Review of dust particle formation, charging, and transport

1. Review of dust particle formation, charging, and transport John Goree The Univ. of Iowa

2. Outline • Particle detection • Particle formation • Particle charging • Particle transport

3. Outline • Particle detection • video imaging • electron microscopy • other • Particle formation • flaking • gas-phase • Particle charging • e & ion collection • electron emission • Particle transport • Forces: • Coulomb • ion drag • radiation pressure • gas drag • thermophoretic • gravity

4. Particle detection • Particle detection • video imaging • electron microscopy • other

5. Particle detection Particles were always there, but you didn’t know it until you looked for them the right way Images from Gary Selwyn: The original discovery that RF plasmas (etching/deposition) Þparticle growth in the gas phase, and particle levitation electron microscopy Laser light scattering G.S. Selwyn, Plasma Sources Sci. Tehcnol. 3, 340 (1994)

6. Particle detection Other detection methods, used in semiconductor mfg: In the reactor exhaust: • Coulter counter • Measure plasma impedance (dust makes a glow discharge impedance more resistive, less reactive) Coulter counter: particle detection and sizing

7. Formation • Particle formation • flaking • gas-phase

8. Formation: flaking • Flakes: • Thin films • deposited on surfaces by sputtering or evaporation • films subsequently crumble J. Winter, Phys. Plasmas 7, 3862 (2000)

9. Formation: tubules • Tubules: • Reported in tokamak T-10, • Growth mechanism? B.N. Kolbasov et al., Phys. Letta A 269, 363 (2000)

10. Formation: gas phase • Gas-phase formation in astrophysics: • Vapor flowing outward from a carbon star cools & nucleates, resulting in dust. • Dust grains then grow by “coagulation” M16 pillar, Credit: NASA, HST, J. Hester & P. Scowen (ASU)

11. Formation: gas phase • Gas-phase formation G. Praburam and J. Goree Cosmic dust synthesis by accretion and coagulation Astrophys. J 1995

12. Formation: gas phase • Cauliflower particles grow in the gas phase: intact fractured Gary Selwyn, IBM, 1989 Ganguly et al., J. Vac. Sci. Technol. 1993

13. Formation: gas phase Explanation proposed for cauliflower shape: The origin of the bumpy shape has been attributed to columnar growth. If true, column size will depend on temperature J.A. Thornton, J. Vac. Sci. Technol. A 11, 666 (1974). columnar growth, for thin films on a planar surface, using sputter deposition

14. Formation: gas phase • Gas-phase formation resulting from sputtering: • Targets were sputtered by Ar+ in a glow discharge • Particles grew in the gas phase D. Samsonov and J. Goree Particle growth in a sputtering discharge J. Vac. Sci. Technol. A 1999

15. Formation: gas phase • Gas-phase formation resulting from sputtering: Growth of carbon particles, from sputtering graphite in an rf discharge D. Samsonov and J. Goree Particle growth in a sputtering discharge J. Vac. Sci. Technol. A 1999

16. Formation: gas phase Particles grown by sputtering tungsten D. Samsonov and J. Goree Particle growth in a sputtering discharge J. Vac. Sci. Technol. A 1999

17. Formation: gas phase Particles grown by sputtering graphite D. Samsonov and J. Goree Particle growth in a sputtering discharge J. Vac. Sci. Technol. A 1999

18. Formation: gas phase Particles grown by sputtering titanium Spherical-shaped primary particles that have coagulated into aggregates consisting of a few spheres. The surface of the particles appears smoother than that of the graphite. D. Samsonov and J. Goree Particle growth in a sputtering discharge J. Vac. Sci. Technol. A 1999

19. Formation: gas phase Particles grown by sputtering stainless steel D. Samsonov and J. Goree Particle growth in a sputtering discharge J. Vac. Sci. Technol. A 1999

20. Formation: gas phase Particles grown by sputtering aluminum D. Samsonov and J. Goree Particle growth in a sputtering discharge J. Vac. Sci. Technol. A 1999

21. Formation: gas phase Particles grown by sputtering copper D. Samsonov and J. Goree Particle growth in a sputtering discharge J. Vac. Sci. Technol. A 1999

22. Formation: gas phase Particles grown by sputtering Growth rate varies tremendously, depending on the material D. Samsonov and J. Goree Particle growth in a sputtering discharge J. Vac. Sci. Technol. A 1999

