Experimental Test of Instability-Enhanced Collisional Friction for Determining Ion Loss in Two Ion Species Plasmas

Abstract

Recent experiments have shown that ions in weakly collisional plasmas containing two ion species of comparable densities nearly reach a common velocity at the sheath edge. A new theory suggests that collisional friction between the two ion species enhanced by two stream instability reduces the drift velocity of each ion species relative to each other near the sheath edge and finds that the difference in velocities at the sheath edge depends on the relative concentrations of the species. It is small when the concentrations are comparable and is large, with each species reaching its own Bohm velocity, when the relative concentration differences are large. To test these findings, ion drift velocities were measured with laser-induced fluorescence in argon-xenon plasmas. We show that the predictions are in excellent agreement with the first experimental tests of the new model.

Ar+andXe+Velocities near the Presheath-Sheath Boundary in anAr/XeDischarge

“…The argon ion temperature is higher for argon ions in an Ar=Xe mixture than for a pure argon discharge. Yip et al [8] measured the ion temperature in the range 37-61 meV which is in a similar range as the simulation results. The velocities of argon and xenon ions versus the distance from the biased plate shifted by the location presheath-sheath boundary are shown in Fig.…”

“…The question of how the velocities of ions are determined at the presheath-sheath boundary in plasmas with multiple-ion species is of fundamental interest in various fields of plasma physics and has gained interest lately [1][2][3][4][5][6][7][8]. In a weakly collisional plasma with a single-ion species, where the ion collisional mean free path is significantly larger than the Debye length, a presheath develops in the plasma and ions are accelerated to the Bohm speed (ion sound speed) at the presheath-sheath edge…”

Experimental test of instability enhanced collisional friction for determining ion loss in two ion species plasmas

“…[8][9][10] The common velocity was the bulk system ion sound velocity. In this paper we address the questions: How does the plasma achieve that result?…”

An analytic expression for the sheath criterion in magnetized plasmas with multi-charged ion species

“…However, experimental results of two-ion species plasmas do not prove this prediction. The measured velocity of each ion species in these experiments was closer to the system sound velocity, c cs , than their individual sound velocities 18,19 …”