Determination of the levitation limits of dust particles within the sheath in complex plasma experiments

Abstract

Experiments are performed in which dust particles are levitated at varying heights above the powered electrode in a RF plasma discharge by changing the discharge power. The trajectories of particles dropped from the top of the discharge chamber are used to reconstruct the vertical electric force acting on the particles. The resulting data, together with the results from a selfconsistent fluid model, are used to determine the lower levitation limit for dust particles in the discharge and the approximate height above the lower electrode where quasineutrality is attained, locating the sheath edge. These results are then compared with current sheath models. It is also shown that particles levitated within a few electron Debye lengths of the sheath edge are located outside the linearly increasing portion of the electric field.

Potential of a plasma bound between two biased walls

“…Time-independent biasing is used in plasma experiments for different purposes, namely for the measure of the ion and electron temperatures with electrostatic probes, 2,3 in plasma thrusters for space propulsion, 4 to study the effect of shear flow on turbulence, [6][7][8][9] for the study of dust particles, 5 and for the control of turbulence in magnetic fusion devices. [10][11][12] A bias may induce local perturbations of the plasma potential.…”

Glow and Dust in Plasma Boundaries

“…The intensity of each pixel (on an 8-bit scale) was then averaged across each horizontal row in order to obtain the emission intensity as a function of the height above the lower electrode. Figure 1a shows both curves obtained at a system pressure of 20 Pa and a driving potential amplitude of V RF = 30 V. The solid vertical line indicates the point where quasi-neutrality breaks, as determined by the growth of the electric force acting on the dust, which corresponds to the sheath edge [4]. The dashed vertical line indicates the point of maximum glow intensity.…”

“…It has been shown that electric force profiles can be used to locate the sheath edge [4]; however, optical emission from a plasma (when no dust is present) is also commonly used to determine the sheath edge. The dominant transition contributing to the optical emission in argon plasma at the low pressures and powers used in the following experiment is the atomic 5p-4s transition.…”

“…In a recent study [4], the vertical electric force, F E (z) = Q d (z)E z (z), acting on particles dropped in a discharge was reconstructed by solving the equation of motion of a particle:…”

“…In this paper, we investigate optical emission in the sheath region of low-pressure complex plasma experiments and compare that to the simultaneously measured force profile obtained from particle dropping experiments, which have previously been employed to locate the sheath edge (the breaking of quasi-neutrality) [4]. Furthermore, the onset of dust density waves is investigated together with the emission originating from the plasma.…”