LIF Measurements of Ar+ and Xe+ in Ar-Xe Plasmas Near the Sheath Boundary with Tunable Diode Lasers

Summary form only given. The Bohm sheath criterion in two-ion species plasma is studied with Laser-Induced Fluorescence (LIF) using two diode lasers. A new LIF scheme in Xe II plasmas which uses the 5p⁴(³P₁)5d[3]_7/2 metastable state is developed with a diode laser. The xenon II transition sequence at 680.574 nm (in air) is adopted for optical pumping to detect the fluorescence from the plasma. To our knowledge, this scheme has never been tried before for LIF measurements in plasmas. The complete scheme is 5p⁴(³P₁)5d[3]_7/2rarr5P⁴(³P₁) 6p[2]_5/2 rarr5p₄(³P₁)6s[1]_3/2, and the detectable photon has lambda = 492.15 nm in air. Plasmas are generated in an unmagnetized dc hot filament discharge chamber confined by surface multidipole magnetic fields. The argon and xenon ion velocity distribution functions are measured as a function of position relative to a negatively biased boundary plate. The structures of the plasma sheath and presheath are measured by an emissive probe. The new Xe II LIF scheme combined with the previously used Ar II scheme1 in Ar-Xe plasmas permits measurements of the ivdfs for both ions. Both the ivdfs provide a solution for the generalized Bohm criterion in two-ion species plasmas.