23. Charging • Particle charging • e & ion collection • electron emission

24. Charging: mechanisms Ielectron collection + Iion collection + Ielectron emission Ielectron collection + Iion collection H+ H+ e- e- + _ e- Charging by collecting electrons and ions only ÞQ < 0 • Electron emission • secondary emission due to e- impact • photoemission • thermionic • ÞQ > 0 Goree, Plasma Sources Sci. Technol. 1994

25. Charging: mechanisms • Particles immersed in a plasma are in charge equilibrium: • Itotal = Ielectron collection + Iion collection + Ielectronemission • Each of these currents depends on the potential V of the particle • Equilibrium: at a “floating potential” V, the currents balance: Itotal = 0 • Q = CV • C = 4pe0a is capacitance of sphere of radius a • Example: Q = 695 efor a = 1mm particle with a surface potential V = 1Volt Goree, Plasma Sources Sci. Technol. 1994

26. Charging: mechanisms Ielectron collection + Iion collection H+ e- _ • Charging by collecting electrons & ions only • Consider a particle that is initially uncharged & is suddenly immersed in a plasma: • Initially it collects electrons more rapidly than ions, due to higher vte • Eventually it reaches equilibrium “floating potential” • Popular model: OML (Orbital motion limited currents), yields a value for Q accurate to a factor of ~2. • V = -2.5 kTe (Hydrogen, Te = Te , for an isolated particle) Goree, Plasma Sources Sci. Technol. 1994

27. Charging: OML predictions • OML charge • Charge µ Te • Charge µ a KQ = -1737 for hydrogen, Te = Ti Goree, Plasma Sources Sci. Technol. 1994

28. Charging: electron depletion • Electron depletion • When the density N of negatively-charged dust is high: • Dust potential is reduced • Dust charge is reduced • Plasma potential is altered Goree, Plasma Sources Sci. Technol. 1994

29. Charging: secondary emission • Secondary electron emission (electron impact) • For mono-energetic electrons: • Yield d • Graphite in bulk: • dm = 1 • Em = 400 eV • For small particles, yield isbigger than for bulk, because of bigger solid angles for secondary electrons to escape particle Goree, Plasma Sources Sci. Technol. 1994

30. Charging : secondary emission Ielectron collection + Iion collection + Ielectron emission H+ e- + e- • Secondary electron emission (electron impact) • For Maxwellian electrons: • Meyer-Vernet* provides formulae for electron current, result: • Polarity of particle’s charge switches from negative to positive • Occurs for Te in range 1 – 10 eV, depending on dm • Other electron emission processes: • photoemission due to UV (very common in space) • thermionic emission (uncommon?) *Meyer-Vernet, Astron. Astrophys. 105,98 (1982)

31. Charging: charging time • Charging time • A particle’s charge: • Can change at a finite rate, as plasma conditions change • Fluctuates randomly as individual electrons & ions are collected • Characteristic time scale is called “charging time, can be defined as: • charge / current of one of the two incident species“floating potential V Kt= -1510 sec for hydrogen, Te = Ti • Typically t»1 msec for a 1 micron grain in a glow discharge Goree, Plasma Sources Sci. Technol. 1994

32. Charging: stochastic fluctuations • Charge fluctuations • Stochastic, due to collection of individual electrons and ions at random times • dQ » 0.5 (Q/e)1/2 Goree, Plasma Sources Sci. Technol. 1994

33. Transport • Particle transport • Forces: • Coulomb • ion drag • radiation pressure • gas drag • thermophoretic • gravity

34. Transport: forces Forces acting on a particle CoulombQEµa Lorentz Q v´ B µa ¬ tiny except in astronomy Ion drag µ a2¬ big for high-density plasmas Radiation pressure µ a2¬ if a laser beam hits particle Gas drag µa2¬ requires gas Thermophoretic force µa2¬ requires gas Gravity µa3¬ tiny unless a > 0.1 mm

35. Transport: ion drag Momentum is imparted to the dust particle _ _ Orbit force: Ion orbit is deflected Collection force: Ion strikes particle

36. Transport: ion drag Voids in particle suspensions are experimental evidence of the ion drag force Glow (image of Ar I spectral line) RF parallel-plate discharge, imaged from the side, zero-g conditions (NASA’s KC-135 airplane) Particles (image of scattered laser light

37. Transport: ion drag • Gas-phase formation resulting from graphite sputtering: • “filamentary-mode” instability, driven by ion drag, for nm size particles • Dust (laser light scattering from a horizontal laser sheet) • Glow D. Samsonov and J. Goree Instabilities in a Dusty Plasma with Ion Drag and Ionization Physical Review E Vol. 59, 1047-1058, 1999

