@phdthesis{Marsen2008,
  author    = {Stefan Marsen},
  title     = {The Spatio-temporal Structure of Electrostatic Turbulence in the WEGA Stellarator},
  journal    = {Die r{\"a}umlich-zeitliche Struktur elektrostatischer Turbulenz im Stellarator WEGA},
  url       = {https://nbn-resolving.org/urn:nbn:de:gbv:9-000497-2},
  year      = {2008},
  abstract  = {The present work is the first work dealing with turbulence in the WEGA stellarator. The main object of this work is to provide a detailed characterisation of electrostatic turbulence in WEGA and to identify the underlying instability mechanism driving turbulence. The spatio-temporal structure of turbulence is studied using multiple Langmuir probes providing a sufficiently high spatial and temporal resolution. Turbulence in WEGA is dominated by drift wave dynamics. Evidence for this finding is given by several individual indicators which are typical features of drift waves. The phase shift between density and potential fluctuations is close to zero, fluctuations are mainly driven by the density gradient, and the phase velocity of turbulent structures points in the direction of the electron diamagnetic drift. The structure of turbulence is studied mainly in the plasma edge region inside the last closed flux surface. WEGA can be operated in two regimes differing in the magnetic field strength by almost one order of magnitude (57mT and 500mT, respectively). The two regimes turned out to show a strong difference in the turbulence dynamics. At 57mT large structures with a poloidal extent comparable to the machine dimensions are observed, whereas at 500mT turbulent structures are much smaller. The poloidal structure size scales nearly linearly with the inverse magnetic field strength. This scaling may be argued to be related to the drift wave dispersion scale. However, the structure size remains unchanged when the ion mass is changed by using different discharge gases. Inside the last closed flux surface the poloidal ExB drift in WEGA is negligible. The observed phase velocity is in good agreement with the electron diamagnetic drift velocity. The energy in the wavenumber-frequency spectrum is distributed in the vicinity of the drift wave dispersion relation. The three-dimensional structure is studied in detail using probes which are toroidally separated but aligned along connecting magnetic field lines. As expected for drift waves a small but finite parallel wavenumber is found. The ratio between the average parallel and perpendicular wavenumber is in the order of 10^-2. The parallel phase velocity of turbulent structures is in-between the ion sound velocity and the Alfv{\`e}nvelocity. In the parallel dynamics a fundamental difference between the two operational regimes at different magnetic field strength is found. At 500mT turbulent structures can be described as an interaction of wave contributions with parallel wavefronts. At 57mT the energy in the parallel wavenumber spectrum is distributed among wavenumber components pointing both parallel and antiparallel to the magnetic field vector. In both cases turbulent structures arise preferable on the low field side of the torus. Some results on a novel field in plasma turbulence are given, i.e. the study of turbulence as a function of resonant magnetic field perturbations leading to the formation of magnetic islands. Magnetic islands in WEGA can be manipulated by external perturbation coils. A significant influence of field perturbations on the turbulence dynamics is found. A distinct local increase of the fluctuation amplitude and the associated turbulent particle flux is found in the region of magnetic islands.},
  language  = {en}
}