Measurements of ion velocity and density in the plasma sheath

Abstract

Using laser-induced fluorescence, the ion velocity and density inside a dc plasma sheath have been measured. A polished planar electrode, biased at −100 V, was aligned so that a laser beam struck it at normal incidence. Using this arrangement, the ion velocity component perpendicular to the electrode surface was measured. By detecting the fluorescence while scanning the laser frequency, a line shape was recorded that had two peaks, due to the Doppler shift from the incident and reflected beams. The separation of the peaks yielded an absolutely calibrated measure of the ion drift velocity, while the height of the peaks gave the ion density. As expected, in the sheath the measured ion density was lower and the velocity was higher than in this plasma. Using these measurements, it was confirmed that the ion flux is conserved in a sheath. The spatial profiles of ion velocity and density in the sheath were used to test a time-independent two-fluid theory, and good agreement was found. The data were also compared to Child’s law, which showed good agreement near the electrode but predicted the density poorly, as expected, near the plasma–sheath boundary.

Plasma energy transport to an electrically biased surface

“…This model without accounting for kinetic mechanisms of collisions is based on the fact that the ions would be a Maxwell-Boltzmann distribution in the absence of an electrostatic field far from the wall. This proposition has been confirmed by measurements of the ion distribution functions from Bachet et al [19], and successful comparisons between the measured ion density and velocity in argon plasma from Goeckner et al [20] and this work, as can be seen later. Since inert gases and low densities of charged particles are considered, chemical reactions and the existence of negative ions can be neglected.…”

“…Deviation can be due to the unavailable ion and electron temperatures at the sheath edge. Good comparisons between the predicted and measured ion density and velocity as a function of potential [20] for an argon plasma and stainless steel workpiece are also shown in Fig. 3.…”

A Guide to Laser-Induced Fluorescence Diagnostics in Plasmas

“…Single photon detection is nonetheless possible, and the above logic thus continues to apply: Goeckner et a1 [15] have measured argon ions by pulsed laser-induced fluorescence with a precision of the order of 10' ~m -~, whilst for molecular species some of our own results are shown in fig 2 at a total concentrationof 2 x 10' cmJ [14], and we shall see later that it is possible to achieve some degree of velocity resolution at this level. Both cases are typical of ion densities in plasma sheaths.…”

Ion flow and sheath physics studies in multiple ion species plasmas using diode laser based laser-induced fluorescence

“…Our work, of course, was not the first to employ an LIF diagnostic to measure ion flow at the sheath edge: the work on a single ion species plasmas began with Gulick et al [17], who found that in a high density magnetized ECR plasma (n e = 3 Â 10 11 cm À 3 , kT e¨1 0 eV) that the ions did not reach the Bohm speed at the boundary. In lower density plasmas (n e 1Â10 9 cm À 3 ), other researchers did find that the flow velocity exceeded C, although Goree et al [18] did not measure the plasma space potential and had to infer the location of the sheath edge, and although Batchet et al and Carrere et al [19,20] did measure the plasma potential, their potential measurements did not specifically locate the sheath edge. However, in all work other than Gulik's, there were unmistakable signatures in the ivdfs that indicated that some were measured between the sheath edge and the material boundary.…”