Smoky Plasma

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Smoky Plasma

• Scott Robertson
• Zoltan Sternovsky
• University of Colorado, Boulder CO
• Thanks to Plasma Science Initiative
• and Mihaly Horanyi

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What is smoke?

• Standard definitions
• Smoke particles are held aloft by weak air
  currents.
• 2) Smoke particle diameter is lt 1 mm.
• My definition
• Time for sedimentation (pressure dependent) is
  longer than the observation time.

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Motivation

• The mesosphere has smoke particles from ablation
  of meteors.
• Meteoritic smoke particles may be nucleation
  sites for noctilucent clouds. (NLC) particles
  (they are also smoke-like).
• Observational evidence (from rockets) is sparse
  for meteoritic smoke.

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The summer polar mesosphere
NLC/PMC Condensation of water into nm sized
aerosols (T lt130K) PMSE strong radar echoes
Aerosols are a dusty plasma Indicator of global
change? Required
longer term observations
NLC
PMC
Donald Petit, Space Station
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Physics issues for NLC layers

• Why are some NLC particles positive?
• Why are there sometimes strong DC and AC electric
  fields?
• Does the mesosphere support dusty plasma waves or
  instabilities?In the lab
• How do we make a smoky plasma?
• How do we diagnose a smoky plasma?
• What experiments can we do? An experiment we
  cannot do strong coupling.

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1. Why are some NLC particles positive?

• Reduced work function?
• Ooops! Positive particles are also observed at
  night.
• Is electron mobility reduced?
• Electrons may have disappeared onto negative
  ions.
• Experiments with UV illumination,
• with recombining smoky plasmas,
• with electronegative gas (SF6 ),
• will help decide among theories.

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2. Why do rockets see strong AC DC electric
fields?

• DC fields arise in NLC
• Reduced electron and ion mobility increases
  resistivity locally interrupting global electric
  circuit.
• Can this be modeled?
• AC fields are seen in electron bite-outs
• Does acoustic turbulence drive electric fields?
• Why are the fields larger in bite-outs?
• Experiments on electron and ion mobility are
  needed, as well as on the coupling of acoustic
  turbulence to electric fields

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3. Does the ionosphere support dusty plasma waves
or instabilities?

• High neutral density heavily damps plasma waves
  in the ionosphere.
• Smoky plasma experiments with high neutral
  density can reveal the role of neutral damping.
• Does Epstein formula correctly give wave damping?

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4. Can we make smoky plasma in the lab

• It has been done!
• RF plasmas in silane grow smoke particles in
  situ, e.g., (Selwyn et al.)
• Metal evaporation sources have been used to
  create nanoparticles, and UV charging was
  observed (Yokota et al., Hazelton and Yadlowsky,
  1980s)

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4a. How to make smoke particles

• Evaporate metal into 5 Torr inert gas.

Source characteristics 3-300 nm
particles Depends on P, T, flow log-normal
distribution 1012 cm-3 of particles 1015
particles / second
Heated crucible with metal
Differential pumping port
Heat shield
Inert gas inlet
C. G. Granqvist and R. A. Buhrman, J. Appl. Phys.
47, pp. 2200-2219 (1976) J. D. Eversole and H. P.
Broida, Phys. Rev. B 15, pp. 1644-1655 (1977).
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4b.Getting smoke particles into the plasma

• Smoke particles are entrained in the gas flow.
• Gas flow rate in the source is too large
• (5 Torr-liters/second), necessitates
  differential pumping.
• 1 of particles are used, but this is 1013 /sec.
• Plasma volume may be 100 liters, so this is 108
  cm-3 / sec.

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4c. Is the smoke confined?

• The positive plasma potential that contains
  electrons can also contain the negative smoke.
• Suppose radius 4 nm (104 atoms). Te 1 eV,
• then Q -8 e.
• Presheath potential 1 eV implies 8 eV potential
  well.
• Gravitation difference for r 4 nm Zn particle
  is 0.1 eV.
• Smoke is electrostatically confined to the
  central presheath region for sizes up to 15 nm
  radius.
• But we must solve self-consistently for the
  presheath with the smoke present.

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4d. Will there be a void?

• Theory says no, Ion drag scales with r2
• Experiment says no, RF discharges with silane
  have no void until particles get big (Kovacevic
  et al. 2000)
• Also, thermophoresis is small Low gas pressure
  does not support ?T

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Photo of experiment
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5. Diagnostics for smoke particles

• Laser light scattering
• Probes
• Mobility and diffusion
• Waves

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5a. Laser diagnostics for particles

• Mie scattering regime applies.Particles are not
  seen individually.Smallest size seen is r 4 nm
  _at_ 107 cm-3 (A. Gallagher lab, Proc. 3rd Intl
  Conf.)s scales as r 6, largest particles are
  seen
• Can we get size information from angular
  dependence of light scattering?Only a
  characteristic size, weighted by r6

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5b. Probes as diagnostics
Magnetic insulation can prevent electron
collection by a positively biased probe, so that
negative smoke particles are collected.
Rocket probe with magnetic insulation
Langmuir probe with magnetic insulation from
current I
I
B
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5c. Mobility and diffusion as a size diagnostic
tool

• In neutral gas, smoke particle density decay time
  reveals diffusivity and hence size

UV flash lamp

I(t)
-
UV charged smoke particles electric drift reveals
mobility (vdrift m E) and hence size.
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5d. Smoky plasma wavescan be a diagnostic

• Dust acoustic mode is easily driven unstable
  Theory Rosenberg, Planet Sp. Sci. 41, 229
  (1993)Most recent Trottenberg, et al., Phys.
  Plasmas 13, 042105 (2006)
• Ion acoustic shocks with dust Nakamura et al.,
  Phys. Rev. Lett. 83, 1602 (1999)
• Dust acoustic solitonsIvlev and Morfill, Phys.
  Rev. E 63, 026412 (2001)Kompaneetz, Tsytovich
  and Morfill, IEEE TPS 32, 561 (2004)

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6a. Smoke plasma recombination experiments
UV induced smoke plasma (not smoky plasma)
UV flashlamp
? Measure recombination and density decay after
the flash.
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6b. Smoky plasma particle and energy balance
Add smoke to hot-filament discharge at 0.5 mTorr
Ar.
What densities can be obtained ne, ni, nd ,
nd/ne? Is recombination on smoke particles or the
wall?
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6c. Smoky plasmawave experiments
Add smoke to hot-filament discharge at 0.5 mTorr
Ar.
Grid antenna
? Launch smoke acoustic waves? solitons?
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6d. Acoustic (ordinary sound) wave experiments in
smoky plasma
Add sound waves for wave coupling.
P gt 1 mTorr
Loud speakers
? How are sound waves and electrostatic waves
coupled?
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6e. Theory for coupling of acoustic and plasma
waves

• is the Epstein drag coefficient that couples
  particles and neutrals.The momentum equation for
  each of the species j (i, e, or a)

Poisson couples all charged species
The electric field driven by the acoustic waves
is (see poster paper)
n0 and u0 are fluctuation air density and
velocity.
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Conclusion

• Look for experimental results
• at the next Dusty Plasma Workshop.