@phdthesis{Windisch2007,
  author    = {Thomas Windisch},
  title     = {Intermittent events and structure propagation in plasma turbulence},
  journal    = {Intermittent events and structure propagation in plasma turbulence},
  url       = {https://nbn-resolving.org/urn:nbn:de:gbv:9-000365-5},
  year      = {2007},
  abstract  = {Turbulence is an omnipresent phenomenon in nature. Its main characteristics, fluctuations under a wide spread of spatial and temporal scales, are observed in neutral fluid or gaseous systems as well as in plasmas. Although the electromagnetic character of a plasma makes plasma turbulence more complex, the basic features of turbulence are universal. In magnetically confined plasmas, fluctuating plasma parameters give rise to turbulent transport of plasma particles and energy, which degrades the confinement and affects key issues of future fusion devices. The intermittent character of this convective particle flux is associated with high-amplitude turbulent structures, called \"blobs\", propagating radially outwards across the magnetic field. Also the intermittent fluctuations observed in the plasma edge of devices with linear magnetic field geometry seem to be related to propagating turbulent structures. The mechanism of the radial propagation is however poorly understood. The present thesis reports on the formation and radial propagation of turbulent structures in the linearly magnetized Vineta helicon device. By imaging the fluctuations in the azimuthal plane with multi-dimensional probe arrays, it is demonstrated that turbulent structures develop out of drift-wave turbulence in the maximum plasma density gradient region. The turbulent structures propagate mainly azimuthally in direction of the background ExB-drift but they exhibit also a strong radial velocity component. The radial propagation is caused by the self-consistent potential of the turbulent structures. They are the main contributors to fluctuation-induced radial transport. In the plasma edge the turbulent structures occur as intermittent density bursts with high amplitudes. A comparison of the experimental findings with numerical three-dimensional fluid simulations with appropriate geometry and boundary conditions reveals consistency, i.e., the formation of the turbulent structures is causally connected with a quasi-coherent drift mode and their radial propagation is caused by the self-consistent electric field that develops due to their three-dimensional dynamics. For a comparison with a fusion plasma, the propagation properties of turbulent structures in the edge plasma of the National Spherical Torus Experiment (NSTX) are investigated and compared with the common propagation models.},
  language  = {de}
}