Waves and Instabilities in Dusty Plasmas

Transcription

1 15 th Topical Conference on RF Power in Plasmas May 20 22, 2003 Jackson Hole, Wyoming Waves and Instabilities in Dusty Plasmas Bob Merlino University of Iowa

2 Outline What is a dusty plasma? Where are dusty plasmas? Charging of dust particles Waves in dusty plasmas

3 Dusty Plasmas Dust represents much of the solid matter in the universe and this component often coexists with the ionized matter forming a dusty plasma. Dust is often present in laboratory plasmas as well either by choice or circumstance.

4 What is a dusty plasma? plasma = electrons + ions - + Plasma small particle of solid matter absorbs electrons and ions becomes negatively charged Debye shielding D + + -

5 Importance of Charged Dust the dust acquires an electrical charge and thus is subject to electromagnetic as well as gravitational forces the charged dust particles participate in the collective plasma processes

6 DUSTY PLASMAS Natural 1. Solar nebula 2. planetary rings 3. interstellar medium 4. comet tails 5. noctilucent clouds 6. Lightning 7. snow Man-made 1. Microelectronic processing 2. rocket exhaust 3. flames 4. fusion devices 5. H bomb

7 Rosette Nebula Our solar system accumulated out of a dense cloud of gas and dust, forming everything that is now part of our world.

8 A flame is a very weakly ionized plasma that contains soot particles. An early temperature measurement in a dusty plasma.

9 Comet Hale-Bopp

10 Spokes in Saturn s B Ring Voyager 2 Nov Cassini- Huygens July 2004

11 Semiconductor Processing System MHz gas dust qe mg E substrate PUMP silane (SiH 4 ) + Ar + O 2 SiO 2 particles

12 Semiconductor Manufacturing Si dust

13 Physics Today August 1994

14 Dusty Plasma DUST

15 Dust Charging Processes electron and ion collection secondary emission UV induced photoelectron emission Total current to a grain = 0 Σ I = I e + I i + I sec + I pe = 0

16 The Charge on a Dust Grain In typical lab plasmas I sec = I pe = 0 Electron thermal speed >> ion thermal speed so the grains charge to a negative potential V S relative to the plasma, until the condition I e = I i is achieved. kte evs Ie = ene exp π a me kte kti evs Ii = eni 1 π a mi kti 2 2 electron repulsion ion enhancement a Q = (4πε o a) V S

17 Typical Lab Plasma For T e = T i = T in a hydrogen plasma V S = 2.5 (kt/e) If T 1 ev and a = 1 µm, Q 2000 e Mass: m m p

18 Dust Charge Measurements 2 Walch, Horanyi, & Robertson, Phys. Rev. Lett. 75, 838 (1995) Diameter (micron) Graphite Glass Electron Energy (ev)

19 Waves in dusty plasmas electrostatic dust ion-cyclotron waves (EDIC) dust ion acoustic waves (DIA) dust ion acoustic shocks (DIAS) dust acoustic waves (DA) Dust cyclotron mode Strongly coupled dusty plasmas

20 Effect of dust on plasma waves the presence of dust modifies the characteristics of the usual plasma modes, even at frequencies where the dust does not participate in the wave motion the dust provides an immobile charge neutralizing background n = n + Z n i e d d

21 Dust Modes new, low frequency (~ few Hz) modes in which the dust grains participate in the wave motion appear in the dispersion relations the dust dynamics can be observed visually since the dust motion can be imaged and recorded on tape

22 Quasineutrality in dusty plasmas For low frequency waves the condition holds in both zero and first order ni = ne + Zdnd defining: ε = n n do io we characterize the dusty plasma using the quantity εz d which is the fraction of negative charge on the dust grains

23 Fluid theory of Low frequency electrostatic waves in dusty plasmas Three component plasma: electrons, ions, negative dust nα I. + ( nαvα) = 0 t vα II. nαmα + nαmα( vα ) vα + qαnα ϕ t q n ( v B) = 0 α α α. i = e + d d III n n Z n

24 New Phenomena in Dusty Plasmas Unlike ordinary plasma, or plasmas containing negative ions, the charge on a dust grain is not constant, but fluctuates with the local plasma potential. This leads to new damping effects and new mechanisms for wave growth.

