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Hans Meister
Transportuntersuchungen im Uebergangsbereich von Einschlussregimen an ASDEX Upgrade
Betreuer: Prof. Dr.-Ing. Kurt Behringer [Experimentelle Plasmaphysik]
Datum der mündlichen Prüfung: 31.10.2000
110 Seiten, deutsch , ISBN 3-89820-181-3, Mensch & Buch Verlag, Berlin, 2001
A promising operation regime for future fusion devices with magnetic confinement based on the tokamak principle is the "advanced tokamak". Showing a good energy confinement and a high fraction of non-inductive driven plasma current, these scenarios have an internal transport barrier (ITB) which enables high ion temperatures in the plasma center. As heat diffusivity coefficients reach neoclassical levels in the plasma center in these scenarios, anomalous, turbulent transport is reduced. Similar to the transport reduction at the plasma edge in H-mode scenarios, theory explains the transport reduction in ITB regimes by E×B flow shear decorrelation of turbulence. In order to experimentally test this prediction, the radial electric field is calculated from measurements of density, temperature, toroidal and poloidal rotation velocity of one ion species using the radial force balance. These profiles are provided from the charge exchange recombination spectroscopy (CXRS) which has been extended by poloidal sight-lines. The CXRS diagnostic at ASDEX Upgrade and its data evaluation procedure is presented. Special attention is paid to the topics of interest for proper interpretation of the observed spectra: exact calibration of the system and the alignment of the sight-lines which gives rise to high corrections due to the energy dependent cross-section of the charge exchange rate coefficient. Methods for considering these corrections and their implementation during data evaluation are described. The discussion of experimental results includes three different regimes, the H-mode, the H-mode with improved confinement and a discharge with internal transport barrier and L-mode edge. Measurements at ASDEX Upgrade show that the radial electric field is always dominated by the toroidal rotation. The pressure gradient plays only a minor role even in regimes with ITB, whereas the poloidal rotation usually alters the profile of the radial electric field and is therefore important for the calculation of the shearing rates. The comparison of ion heat diffusivity coefficients, E×B shearing rates and maximum linear growth rates for the ion temperature gradient instabitlity in these regimes support the assumption that turbulent transport inside the ITB is suppressed, even when taking the high experimental uncertainties into account.