On the use of microscopic test particles for non-conventional plasma sheath diagnostics

Summary form only given. The idea to use microscopic test particles as electrostatic and thermal probes in complex plasmas has been consequently developed during the last years^1-3. Due to the force balance of the particles, however, it is very difficult to change their position in the plasma sheath without changing the external and internal plasma parameters. Recently, experiments have been performed where the confined particles are affected by additional centrifugal force⁴ or by laser radiation⁵. In the present study for the first time a macroscopic optical manipulation system for micro-particles in a plasma has been realized, which is based on the principle of laser tweezers⁶. The particles have been successfully trapped in the focus of a split infrared laser beam whereas the focus length was several tens of centimeters. By vertical motion of the rf-electrode the fixed particle could be shifted to a certain extent through the sheath in front of the electrode. By this non-invasive method it is possible to perform flexible investigations without changing or disturbance of the plasma conditions. The evaluation of the affected force balance (in pN range) yields information about the potential and electric field at arbitrary positions in the sheath.