Charging of Dust in a Plasma with Negative Ions

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Charging of Dust in a Plasmawith Negative Ions
11th Workshop on the Physics of Dusty
Plasmas Williamsburg, VA June 28 July 1, 2006

• Su-Hyun Kim and Bob Merlino
• The University of Iowa
• Supported by DOE

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Positive Dust

• Photoelectric charging
• Secondary emission
• Ti gt (mi/me) Te
• If electrons areattached to heavynegative ions

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Dust in plasmas with negative ions

• the plasma environment of the nighttime polar
  mesosphere is dominated by negative and positive
  ions, few electrons, and nanometer size particles
• microelectronic manufacturing devices using
  reactive gases, such as silane,SiH4, have large
  concentrations of negative ions and dust
  particles

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Plasma densities in the polar mesosphere
M. Rapp et. al, GRL 32, L23821 (2005)
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Mesospheric particle detector current
The detector currents correspond to positive
charge densities, eZN 100 e/cm-3
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Hydrogenated amorphous silicon (a-SiH)
semiconductors

• for liquid crystal displays and solar cells
• made by PECVD in rf discharges using
  hydrogen-diluted silane
• SixHm (x ?200) nm size particles also form and
  incorporate into the depositing film
• particles must be neutral or to get out
• in these electronegative discharges neltltni
• effect of ve ions must be considered

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STM image of a nanoparticle on a 4.2 nm thick
a-SiH film
nanoparticle
film
substrate
Tannenbaum, et. al, APL 68, 1705 (1996)
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Charging of dust with positive ions, negative
ions and electronsMamun Shukla, PoP 10,1518
(2003)
Vs Vf ? Vp
Vs lt 0
Vs gt 0
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Solutions to the charging equations
n ne n ? ? ne/n ? n (1
?)n
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Experiment

• negative ion plasma, with ne ltlt n
• dust particles

• Q machine plasma
• K ions
• Te Ti ? 0.2 eV
• admit SF6 gas to form
• negative ions
• Disperse hollow glass
• microspheres (35 mm)
• using rotating cylinder

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Negative ion plasmas in a Q machine
very effective due to low Te
N. Sato, in K Q machine
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Langmuir curves before and after dust added
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Data analysis

• Analysis of the Langmuir probe currents can be
  used to determine how the charge in the plasma is
  divided between free /- ions, free electrons
  and dust particles.
• Charge neutrality in dusty plasma
• en Qnd ene en

h is determined from the changes in negative and
positive probe currents when dust is introduced
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Data analysis
if e ne/n ltlt1 (electrons attached to SF6)
The sign of R R determines the sign of the
dust charge, Q
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Results
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Qnd/eno vs. e
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Langmuir Probe floating potential (relative
tothe plasma potential) vs. P(SF6)
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Summary and Conclusions

• dust charging in a plasma with negative ions has
  been studied experimentally
• the addition of negative ions reduces the density
  of electrons leading to a reduction in the
  (negative) charge on dust
• conditions have been established which cause
  positive charging of dust, in a plasma with light
  ions and heavy - ions

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