle and particle-plasma interaction • Waves • Finite systems and normal modes For an extended introduction see: www5.physik.uni-greifswald.deCharging plasma Q Q 0 0 ions electrons In typical discharges: Particle will be charges negatively due to higher mobility of electronsOML currents Probe theory of Langmuir and Mott-Smith 1929 Ion current     8kT eΦ Φ Φ Φ 2 i   I = = = = π π π πa n e 1 − − − − i i     π π π πm kT i  i  increased collection cross section geometry thermal velocities density reduction Boltzmann factor   8kT eΦ Φ Φ Φ 2 e I = = π πa n e exp  = = π π e e electron     π π π πm kT e  i  and Electron current ion currentsParticles as floating probes I (Φ Φ Φ Φ ) = = = = 0 ∑ ∑ ∑ ∑ q fl q  eΦ Φ  eΦ Φ Φ Φ  Φ Φ    T m n fl fl With OML collection e i e     1− − − − = = = = exp       currents only kT T m n kT i i e i  e    Values of eΦ Φ Φ Φ /kT for n =n fl e e i T /T 1 10 20 100 e i -2.50 -1.91 -1.70 -1.24 H Φ Φ Φ Φ ≈ ≈ ≈ ≈ − − − −2kT /e fl e -3.05 -2.39 -2.16 -1.65 He -3.99 -3.24 -2.99 -2.41 ArLimitations of OML description • collisionless (ion) trajectories none of that is • Maxwellian velocity distribution met in „real“ discharges • isotropic Often: ion drift velocity u (much) larger than i ion thermal velocity v SHEATH th,i e ex xa ac ct t   8kT eΦ Φ Φ Φ 2 i solution I = = π πa n e 1 − −  = = π π − − i i   π π π πm kT i  i    approximation   2eΦ Φ Φ Φ 2 I = = = = π π π πa n eu 1 − − − −  i i i 2     m u  i i The capacitance model Particle as a spherical capacitor Q = = C Φ Φ = = Φ Φ fl of capacitance C Capacitance in vacuum C = = = = 4π π π πε ε ε ε a 0 For a particle of a=1µm: 700 e per Volt With the rule-of-thumb approximation: Φ Φ Φ Φ=2kT /e e Q = = = = 1400a T μ μ μ μm e,eVOther charging currents Photoelectron emission Particle can become (UV radiation) positively charged electrons hν ν ν ν 2 2 S Se ec co on nd da ar ry y e elle ec ct tr ro on n e em miis ss siio on n II = = = = = = = = π π π π π π π πa a e eμ μ μ μ μ μ μ μ Γ Γ Γ Γ Γ Γ Γ Γ Φ Φ Φ Φ Φ Φ Φ Φ 0 0 ν ν ν ν fl ν ν ν ν 2 I = = = = π π π πa eμ μ μ μ Γ Γ Γ Γ exp(− − − −eΦ Φ Φ Φ /kT ) Φ Φ Φ Φ 0 ν ν ν ν fl p fl ν ν ν ν electrons   E E e e   δ δ δ δ(E ) = = = = 7.4δ δ δ δ exp − − − − e m   E E m m  Charging time scale kT 1 i τ τ τ τ = = = = RC τ τ = = 4π πε ε a τ τ = = π πε ε i 0 2 e π π π πa en v i th,i Time constant for charging of a capacitor 1/I C U Smaller particles 1 τ τ τ τ ∝ ∝ ∝ ∝ i are charged slower a 1 µs Plasma timeSummary Charging • Micrometer sized particles carry 3 4 10 to 10 elementary charges • Charging time: microseconds: Charge in dynamical equilibrium • • C Ch ha ar rg ge e t to o m ma as ss s r ra at tiio o Q Q/ /m m e ex xt tr re em me elly y s sm ma allll: : slow timescalesForces on dust particles • Gravity • Electric field force • Thermophoresis • Ion Drag • Neutral DragGravity r r v 4 3 F = = = = mg = = = = π π π πρ ρ ρ ρa g 3 What else needs to be said ?