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Experimental data obtained with emissive probes and Langmuir probes show that the plasma potential profile in the presheath scales as -ephi /T(e)= sqrt[(x(0) -x)/lambda ], consistent with ion flux conservation, and that the sheath consists of a transition region and an electron-free collisionless sheath with thicknesses scaling as lambda( 1/5)lambda (4/5 )(D) and lambda(D )(ephi/ T(e))(3 /4), respectively, where lambda is the ion-neutral collision length. Results support Rieman's presheath and transitional region model [Phys. Plasmas 4, 4158 (1997)]]. The potential drop over the presheath and transition sheath region were the order of T(e) /e and 2T(e )/3e, respectively, increasing with increasing pressure.

The Bohm sheath criterion in single-and two-ion species plasma is studied with laser-induced fluorescence using a diode laser. Xenon is added to a low pressure unmagnetized dc hot filament argon discharge confined by surface multidipole magnetic fields. The Ar II transition at 668.614 nm is adopted for optical pumping to detect the fluorescence from the plasma and to measure the argon ion velocity distribution functions with respect to positions relative to a negatively biased boundary plate. The structures of the plasma sheath and presheath are measured by an emissive probe. The ion concentrations of the two-species in the bulk plasma are calculated from ion acoustic wave experiments. Results are compared with previous experiments of Ar-He plasmas in which the argon ions were the heavier ion species. Unlike the previous results, the argon speed is slower than its own Bohm velocity near the sheath-presheath boundary in the Ar-Xe plasma where argon ions are the lighter ion species. We argue that this result is consistent with the behaviour of the helium ion required by the generalized Bohm criterion in the previous experiments with Ar-He plasmas. Further, our results suggest that the measured argon ion speed approaches the ion sound speed of the system.

Potential variations in bulk plasma, presheath and sheath describe the plasma potential profile responsible for ion acceleration out of weakly ionized, weakly collisional plasma. Experiments with emissive probes, Langmuir probes, laser induced fluorescence (LIF) and Mach probes in a multi-dipole plasma show that the presheath potential near a negatively biased plate scales as −eφ/T e = √ (x 0 − x)/λ and is insensitive to the value of the plate bias. Plasma parameters were chosen so that ε = λ D /λ ≈ 0.02 → 0.06 and 0.2 < λ/L < 0.6, where λ is the ion-neutral mean free path, λ D is the Debye length and L is the plasma length. A Child-Langmuir-like sheath with width scaling as λ D (eφ/T e ) 3/4 with a T e /λ D initial boundary electric field has been measured. A transition region, in which the plasma starts to deviate from being quasi-neutral, is observed between the presheath and the Child-Langmuir sheath. The transition region is approximately 2λ D or λ 1/5 λ 4/5 d . Mach probe data, calibrated by LIF data, suggest that the average ion velocity reaches the Bohm velocity inside the transition region and significantly exceeds the Bohm velocity at the Child-Langmuir sheath/transition boundary.