Plasma sheath diagnostics by micro-particles of different sizes R. Basner 1 , G. Thieme 1 , F. Sigeneger 1 , H. Kersten 2 , G. Schubert 3 , H. Fehske 3 1 INP Greifswald, F.-L.-Jahn-Str. 19, 17489 Greifswald, Germany 2 CAU Kiel, IEAP, Olshaussenstr. 40 24098 Kiel, Germany 3 EMAU Greifswald, IfP, Domstr. 10a,17489 Greifswald, Germany Introduction The interface between a plasma and its surrounding surfaces (walls, electrodes, substrates) is formed by a self-organised structure, called the plasma-sheath. In plasma diagnostics a rel- atively large uncertainty exists for the determination of the structure of these plasma sheaths near the surface. To gain additional insight, micro-sized particles can be used as electrostatic probes. Due to electron and ion fluxes in the plasma, these particles acquire a negative surface charge, allowing for trapping them within the plasma sheath. A multitude of forces act on the particles, which have been discussed extensively in literature [1]. The particles will attain an equilibrium position, where the sum of all acting forces vanishes. In our case, the system is dominated by gravitational and electrostatic forces, while neutral and ion drag, thermophoresis and photophoresis are of minor importance. The levitated particles react sensitively to changes in the plasma sheath [2], making them suitable electrostatic probes. This approach has been suc- cessfully demonstrated in front of the powered electrode of a capacitively coupled rf-discharge [3]. In the present work, we focus on the behaviour of dust grains in front of the grounded elec- trode. We determine their equilibrium position and resonance frequency, whereby we calculate the electric field and particle charge. The sheath structure in front of a grounded surface is of importance in plasma technology for the treatment of substrate surfaces. Experiment The experimental setup is shown in figure 1. A typical asymmetric, capacitively coupled rf-plasma in argon (1- 10 Pa) is employed to charge the particles which are spherical melamine- formaldehyde (MF) particles of 0.5, 1, 5, and 10μ m in diameter. The cylindrical reactor vessel with 40 cm in diameter and 50 cm in height contains two electrodes (diameter 13 cm) in a distance of 10 cm. The upper electrode is rf driven with a power of 10 W. The lower electrode is a so called adaptive electrode (AE). It consists of 101 identical square segments (7×7 mm 2 ) surrounded by 4 larger segments and an outer ring electrode. Each segment can be biased inde- pendently with dc and/or ac voltage of up to ±100 V and frequencies of maximum 50 Hz. This arrangement allows distinct local manipulations of the plasma sheath to create different static