The Dynamics of Electrons in Linear Plasma Devices and Its Impact on Plasma Surface Interaction

Abstract

Bibliografische Information der Deutschen Nationalbibliothek. Die Deutsche Nationalbibliothek verzeichnet diese Publikation in der Deutschen Nationalbibliografie; detaillierte Bibliografische Daten sind im Internet über http://dnb.d-nb.de abrufbar. AbstractFor the viable operation of a thermonuclear fusion reactor, large pressure gradients are necessary in the edge of a magnetically confined plasma device. Turbulent cross-field transport across these gradients severely limits the durability of the current reactor designs by the erosion of plasma facing components. The convective character of this transport is caused by intermittently occurring plasma filaments, which are observed with similar characteristics in the edge of a wide range of plasma devices. Besides the ballistic plasma transport, filaments lead to pressure fluctuation statistics with a positive skewness and kurtosis, i.e. short, local spikes in density and temperature time traces. Erosion predictions based on time-averaged measurements underestimate dynamic effects on erosion mechanisms, e.g. the sputtering yield, due to its non-linear temperature dependence.The aim of this thesis was the development of a time-resolved laser Thomson scattering diagnostic system to investigate plasma dynamics leading to and during intermittent transport events, which were identified at the linear plasma generator PSI-2 in high powered, steady state Deuterium discharges with fast visual imaging. To retain the temporal information in the Thomson scattering setup, the signal was recorded in a triple grating spectrometer on a shot-to-shot basis with a photon counting method. The synchronization of the Nd:YAG laser and spectrometer to a fast framing camera allowed the novel usage of conditional averaging to create Thomson spectra. The selection of a subset of laser pulses with conditions in time and space was based on the plasma state characterized by 100 captured frames around each laser pulse with a time resolution of 3 µs.Time-averaged Thomson scattering profiles of plasma density and temperature were obtained for Deuterium, Helium, Neon and Argon discharges at different high power settings and a reasonable agreement was found with results from the Langmuir probe as the standard diagnostic tool at PSI-2. Standalone fast camera measurements were used to characterize the dynamics in the visible wavelength range and found distinct mode structures and slower oscillations with increasing ion mass. Two synchronized fast cameras showed that these mode are elongated along the magnetic field.The application of conditional averaging on Thomson scattering was successfully tested Argon discharges, where rotating brightness fluctuations with a 75 µs correlation time were found to correspond to a 20% temperature fluctuation amplitude around the temporal mean at the profile maxima. Subsequently, filaments in the edge of high power, low gas-feed Deuterium discharges were selected by conditional averaging and a significant temperature increase was found in the edge of Thomson scatterin...