Fundamentals and applications of Langmuir probe diagnostics in complex plasmas
This thesis explores the fundamentals and first applications of spatially-resolved Langmuir probe diagnostics in dusty plasmas with respect to laboratory and microgravity experiments. The basics of dusty plasmas and the subtleties of Langmuir probe theory in low density plasmas with weak collisionality are described. The investigations address the following topics:
The major results can be summarized as follows:
An automatic fitting algorithm for the Langmuir probe data based on the ABR-Model was developed that includes corrections for ion-neutral collisions. The competing OML-model was found to be not valid in given discharge regime. The random sampling technique to obtain probe characteristics was optimized to avoid dust contamination of the probe. An important discovery was the observation of ``anti-voids'' around probes and their explanation by quantitative modeling of the acting ion drag and electric field forces on dust particles in the presheath regime. From the understanding of this mechanism the formation of dust layering and its relationship to the gradients of exerted forces could be explained by direct comparison between model and experimental observations. The marking of the equilibrium point by dust tracers was introduced as a novel technique to visualize the sheath boundaries. The quantitative comparison of probe diagnostics with SIGLO plasma simulations shows now a reasonable agreement, which is an important prerequisite for future applications of the Langmuir probe in the IMPF experiments aboard the International Space Station.