1. Dusty plasmas in basic science, astronomy, industry & fusion John Goree The Univ. of Iowa

2. The growth of dusty plasmas as a field of research

3. Outline 1.What is dust? 2.Formation of dust Fusion Industry Astronomy 3.Dust charge 4.Forces acting on dust 5.Some physics experiments: Voids under microgravity conditions Strongly-coupled vs. Weakly-coupled Plasmas Waves & Instabilities Shear flow Wakes

4. What is dust? “Dust” = small particles of solid matter, 10 nm – 1 mm, usually dielectric Astronomy: “dust” M16 pillar Credit: NASA, HST, J. Hester & P. Scowen (ASU) Semiconductor industry “particulates” or “particles” G.S. Selwyn, Plasma Sources Sci. Tehcnol. 3, 340 (1994)

5. Safety Issues for fusion Radiological Dust: activated retains tritium ITER safety limit: 350 kg Tungsten dust Fire & chemical explosion Hydrogen: stored in dust released during accidental exposure to: air steam ITER safety limit: 6 kg dust allowed on hot surfaces Phil Sharpe Fusion Safety Program, Idaho National Laboratory Dust in Fusion Plasmas Workshop 2005 What is dust?

6. Formation of dust 1.What is dust? 2.Formation of dust Fusion Industry Astronomy 3.Dust charge 4.Forces acting on dust 5.Some physics experiments: Voids under microgravity conditions Strongly-coupled vs. Weakly-coupled Plasmas Waves & Instabilities Shear flow Wakes

7. Formation Produced in the gas phase Nucleation Coagulation Purchased from vendor What’s the source of dust in a plasma? Produced on surfaces Flaking of deposited films Bubbles & blistering of surfaces

8. Fusion: various shapes of dust collected from the Tore Supra tokamak Phil Sharpe Fusion Safety Program, Idaho National Laboratory Dust in Fusion Plasmas Workshop 2005 Formation: tokamaks Composition is mainly: carbon constituents of stainless steel

9. Formation: tokamaks Tungsten dust formation: flaking from He bubbles N. Ohno, S. Takamura, Dai. Nishijima “Formation and Transport of Dust in the Divertor Plasma Simulators” Dust in Fusion Plasmas Workshop 2005 Divertor Plasma Simulator NAGDIS-II 2  m Surface Temp.: 2200 K Flux: 8.3×10 22 m -2 s -1 Ion Energy:15 eV Time: 10 4 s

10. Dust Poloidal Limiter High Z dust is emitted from the Mo poloidal limiter. Observation of High Z Dust in TRIAM-1M by Fast Framing Camera, 4500 fps Formation: tokamaks N. Ohno, S. Takamura, Dai. Nishijima “Formation and Transport of Dust in the Divertor Plasma Simulators” Dust in Fusion Plasmas Workshop 2005

11. A lesson from the semiconductor industry Particles were always there, but you didn’t know it until you used the right diagnostics: G.S. Selwyn, Plasma Sources Sci. Tehcnol. 3, 340 (1994) camera imaging in-situ electron microscopy ex-situ

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

13. Formation: gas phase Gas-phase formation G. Praburam and J. Goree Astrophys. J 1995

14. Formation: gas phase Cauliflower particles grow in the gas phase: Gary Selwyn, IBM, 1989 Ganguly et al., J. Vac. Sci. Technol. 1993 intactfractured

15. Formation: gas phase Particles grown by sputtering tungsten D. Samsonov and J. Goree J. Vac. Sci. Technol. A 1999 300 nm Coagulated particles consisting of 3+ cauliflowers

16. Formation: gas phase Particles grown by sputtering graphite D. Samsonov and J. Goree J. Vac. Sci. Technol. A 1999

17. Formation: gas phase Particles grown by sputtering aluminum D. Samsonov and J. Goree J. Vac. Sci. Technol. A 1999

18. Polymer microspheres: melamine-formaldehyde diameter 8.09  0.18  m used in basic science experiments introduced into plasma with a “salt shaker” Formation: purchased from vendor

19. Outline 1.What is dust? 2.Formation of dust Fusion Industry Astronomy 3.Dust charge 4.Forces acting on dust 5.Some physics experiments: Voids under microgravity conditions Strongly-coupled vs. Weakly-coupled Plasmas Waves & Instabilities Shear flow Wakes

