1 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 DPP06 Meeting of The American Physical Society Thursday Nov. 2 — V
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 a = dust radius s = eV s /kT e Q=4 o aV s electron current ion current from OML theory
4 Charging of dust with positive ions, negative ions and electrons [ Mamun & Shukla, PoP 10,1518 (2003)] V s > 0 V s < 0 V s V f V p
5 Solutions to the charging equations n + = n e + n – = n e /n + n – = (1– )n +
6 Experiment negative ion plasma, with n e << n + dust particles Q machine plasma K + ions T e = T i 0.2 eV admit SF 6 gas to form negative ions Disperse hollow glass microspheres (35 m) using rotating cylinder
7 Negative ion plasmas in a Q machine very effective due to low T e N. Sato, in K Q machine
8 Langmuir curves before and after dust added P(SF 6 ) =0 4.6 x x10 -4 I0I0 II I +0 I+I+
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 + + Qn d = en e + en – is determined from the changes in negative and positive probe currents when dust is introduced
10 if = n e /n + <<1 (electrons attached to SF 6 ) The sign of R – – R + determines the sign of the dust charge, Q Data analysis
11 Results
12 Qn d /en o = n e /n + Qn d /en o vs.
13 Langmuir Probe floating potential (relative to the plasma potential) vs. P(SF 6 )
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