Fundamentals and applications of Langmuir probe diagnostics in complex plasmas

Klindworth, Markus

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 design of an rf compensated miniature Langmuir probe with two-axis drive, which matches the constraints for experiments under microgravity
  • the evaluation and adaption of available probe theories for the analysis of probe data under typical discharge conditions used for complex plasma experiments
  • the quantification of possible feedback of the probe on the plasma and verification of the developed Langmuir probe system
  • the investigation of effects of the probe on the surrounding particle distribution
  • the demonstration of first probe diagnostics applications in dusty and dust-free plasmas

    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.

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    Citation style:

    Klindworth, Markus: Fundamentals and applications of Langmuir probe diagnostics in complex plasmas. 2005.


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