20. Charging: mechanisms Charging by collecting electrons and ions only  negative charge I electron collection + I ion collection H+H+ e-e- _ Goree, Plasma Sources Sci. Technol. 1994 I electron collection + I ion collection + I electron emission H+H+ e-e- e-e- + Electron emission secondary emission due to e - impact photoemission thermionic  positive charge

21. Charging: mechanisms Particles immersed in a plasma collect currents: I total = I electron collection + I ion collection + I electron emission Each of these currents depends on the potential V of the particle Goree, Plasma Sources Sci. Technol. 1994 Equilibrium: I total = 0 at the “floating potential” V: Q = CV C =      a is capacitance of sphere of radius a H+H+ e-e- e-e- a surface potential V

22. Charging: mechanisms Charging by collecting electrons & ions only Consider a particle that is suddenly exposed to plasma: Initially it collects electrons more rapidly than ions, due to higher v te Eventually it reaches equilibrium “floating potential”: Hydrogen, T i = T e V = -2.5 kT e Example: Parameters: T e = 1 eV a = 1  m Charge: Q = - 1737 e Goree, Plasma Sources Sci. Technol. 1994 I electron collection + I ion collection H+H+ e-e- _

23. Forces 1.What is dust? 2.Formation of dust Fusion Industry Astronomy Pure physics 3.Dust charge 4.Forces acting on dust 5.Some pure physics experiments: Strongly-coupled vs. Weakly-coupled Plasmas Waves & Instabilities Shear flow Wakes

24. Forces Forces acting on a particle Ion drag  a 2  big for high-density plasmas Radiation pressure  a 2  if a laser beam hits particle Gas drag  a 2  requires gas Thermophoretic force  a 2  requires gas Coulomb QE  a  provides levitation Lorentz Q v  B  a  tiny except in astronomy Gravity  a 3  tiny unless a > 0.1  m

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

26. Void is due to ion drag D. Samsonov and J. Goree Instabilities in a Dusty Plasma with Ion Drag and Ionization Physical Review E Vol. 59, 1047-1058, 1999 Dust (laser light scattering from a horizontal laser sheet) Glow Ion drag force Plasma: RF parallel-plate glow discharge argon gas Dust: nm size carbon grown by sputtering graphite target

27. Void is due to ion drag D. Samsonov and J. Goree Instabilities in a Dusty Plasma with Ion Drag and Ionization Physical Review E Vol. 59, 1047-1058, 1999 Dust (laser light scattering from a horizontal laser sheet) Glow Ion drag force Plasma: RF parallel-plate glow discharge argon gas Dust: nm size carbon grown by sputtering graphite target

28. How ion drag produces a void: 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 Ion drag force Ionization source Positive plasma potl Outward ion flow dustvoid

29. 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 E. C. Whipple, Rep. Prog. Phys. 44, 1198 (1981) Two contributions: Orbit force (this is the usual drag force for Coulomb collisions, except that ln  is problematic) Collection force (ions actually strike the particle) Depends on ion velocity u i Force  n i Orbit force from Rosenbluth potential Collection force from OML model T e / T i = 60, mi = 40 amu, D = 130  m Ion drag is normalized by 4  n i a 2 T e / (T i /T e ) 0.5  V -2  V  V 2 Ion drag force

30. 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 Fusion edge plasma parameters: T e = T i, deuterium mass T e / T i = 1, m i = 2 amu, D = 13  m Ion drag is normalized by 4  n i a 2 T e / (T i /T e ) 0.5 Ion drag force

31. Gas drag molecular-flow regime Epstein: N gas gas atom: number density m gas mass c gas mean thermal speed V velocity of particle with respect to the gas  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). Gas drag force Stokes-flow regime

32. Radiation pressure force Radiation pressure B. Liu, V. Nosenko, J. Goree and L. Boufendi, Phys. Plasmas (2003). incident laser momentum imparted to microsphere transparent microsphere

33. Physics experiments 1.What is dust? 2.Formation of dust Fusion Industry Astronomy Pure physics 3.Dust charge 4.Forces acting on dust 5.Some physics experiments: Microgravity conditions Strongly-coupled vs. Weakly-coupled Plasmas Waves & Instabilities Shear flow Wakes

34. Physics experiments Remainder of this talk: All experiments performed with polymer microspheres

35. Equipotential Contours (RF glow discharge) electrode positive potential electrode With gravity, particles sediment to high- field region  2-D layer Without gravity, particles fill 3-D volume QE mg Microgravity conditions

