Measurements of Ar+ and Xe+ velocities near the sheath boundary of Ar–Xe plasma using two diode lasers

The Bohm sheath criterion in single- and two-ion species plasmas is studied with laser-induced fluorescence (LIF) using two diode lasers in Xe and Ar–Xe plasmas. The plasmas are generated in a low pressure unmagnetized dc hot filament discharge confined by surface multidipole magnetic fields. Two LIF schemes are employed to measure the argon and xenon ion velocity distribution functions near a negatively biased boundary plate. The results show that the argon and xenon ion velocities approach the ion sound speed of the system near the sheath-presheath boundary and satisfy the generalized Bohm criterion.