Determination of the Electric Field by Particle Tracking in a Plasma Sheath Region during Free Fall

In this work we present an experiment in which we injected microspheres at low pressure into a capacitively coupled argon plasma chamber. The setup was located in the top point of the Einstein-Elevator drop tower in Hannover, Germany, where the microparticles reached their equilibrium position above the lower electrode during 1 g . During the fall, the trajectories of the microparticles, which were driven by the electric force, the neutral drag force and some residual gravitational force, were recorded. In addition, simulations of the plasma conditions were performed with commercial software to determine the microparticle charges via an orbital motion limit theory approach, taking into account the charge exchange in ion-neutral collisions. Based on the calculated position dependence of the microparticle charges and the electric force, the electric field present in the plasma sheath region was finally determined.