38. Transport: ion drag • Gas-phase formation resulting from graphite sputtering: • “great void” instability, driven by ion drag, for nm size particles • Dust (laser light scattering from a horizontal laser sheet) • Glow D. Samsonov and J. Goree Instabilities in a Dusty Plasma with Ion Drag and Ionization Physical Review E Vol. 59, 1047-1058, 1999

39. Transport: ion drag • Ion drag force J. Goree, G. E. Morfill, V. N. Tsytovich and S. V. Vladimirov, Theory of Dust Voids in Plasmas, Physical Review E Vol. 59, 7055-7067, 1999

40. Transport: ion drag Collection force from OML model Orbit force from Rosenbluth potential • Ion drag force • Two contributions: • Orbit force (this is the usual drag force for Coulomb collisions, except that lnL is problematic) • Collection force (ions actually strike the particle) • Depends on ion velocity ui • Force µni µ V µ V2 µ V-2 Te / Ti = 60, mi = 40 amu, lD = 130 mm Ion drag is normalized by 4 p ni a2 Te / (Ti/Te)0.5 J. Goree, G. E. Morfill, V. N. Tsytovich and S. V. Vladimirov, Theory of Dust Voids in Plasmas, Physical Review E Vol. 59, 7055-7067, 1999 E. C. Whipple, Rep. Prog. Phys. 44, 1198 (1981)

41. Transport: ion drag • Ion drag force • Fusion edge plasma parameters: • Te = Ti, deuterium mass Te / Ti = 1, mi = 2 amu, lD = 13 mm Ion drag is normalized by 4 p ni a2 Te / (Ti/Te)0.5 Data computed March 2005 using the same code as in J. Goree, G. E. Morfill, V. N. Tsytovich and S. V. Vladimirov, Theory of Dust Voids in Plasmas, Physical Review E Vol. 59, 7055-7067, 1999

42. Transport: ion drag • Gettering particles: • Design your ion flow so that it pushes particles where you want them to go

43. Transport: radiation pressure transparent microsphere incident laser momentum imparted to microsphere Radiation pressure • Radiation pressure force µa2 • q dimensionless • (q = 1 if all photons are absorbed) • Ilaserlaser intensity • c speed of light • B. Liu, V. Nosenko, J. Goree and L. Boufendi, Phys. Plasmas (2003).

44. Transport: radiation pressure Next: Experimental demonstration of the force due to radiation pressure

45. Transport: radiation pressure • two Ar+ laser beams: • 0.61 mm width • rastered into vertical sheets

46. Transport: radiation pressure undisturbed monolayer Ar+ laser pushes particles Transport: radiation pressure medium power: plastic deformation, flow highpower: melting the lattice low power: slow deformation, rotation

47. Transport: radiation pressure • Gettering particles: • Not very practical to use radiation pressure force; it requires intense cw laser light • High-power pulsed lasers (~ 1J YAG lasers) have been used to explode dust particles suspended in a plasma. This isn’t the radiation pressure force, but it works. Maybe not very practical.

48. Transport: gas drag Gas drag • Gas drag force µa2 • Epstein expression: • Ngasgas atom: number density • mgasmass • cgasmean thermal speed • V velocity of particle with respect to the gas • d dimensionless, ranges from 1.0 to 1.442 • P. Epstein, Phys. Rev. 23, 710 (1924). • M. J. Baines, I. P. Williams, and A. S. Asebiomo, Mon. Not. R. Astron. Soc. 130, 63 (1965).

49. Transport: gas drag Demonstration of gas drag • Track the motion of a single microsphere in a glow discharge • Particle is levitated by a sheath electric field • Sheath is curved Þ harmonic confining potential • Apply a laser beam, and then turn the laser • Observe damped harmonic motion B. Liu, V. Nosenko, J. Goree and L. Boufendi, Phys. Plasmas (2003).

50. Transport: gas drag Gas drag demonstration: • Track the motion of a single microsphere in a glow discharge • Particle is levitated by a sheath electric field • Sheath is curved Þ harmonic confining potential • Apply a laser beam to push the particle away from the bottom of the harmonic well, and then turn the laser off • Observe damped harmonic motion B. Liu, V. Nosenko, J. Goree and L. Boufendi, Phys. Plasmas (2003).