AVS 50th International Symposium
    Plasma Science and Technology Thursday Sessions
       Session PS-ThA

Paper PS-ThA7
The Study of Ion Drift Velocities and Instabilities in Presheaths in Two Ion Species Plasmas

Thursday, November 6, 2003, 4:00 pm, Room 315

Session: Plasma Diagnostics: Mechanisms
Presenter: X Wang, University of Wisconsin, Madison
Authors: X Wang, University of Wisconsin, Madison
E. Ko, University of Wisconsin, Madison
N. Hershkowitz, University of Wisconsin, Madison
Correspondent: Click to Email
The presheath is a region of weak electric field that accelerates ions into the sheath at the plasma boundary. The experiments were performed in multi-dipole DC plasmas with He-Ar gas mixtures (P@sub total@ @>=@ 1.0mTorr, n@sub e@ @>=@ 1x10@super 9@cm@super -3@, T@sub e@ @<=@ 2eV). The concentration of ion species in the two ion species plasmas was determined by measuring ion acoustic wave phase velocity and electron temperature in the bulk region.@footnote 1@ To measure ion drift velocities in the presheath, an ion acoustic wave was launched by both a continuous sinusoidal wave and a pulse, and detected by a cylindrical probe with a boxcar averager. Ion drift velocities were measured in pure Ar plasma. Based on the dispersion relation in the presheath for multi-ion species plasma and phase velocity measurements in He-Ar plasma, the relationship between Ar and He ion drift velocities was determined. Using Ar ion drift velocities from LIF data,@footnote 2@ the He ion drift velocities were determined. In two ion species plasmas, instabilities can be excited by the two ion streams with different drifting velocities that are created in the presheath. Instabilities changing with different partial pressure, positions and discharge current were observed by using a cylindrical probe biased to collect either ion saturation current or electron saturation current and a spectrum analyzer. The frequency of instabilities is ~1.0MHz and wavelength is ~5.0mm compared to ~3.0cm of presheath length. ** Work supported by US DOE grant DE-FG02-97ER54437. @FootnoteText@@footnote 1@A. M. Hala and N. Hershkowitz, Rev. Sci. Instrum. 72, 2279 (2001).@footnote 2@G. D. Severn, Xu Wang, Eunsuk Ko and N. Hershkowitz, Phys. Rev. Lett. 90, 145001 (2003).