36. To obtain a 3D dust suspension, use zero g conditions: Parabolic flights, NASA KC-135

37. Microgravity conditions

38. Parabolic flights, NASA KC-135 video Microgravity conditions

39. “strongly coupled” dusty plasma:Q big star interior: r small pure-ion plasma:T small Strongly-coupled vs. weakly-coupled plasmas  > 1 plasma is like a solid or a liquid  << 1 plasma is like a gas Coulomb coupling parameter:

40. Physics experiments Next: Waves in a weakly-coupled dusty plasma

41. Parameter: gas: Argon p = 1.0.. 2.5 Pa n i = 10 15 m -3 B = 20.. 80 mT dust: MF-spheres d = 1 µm n d = 0.5.. 3 x 10 11 m -3 particles anodic plasma anode 3 cm RF-discharge camera dust tray plasma columnprobes Anode: U A = 50.. 100 V I A = 3.. 12 mA Dusty Plasma Research, A. Piel, 2005 41 Courtesy Alexander Piel, Kiel University, Germany, 2005 Dust acoustic wave experiment: Kiel Univ.

42. 15 mm Time lapse 1:10 p = 2.5 Pa I A = 10 mA B = 20 mT Dust acoustic wave experiment: Kiel Univ. Dusty Plasma Research, A. Piel, 2005 42 Courtesy Alexander Piel, Kiel University, Germany, 2005

43. Physics experiments Next: Shear flow in a strongly-coupled dusty plasma (plasma crystal).

44. Shear flow in a 2D dusty plasma two Ar + laser beams: 0.61 mm width rastered into vertical sheets

45. Ar + laser pushes particles low power: slow deformation, rotation medium power: plastic deformation, flowhigh power: melting the lattice undisturbed monolayer Transport: radiation pressure Shear flow in a strongly-coupled dusty plasma

46. Zoom-in view A 2D liquid, observed at an atomistic level Shear flow in a strongly-coupled dusty plasma

47. Video data: Data recorded: x & v for each particle i.e., a kinetic approach Next step in analysis: convert to a continuum approach, by spatial averaging Shear flow in a strongly-coupled dusty plasma particle’s x,y position measured in each video frame

48. Velocity profiles Shear flow in a strongly-coupled dusty plasma

49. Navier-Stokes equation vfluid velocity  areal mass density (2D) ppressure (2D)  /  kinematic viscosity (2D)  second viscosity (2D) E gas drag

50. Navier-Stokes equation reduces to: kinematic viscosity E gas drag coefficient Our experiment: 2D  /  t = 0  /  x = 0 v y = 0  Navier-Stokes equation

51. Velocity profiles fit to Navier-Stokes

52. Results: viscosity vs. inverse temperature water at STP (3D) high temperature viscosity has a minimum low temperature  y  1/T y

53. Physics experiments Next: Waves in a strongly-coupled dusty plasma

54. Waves: two modes in a lattice

55. S & P waves in seismology Only the P wave passes through the core of Earth – because the core is liquid

56. Frequency   Theory for a triangular lattice,  0° Wang, Bhattacharjee, Hu, PRL (2000) wavenumber ka/  acoustic limit compressional shear Wave dispersion relation – 2D triangular lattice

57. Longitudinal wave 4mm k Laser incident here f = 1.8 Hz Nunomura, Goree, Hu, Wang, Bhattacharjee Phys. Rev. E 2002

58. Random particle motion No Laser! 4mm S. Nunomura, Goree, Hu, Wang, Bhattacharjee, Avinash PRL 2002

59. Wave spectrum & sinusoidally-excited waves S. Nunomura, Goree, Hu, Wang, Bhattacharjee, Avinash PRL 2002

60. Mach Cones Courtesy of Dan Dubin, UCSD

61. Mach cone angle C = U Sin   U Supersonic disturbance Acoustic wavefronts cone Courtesy of Dan Dubin, UCSD

62. Lateral wake Transverse Wake Ship’s wake Courtesy of Dan Dubin, UCSD

63. water  air Wake pattern is determined by dispersion relation Mach cone Lateral & transverse wakes Has both features: Mach Cone Lateral & transverse wakes plasma crystal Dan Dubin, Phys. Plasmas 2000 Wakes in a dusty plasma

