Measurement of the Vertical Nonuniformity of the Plasma Sheath in a Complex Plasma

Abstract

Abstract-Employing an attenuated oscillation method, the anisotropic interaction force between two vertically aligned dust particles located in the sheath of a complex plasma was measured experimentally based on a linear approximation to the interaction force. Experimental data shows that although both particles experience a repulsive interaction force, the upper particle experiences a stronger magnitude force than does the lower. This result can be explained by the ion wakefield since the lower particle resides within the ion wakefield generated by the streaming ions passing through the sheath and around the upper particle.

Experimental measurement of velocity correlations for two microparticles in a plasma with ion flow

“…Vertically aligned particles, however, are levitated by an electric field that is stronger for the lower microparticle so that the two have different charges as was reported in Refs. [34,35,38,[68][69][70]. In the experiment by Carstensen et al [38], it was estimated that for their experimental conditions, the lower microparticle particle had a charge only 78% as large, i.e., Q β /Q α = 0.78.…”

“…For example, Melzer used laser manipulation to perturb a vertically aligned pair of microparticles to observe an instability arising from the wake effect [31]. Kong and co-workers [34,35] and Carstensen et al [38] studied oscillatory modes for two vertically aligned particles while manipulating them by modulating the voltage on the lower electrode. To study the oscillatory modes we do not perform any manipulation but rely instead on the natural Brownian motion, which can be enhanced by wake-driven instabilities.…”

“…The wake effect in dusty plasmas has been studied theoretically and in experiments [27][28][29][30][31][32][33][34][35][36][37][38][39][40][41]. The experiments reveal a tendency for microparticles to align with the ion flow.…”

“…These experiments with just two microparticles have a special value because a system with two particles is the smallest one that can include interactions among them. These experiments include an experimental estimation of microparticle charge [34,38], studies of the bistability of the orientation of the microparticle pair with respect to the direction of the ion flow [32], and the oscillatory modes [33][34][35]58]. Wake effects tend to greatly influence experiments with only two microparticles, which has been investigated theoretically as well [6,[59][60][61].…”