@phdthesis{Rudischhauser2021,
  author    = {Lukas Rudischhauser},
  title     = {Langmuir Probes in the Wendelstein 7-X Test Divertor Unit},
  journal    = {Langmuir Sonden im Test-Divertor von Wendelstein 7-X},
  url       = {https://nbn-resolving.org/urn:nbn:de:gbv:9-opus-57371},
  pages     = {223},
  year      = {2021},
  abstract  = {This thesis describes how the data of the Langmuir probes in the Wendelstein 7-X (W7X) Test Divertor Unit (TDU) were evaluated, checked for consistency with other diagnostics and used to analyse plasma detachment. Langmuir probes are an electronic diagnostic, and were among the first to be used in plasma physics to determine particle fluxes, potentials, temperatures and densities. W7X is a large, advanced stellarator, magnetic confinement fusion experiment, operated at the Max-Planck-Institut for Plasma Physics(IPP) in Greifswald, Germany. Its TDU is an uncooled graphite component, shaped and positioned to intercept the convective heat load of the plasma. Detachment describes a desirable operation state of strongly reduced loads on this component. The evaluation of Langmuir probe data relies heavily on models of the sheath, formed at the interface between plasma and a solid surface, to infer plasma parameters from the directly measured quantities. Multiple such models are analysed, generalised, and adapted to our use case. A detailed comparison is made to determine the most suitable model, as this choice strongly affects the predicted parameters. Special attention is paid to uncertainties on the parameters, which are determined using a Bayesian framework. From the inferred parameters, heat and particle fluxes are calculated. These are also indirectly measured by two other, camera-based diagnostic systems. Observations are compared to test the validity of assumptions and calculations in the evaluation of all three diagnostics by checking their results for consistency. The first comparison, with the infrared emission camera system, shows good agreement with theoretical predictions and reported measurements of the sheath transmission factor, for which we derive and measure a value in W7X. Parameter dependencies in the quality of this agreement hint at remaining issues. The second comparison, with the Hydrogen alpha photon flux camera system, shows significant discrepancy with expectations. These are argued to originate from systematic differences in the measurement locations, which are quantified and related to the magnetic topology. Langmuir probe observations of individual discharges are analysed to discuss conditions under which detachment occurs, transition into that state and fluctuations observed prior to and during it. A spatial parametrisation of the data is developed and used to facilitate this. These observations contribute to the larger aim of understanding particle balance control and fusion plasma edge processes.},
  language  = {en}
}