The effect of negative ions on the charging of dust in a plasma

1
The effect of negative ions on the charging of
dust in a plasma
DPP06 Meeting of The American Physical
Society Thursday Nov. 2 V01.00001

• Bob Merlino and Su-Hyun Kim
• The University of Iowa
• Supported by The U. S. Dept. of Energy

2
Charging of dust in a plasma

• in typical laboratory plasmas, dust grains
  acquire a negative charge due to the preferential
  attachment of the more mobile electrons
• if the dust is immersed in an environment of UV
  radiation, it can charge positively due to
  photoelectron emission
• dust grains may also acquire a net positive
  charge due to secondary electron emission

3
For H-e plasma
electron current
ion current from OML theory
a dust radius ?s eVs/kTe Q4??oaVs
4
Charging of dust with positive ions, negative
ions and electronsMamun Shukla, PoP 10,1518
(2003)
Vs ? Vf ? Vp
Vs lt 0
Vs gt 0
5
Solutions to the charging equations
n ne n ? ? ne/n ? n (1
?)n
6
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

7
Negative ion plasmas in a Q machine
very effective due to low Te
N. Sato, in K Q machine
8
Langmuir curves before and after dust added
9
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
10
Data analysis
if e ne/n ltlt1 (electrons attached to SF6)
The sign of R R determines the sign of the
dust charge, Q
11
Results
12
Qnd/eno vs. e
13
Langmuir Probe floating potential (relative
tothe plasma potential) vs. P(SF6)
14
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