25 Fluid theory: mode frequencies for ion and electron modes we treat the dust as an immobile negative background for dust modes we can neglect the electron and ion inertia terms For excitation conditions (growth rates, critical drifts, etc.) we must use kinetic theory

26 Dust Ion Acoustic Mode DIA: ion-acoustic wave modified by dust v p Dispersion relation: ω kt kt = = + K m m Z i e i i 1 d ( ε ) 1 2 v p = 0 1 εζ C DIA

27 DIA Kinetic Theory Dust acoustic waves are normally heavily Landau damped in a plasma with T e = T i. However the presence of negatively charged dust can drastically reduce the damping T e δ π me Te 2T i γ = ωr δ δ e 8 + mi T i 1 δ = 1 = εz d - γ εz

28 Dust Ion Acoustic Wave Experiment Ta Hot Plate LANGMUIRPROBE rotating dust dispenser K oven end plate probe 1.5 DIA 1.0 DIA PHASE VELOCITY phase speed (a) ε Z d k i /k r Ki/K r (b) ε Z d

29 DIA - Conclusion Ion acoustic waves which would otherwise not propagate in a plasma with T e = T i can propagate in a plasma with a sufficient amount of negatively charged dust. In the presence of negative dust, the wave phase velocity increases, decreasing the effect of ion Landau damping.

30 Experimental setup Rotating Dust Dispenser HP Cs B EP G LP (a) PG n (b) z

31 DIA Shocks results

32 DIA Shocks results

33 EDIC: fluid theory Electrostatic ion-cyclotron waves excited by electron current along the magnetic field Propagate at large angle to B ω 2 kti kt =Ω + K e + m m (1 εz 2 2 ci =Ω ci K CDIA i i d

34 Electrostatic dust ion-cycloton instability (EDIC)

35 EDIC- kinetic theory results EIC instability driven by current along B As more negative charge is carried by the dust, the critical drift needed to excite the instability decreases the instability is easier to excite in a dusty plasma v de /v eth δ = 1/(1 εz d ) V. W. Chow & M. Rosenberg, Planet. Space Sci. 44, 465 (1996)

36 Dust acoustic waves Dust dynamics Electrons & Ions nd ( ndvd) + = 0 t x vd vd nd ϕ mn d d + vd ktd ezn d d 0 t x + = x x ne ϕ n+ ϕ kte ene = 0; kt+ + en+ = 0 x x x x Quasineutrality n = n + Z n + e d d

37 Combining the dust momentum equation with the plasma equations we see that (for the case of cold dust, T d =0). vd d d e mn = ( p+ p+ ) x x where p e + p + is the total pressure due to electrons and ions. In the dust acoustic wave the inertial is provided by the massive dust particles and the electrons and ions provide the restoring force

38 DA Dispersion relation Monochromatic plane wave solutions for T e = T i = T kt fλ = CDA = Zd md 1 1+ ε ε where ε = n do /n +o dust mass

39 DUST IN A GLOW DISCHARGE QE Anode Glow Plasma B E Anode N 2 mg Dust Tray Vacuum vessel Levitated Grains Dust: kaolin (aluminum silicate) PS +

40 Dust Acoustic Wave Image wavefronts DA Movie

41 Dust Acoustic Wave Dispersion Relation K (cm -1 ) theory ω (s -1 )

42 Electrostatic dust cyclotron mode EDIC involves cyclotron motion of the dust magnetized dust Dispersion relation: ω =Ω + K kt + εz d 2 cd d md + Ti Te ε Zd =Ω + KC cd DA 1 1 ( / )(1 )

43 Gyroradius of dust particles r d = mv d d d ez B kt 3 d md a, Zd a, vd = m d rd a 1 2

44 Gyroradius of dust particles 10 1 [Td=0.025 ev] [Td=1.0 ev] [Td = 0.1 ev] 0.1 B = 800 G dust radius (microns)

45 Solid state dusty plasmas In a typical plasma 2 ez 2 4πε od Γ = << 1 kt d In a dusty plasma the interaction energy is 2 multiplied by Z d which can be very large, so that Γ > 1 is possible The dust grains may then arrange themselves in a regular lattice.

46 Coulomb Crystal John Goree Univ. Iowa triangular lattice with hexagonal symmetry

47 Waves in strongly coupled dusty plasmas The presence of short scale correlations gives rise to novel modifications of the collective behavior Both compressional and transverse shear waves are possible

48 Compressional and shear waves

49 Summary/Conclusions Dusty plasmas are not uncommon in the lab and are ubiquitous in the Universe Presence of dust modifies both the excitation and propagation of plasma waves New, very low frequency dust modes Collective fluctuations in dusty plasmas may provide mechanism for structuring