Transcript Slide 1

DPP06 Meeting of The American Physical Society
Thursday Nov. 2 — V01.00001
The effect of negative ions on
the charging of dust in a plasma
Bob Merlino and Su-Hyun Kim
The University of Iowa
Supported by The U. S. Dept. of Energy
1
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
2
For H+-e– plasma
 kTe  eV / kT
 kT   eVs 
2
2
 ne 
e
4

a

n
1

4

a
0

 
 

 2 m   kT 
 2 me 
1/ 2
1/ 2
s
electron current
e
ion current from OML theory
a = dust radius
s = eVs/kTe
Q=4oaVs
3
Charging of dust with positive ions,
negative ions and electrons
[Mamun & Shukla, PoP 10,1518 (2003)]
Ie  I   I   0
Vs  Vf  Vp
Vs < 0
 kTe  eV / kT
 kT  eV / kT
 kT   eVs 
 ne 
 n 
 n 
 e
 e
 1 
0
 2 m 
 2 m   kT 
 2 me 
1/ 2
1/ 2
s
1/ 2
e
s

Vs > 0
 kTe   eVs 
 kT   eVs 
 kT   eV / kT
1


n
1


n
0
 
 
 
 
 e
 2 m   kT 
 2 m 
 2 me   kTe 
1/ 2
 ne 
1/ 2
1/ 2
s

4
Solutions to the charging equations
n+ = ne + n–

 = ne/n+  n– = (1– )n+
K  / SF6
5
Experiment
• negative ion plasma, with ne << 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
6
Negative ion plasmas in a Q machine
SF6  e  SF


6
very effective due to low Te
N. Sato, in K
Q machine
5
I[P(SF6=0)
I[P(SF6=6e-6)
I[P(SF6=8e-6)
I[P(SF6=1e-5)
I[P(SF6=1.5e-5)
I[P(SF6=2e-5)
I[P(SF6=5e-5)
I[P(SF6=1e-4)
I[P(SF6=2e-4)
I[P(SF6=4e-4)
mSF 
6
mK 
146

39
K+_SF6-_dat_1
4
3
2
1
0
-10
-5
0
5
-1
Probe Voltage [V]
10
7
Langmuir curves before and after dust added
1.2
P(SF6) =0
No dust
1
0.8
0.6
With dust
0.4
0.06
7 x10-4
0.2
I0
0.04
-10
-5
-0.2
5
10
Bias Voltage (V)
0.02
I
0
-10
4.6 x10-5
0.15
No dust
-5
0
5
Bias Voltage (V)
10
-0.02
I+
-0.04
0.1
I+0
-0.06
0.05
With dust
-10
-5
5
10
Bias Voltage (V)
-0.05
8
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–
Qnd en  ne n   1,
  n n
 is determined from the changes in negative and
positive probe currents when dust is introduced
9
Data analysis
if  = ne/n+ <<1 (electrons attached to SF6)
n R I  I 0
 

n R I  I 0
 Qnd 
I I


  R  R 
I 0 I 0
 en0  0
The sign of R– – R+ determines
the sign of the dust charge, Q
10
Results
11
Qnd/eno vs. 
for   1
Qnd
 (  1) R  R
en0
where  
Qnd/eno
Correction for finite n e
kTe me
kT m
 = ne/n+
12
Langmuir Probe floating potential (relative to
the plasma potential) vs. P(SF6)
13
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
14