Laser-induced fluorescence measurements of argon ion velocities near the sheath boundary of an argon–xenon plasma

Abstract

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.

Global model analysis of negative ion generation in low-pressure inductively coupled hydrogen plasmas with bi-Maxwellian electron energy distributions

“…is the effective electron temperature (in electron volts) in the form of the densityweighting harmonic average used to calculate the Bohm velocity, 71 T H À is the negative ion temperature (in Kelvin), and m H þ n is the mass of the positive ion species. The expression describing the effective area of a cylindrical ICP with a multicusp magnetic configuration surrounding the chamber is 14,15…”

Deduction of edge electron density with multiply charged ions in ORNL volume-type electron cyclotron resonance ion source

“…Recently, the mean velocities of all different ionic elements at the sheath edge was found to be very close to the ion sound speed of the system of all particles by using the laser-induced fluorescence method. 11,12 Based on these new results, a generalized Bohm criterion 8 for plasmas containing different charge-state ions can be modified as n es / T e0 ജ ͚ j j 2 n js / m js 2 , where n es , n js , and js are the electron and j-charged ion densities, and the sheath velocity of the j-charged ion, respectively, at the sheath edge. Then js are assumed to be equal to the sound speed ͑ s ͒ of the multiion system at the sheath edge, s 2 ജ ͚ j ͑jn js / n es ͒C sj 2 , which can be used for the ion velocities at the sheath boundary.…”

Experimental studies of transverse metastable ion velocity distribution functions in the presheath of a weakly collisional argon plasma

“…2͒ chamber which is described in detail elsewhere. 21 The cylindrical surface of the chamber was surrounded by rows of permanent magnets of alternating polarity to enhance the plasma density and uniformity. Energetic electrons, thermionically emitted from hot thoriated-tungsten filaments biased at −60 V with respect to the grounded chamber, maintained the electrical discharge in the argon gas.…”