AVS 52nd International Symposium
    Plasma Science and Technology Monday Sessions
       Session PS-MoM

Paper PS-MoM11
Sheaths and Pre-sheaths in Collisionless and Collisional Active Plasmas: Planar and Cylindrical Probes

Monday, October 31, 2005, 11:40 am, Room 302

Session: Plasma Diagnostics
Presenter: F. Iza, Pohang University of Science and Technology (Postech), S. Korea
Authors: F. Iza, Pohang University of Science and Technology (Postech), S. Korea
J.K. Lee, Pohang University of Science and Technology (Postech), S. Korea
Correspondent: Click to Email
Ion kinetics in the sheath and pre-sheath of planar and cylindrical probes have been studied by means of particle-in-cell computer simulations. Low temperature argon discharges with Maxwellian electrons have been simulated in collisionless and collisional regimes. As pressure increases, the sheath, i.e. the region of positive space charge surrounding the probe, becomes collisional and the velocity of the ions entering the sheath falls below the Bohm velocity (u@sub B@). For planar probes, ions enter the sheath with a velocity given approximately by u@sub B@(1+4@lambda@@sub D@/@lambda@@sub i@) where @lambda@@sub i@ is the ion mean free path and @lambda@@sub D@ the Debye length at the sheath edge. This relation differs from that given in@footnote 1@ because the electric field boundary condition at the sheath edge used in@footnote 1@ corresponds to a field reached well inside the sheath. For a floating planar probe, the voltage drop across the sheath increases with pressure to balance the electron and ion fluxes and the ion flux is almost independent of pressure despite the variations in ion velocity. As observed experimentally,@footnote 2@ simulation results show that the voltage across the presheath can be significantly larger than half electron temperature. For planar probes, this voltage depends non-linearly on the electron temperature and increases rapidly for electron temperatures below 2eV. For cylindrical probes, however, the voltage across the presheath can be drastically reduced by the geometrical increase of current density as ions approach the probe. The floating potential and the ion velocity at the sheath edge decrease with decreasing probe radius and for thin probes (r@sub p@<@lambda@@sub D,i@), the voltage drop across the presheath is negligible (<<0.5T@sub e@). @FootnoteText@ @footnote 1@ V A Godyak and N Sternberg, IEEE Trans. Plasma Sci. 18 (1990) 159-168.@footnote 2@ L Oksuz and N Hershkowitz, Plasma Sources Sci. Technol. 14 (2005) 201-208.