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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. 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. Charging of dust in a plasma.

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

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  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

  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 Q=4oaVs

  4. Charging of dust with positive ions, negative ions and electrons[Mamun & Shukla, PoP 10,1518 (2003)] Vs Vf  Vp Vs < 0 Vs > 0

  5. Solutions to the charging equations n+ = ne + n– = ne/n+ n– = (1– )n+

  6. negative ion plasma, with ne << n+ dust particles Experiment • 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. P(SF6) =0 7 x10-4 I0 I 4.6 x10-5 I+ I+0 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+ <<1 (electrons attached to SF6) The sign of R– – R+ determines the sign of the dust charge, Q

  11. Results

  12. Qnd/eno e = ne/n+ 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

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