64. V/C L = 1.17 Mach cone excitation Nosenko et al. PRL 2002

65. Speed map for compressional Mach cone particle speed v (  m/s)

66. The Early History of Dusty Plasmas The first observations of a dusty plasma in the laboratory were made by Langmuir. He reported these observations on September 18, 1924 at an address at the Centenary of the Franklin Institute in Philadelphia. “... we have observed some phenomena of remarkable beauty which may prove to be of theoretical interest.” Langmuir, Found and Dittmer, Science, vol. 60, No. 1557, p 392 (1924)

67. A 2 – 4 Torr Argon tungsten globules 0.01 -0.1 mm negative particles C S Langmuir’s Streamer Discharge

68. Langmuir’s Observations small tungsten ‘globules’ were sputtered into the discharge from the filament these globules could be made to move upward and their motions could easily be followed visually by concentrating a beam of sunlight into the tube, he could see a ‘very intense scattering’ from the particles

69. Langmuir’s conclusions Langmuir concluded that since the walls of the tube are negatively charged, the particles must also be negatively charged because they do not deposit on the walls the negatively charged particles is surrounded by a positive ion shielding cloud the negative particles can lose their charge when moving through an ion sheath, and the resulting neutral particles can condense into larger solid particles

72. Formation: gas phase Gas-phase formation resulting from graphite sputtering: Graphite targets were sputtered by Ar+ in a glow discharge Particles grew in the gas phase Particles (white) are imaged here resting on the graphite lower electrode G. Praburam and J. Goree Cosmic Dust Synthesis by Accretion and Coagulation Astrophysical Journal Vol. 441, pp. 830-838, 1995

73. Formation: gas phase Gas-phase formation resulting from graphite sputtering: Graphite targets were sputtered by Ar+ in a glow discharge Particles grew in the gas phase Particles (white) are imaged here resting on the graphite lower electrode G. Praburam and J. Goree Cosmic Dust Synthesis by Accretion and Coagulation Astrophysical Journal Vol. 441, pp. 830-838, 1995

74. Formation: gas phase Gas-phase formation resulting from graphite sputtering: Graphite targets were sputtered by Ar+ in a glow discharge Particles grew in the gas phase Particles (white) are imaged here resting on the graphite lower electrode G. Praburam and J. Goree Cosmic Dust Synthesis by Accretion and Coagulation Astrophysical Journal Vol. 441, pp. 830-838, 1995

75. Formation: gas phase D. Samsonov and J. Goree Particle growth in a sputtering discharge J. Vac. Sci. Technol. A 1999

76. Log lambda used in code log_lambda = max([3.,alog(debye_length / max([b_c,b_pi]))]) ; John's ad-hoc Coulomb logarithm ; corresponds to impact parameters ranging from ; the one that causes pi/2 scattering or collection on grain ; whichever is bigger, to the Debye length ; the outermost max function assures a nearly zero log lambda if the ; Debye length is shorter than the other length ; minimum value of 3 is suggested by Tsytovich (private communication)

77. 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

78. 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

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

80. 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

81. 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

82. 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

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

84. 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

85. Charging: secondary emission Secondary electron emission (electron impact) For mono-energetic electrons: Yield  Graphite in bulk:  m = 1 E m = 400 eV For small particles, yield is bigger than for bulk, because of bigger solid angles for secondary electrons to escape particle Goree, Plasma Sources Sci. Technol. 1994

86. Charging : secondary emission 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 T e in range 1 – 10 eV, depending on  m Other electron emission processes: photoemission due to UV (very common in space) thermionic emission (uncommon?) *Meyer-Vernet, Astron. Astrophys. 105,98 (1982) I electron collection + I ion collection + I electron emission H+H+ e-e- e-e- +

87. Charging: charging time Typically   1  sec for a 1 micron grain in a glow discharge Goree, Plasma Sources Sci. Technol. 1994 K   = -1510 sec for hydrogen, T e = T i 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

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

89. Navier-Stokes equation vfluid velocity  areal mass density (2D) ppressure (2D)  /  kinematic viscosity (2D)  second viscosity (2D) E gas drag

90. Comparison: experiment & MD simulation ●  equilibrium simulation ▲ non-equilibrium experiment this talk normalized by inverse temperature   1/T

91. Wave spectrum & theory S. Nunomura, Goree, Hu, Wang, Bhattacharjee, Avinash PRL 2002

92. Formation: tokamaks Dust formation: flaking J. Winter, Phys. Plasmas 7, 3862 (2000)