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Reich, Matthias (2005): A new diagnostic for ASDEX Upgrade edge ion temperatures by lithium-beam charge exchange recombination spectroscopy. Dissertation, LMU München: Faculty of Physics
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Abstract

This thesis work investigates the measurement of ion temperatures at the edge of a magnetically confined plasma used for fusion research at the ASDEX Upgrade tokamak operated by Max-Planck-Institut für Plasmaphysik in Garching. The tokamak is the most advanced concept in toroidal magnetic confinement fusion. The H-mode plasma regime, default scenario of the next step experiment ITER, is characterized by an edge transport barrier, which is not yet fully explained by theory. Experimentally measured edge ion temperature profiles will help to test and develop models for these barriers. Transport theory on a basic level is introduced as background and motivation for the new diagnostic. The standard model for an edge plasma instability named "edge localized mode" (ELM) observed in H-mode is described. The implementation of a new diagnostic for ion temperature measurements with high spatial resolution in the plasma edge region, its commissioning and the validation of the measurements comprises the main part of this work. The emission of line radiation induced by charge exchange processes between lithium atoms injected by a beam source and fully ionized impurities (of C and He) is observed with a detection system consisting of spectrometers and fast cameras. Due to the narrow beam (1 cm) and closely staggered optical fibers (6 mm), unprecedented spatial resolution of edge ion temperatures in all major plasma regimes of the ASDEX Upgrade tokamak was achieved. The spectral width of the line radiation (He II at 468.5 nm and C VI at 529.0 nm) contains information about the local ion temperature from thermal Doppler-broadening, which is the dominant broadening mechanism for these lines. The charge-exchange contribution to the total line radiation locally generated by the lithium is determined by gating the beam. Fitting a Gaussian model function to the local line radiation results in absolute line widths which can be directly converted into a temperature. The equilibration of impurities with the main plasma is fast enough that the assumption of nearly identical temperatures as the main plasma is justified. Corrections for systematic line broadening effects from collisional mixing and Zeeman broadening are incorporated by model calculations using existing routines for the involved atomic physics. Time resolution of the diagnostic is still not suffcient to resolve ELM events, but measuring between ELMs is possible if their frequency is low. L-mode plasmas with and without additional heating can be reliably diagnosed with a time resolution depending on the lithium beam intensity and plasma density, in best cases down to 100 ms. It was shown that diagnostic He puffing can be used to enhance the signal-to-noise ratio. Results from L-mode plasmas with electron heating show that ion temperatures can be significantly different from electron temperatures at the edge. For the verification of the new ion temperatures, comparison with data from already established diagnostics was done. In neutral beam heated L-mode and various H-mode plasmas the ion temperatures agree with those from a similar diagnostic measuring in the core using heating beams where both diagnostics overlap. They can be combined to form a complete ion temperature profile over the whole plasma radius. In a first application, transport coefficients have been determined by interpretative modeling for an ohmic plasma. In summary, a new method for measuring ion temperatures in the edge of a magnetically confined fusion plasma has been established. The results provide an important input to further understanding of transport in these plasmas.