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Schirmer, Jasmine (2005): Plasma Turbulence Studies Using Correlation Doppler Reflectometry on the ASDEX Upgrade Tokamak. Dissertation, LMU München: Faculty of Physics
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Abstract

One of the major goals of the tokamak fusion program is the understanding and control of plasma turbulence. Turbulence causes additional radial transport of heat and particles to the tokamak vessel walls, thereby degrading the overall confinement of the plasma. Diagnostics for the study of tokamak turbulence are unfortunately scarce and limited in what they can measure. A new diagnostic technique, Doppler reflectometry, has been developed for measurements of plasma rotation profiles and turbulence properties. It is a type of microwave radar technique which uses the back-scatter of microwaves from a radial position in the plasma where the refractive index equals zero. In this thesis work, the technique is extended for turbulence correlation measurements by adding a second Doppler reflectometer channel where two microwave beams are launched into the plasma with a small frequency difference. A radial correlation Doppler reflectometer system can provide simultaneous measurements of the plasma radial electric field Er and its shear (both parameters are believed to be fundamental for suppressing turbulence) together with measurements of the properties of the plasma turbulence, such as the radial correlation length of the turbulence Lr. These measurements are explored in this thesis work for a wide range of plasma conditions. It was found that Er and its associated shear are indeed linked to plasma confinement. Their absolute values increase with confinement at the plasma edge. An increase in the absolute value of Er shear was also detected at the same plasma edge region where a decrease in radial correlation lengths of the turbulence was measured. This observation is in agreement with theoretical models which predict that an increase in the absolute shear suppresses turbulent fluctuations in the plasma, leading to a reduction in Lr. Measurements of Lr versus the perpendicular wavenumber of the turbulence were also obtained, which prompted an investigation of the correlation Doppler reflectometer response function using a 2-dimensional finite difference time domain (FDTD) code. The simulation results show that the magnitude of the measured Lr is dependent on the radial, poloidal and perpendicular wavenumbers of the turbulence.