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 approximately reach a common velocity at the sheath edge equal to the bulk plasma ion sound velocity. A recent theory [S. D. Baalrud, C. C. Hegna, and J. D. Callen, Phys. Rev. Lett. 103, 205002 (2009)] suggests that this is a consequence of collisional friction between the two ion species enhanced by the two stream instability. The theory finds that the difference in velocities at the sheath edge depends on the relative concentrations of the two ions. The difference in velocities is small, with both species approaching to the bulk sound velocity, 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, drift velocities of Ar and Xe ions were measured with laser-induced fluorescence in Ar-Xe and He-Xe plasmas and combined with ion acoustic wave and plasma potential data. In addition, electron temperature was varied by a Maxwell demon [K. R. MacKenzie et al., App. Phys. Lett. 18, 529 (1971)]. The predictions were found to be in excellent agreement with the experimental data. The generalized Bohm criterion in two ion species plasmas is also verified in a wider variety of relative ion concentrations.

Influence of ion streaming instabilities on transport near plasma boundaries

“…It also includes determining a T e /T i threshold for instability in a presheath by setting ∆V = |c s1 − c s2 |. Figure 8 provides a further quantification of the stability boundaries for Ar + -Xe + and He + -Xe + mixtures, which are common in experiments [10,11]. This shows the stability boundary for the critical differential flow ∆V c at several values of T e /T i .…”

Comment on “Ar+andXe+Velocities near the Presheath-Sheath Boundary in anAr/XeDischarge”

“…Several experiments [2,[4][5][6] have shown that when the densities of Ar þ and Xe þ are similar, the speed of Ar þ is significantly slower than its sound speed, and the speed of Xe þ is significantly faster than its sound speed. The most recent experiments [5,6] have shown that the density ratio of ion species is also important. When the density ratio is large or small, individual sound speeds are measured, but when the ratio is near unity, the speeds merge toward the system sound speed.…”

“…The theory uses this threshold and the generalized Bohm criterion to determine the speed of each species at the sheath edge [8]. The theoretical predictions have been tested experimentally in both Ar=Xe [5] and He=Xe [6] plasmas. In both cases, good agreement is found over a broad range of ion density ratios.…”

“…In both cases, good agreement is found over a broad range of ion density ratios. Experiments have also directly measured the two-stream instabilities predicted by the theory [6,9]. There is substantial evidence that Coulomb collisions and ion-ion beam instabilities are important in these plasmas, but neither effect is included in [1].…”

Positive ion speed at the plasma–sheath boundary of a negative ion‐emitting electrode

“…[28][29][30][31] One of the elegant results in plasma physics is the Bohm criteria [32] that define the minimal speed with which the positive ions enter the sheath boundary. Although the Bohm speed was originally derived for electropositive plasmas, its significance in multi-component plasmas including negative ions [33][34][35] has been comprehended both theoretically and via experiments. A significant number of theoretical/experimental studies on sheath/pre-sheath behaviour have been largely devoted to negative ions produced in the bulk plasma.…”