1. Alex.A. Samarian and Brian.W. James School of Physics, University of Sydney, NSW 2006, Australia Sheath edge location The charge of dust particles in sheath region Dust Motion in rf-Discharge Plasma

2. 2 Sheath model Different features of the sheath and plasma areas Sheath edge location is one of the most important parameters in many sheath models Plasma Sheath Electrode n e (z) n i (z)  (z) zeze zbzb Cross section of sheath in z-dir perpendicular to electrode E(z)

3. 3 Charging of dust particle Ion and electron fluxes a) electrons b) ions

4. 4 Equilibrium of dust particles F Gr av F El ec F Ion Drag F Thermoph oretic F Neut Drag Force Diagram F Grav F Elec F Ion Drag F Thermophoretic F Neut Drag Force Diagram Temperature Gradient Temperature Gradient Neutral Flux is the screening length Where and z Electric Field Gradient Electric Field Gradient Ion Flux

5. 5  (z) E(z) zeze zbzb Electrode Plasma Sheath for  i =10 7 s -1 for  i = 5x10 6 s -1 for  i = 10 6 s -1 Normalised difference between grain equilibrium position & sheath edge position as a function of grain radius for different values of ion plasma frequency  

6. 6 Front View Experimental setup

7. 7 Back View Experimental setup

8. 8 Experimental Setup Experimental chamber and image of test dust particles levitated above the electrode The test grains are generated in the discharge (power up to 200W, pressure up to 1 torr) by electrode sputtering.

9. 9 Main parameters RF discharge 15 MHz Pressure from 10 to 400 mTorr Input power from 15 to 200 W Self-bias voltage from 5 to 180V Carbon (C) particles’ diameter ~ 1 μm Melamine formaldehyde - 2.79 μm ± 0.06 μm Melamine formaldehyde - 6.13 μm ± 0.10 μm Argon plasma T e ~ 2 eV, V p =50 V & n e ~ 10 9 cm -3

10. 10 Result of Measurements Measured electron temperature T e (points with error bars) and discharge emission I (solid line) for P = 90m Torr, W = 80 W. The dashed line shows the equilibrium position (h eq =10.8 mm) of test grains (a~350 nm). Rf-compensated single Langmuir probe Electron temperature T e Ocean optics spectrometer PC2000 discharge emission I

11. 11 Result of Measurements Position of sheath edge (h b ) vs pressure at different rf-input powers – 35W – 60W – 100W

12. 12 Result of Measurements Estimation of dust charge from vertical equilibrium hqhq E E eq h eq hbhb 2(V b -V p )/h b V(h)=h 2 V b /h b 2 +V p -V b E=2(V p -V b )(h b -h eq )/h b 2 Z d =2  a 3 h b 2 /3e (V p -V b )(h b -h eq ) Dust particle radius, (  m) Charge on dust particle (e) From vertical equilibrium From probe data 0.95 5.3x10 3 3.6x10 3 1.391.1x10 4 5.8x10 3 3.074.2x10 4 1.2x10 4 hbhb

13. 13 Summary Spatial location of sheath edge in planar rf- discharge has been determined using test dust grains. The diagnostic technique is based on measurement of the equilibrium position of fine (<500nm) dust grains levitated above the powered electrode of the rf-discharge. Using the resulting value of sheath dimension the charge on dust particles in the sheath region has been estimated.