Investigation of geodesic acoustic mode flow oscillations using Doppler reflectometry in ASDEX Upgrade

One of the most important scientific challenges of today is the development of new technologies to satisfy the world's growing demand for energy in the face of declining fossil resources. One promising approach to providing energy is nuclear fusion, which in its most technically advanced application uses a hot plasma that is magnetically confined in a toroidal chamber known as a tokamak. Its efficiency, however, is limited by the transport of particles and heat due to turbulence in the edge of the fusion plasma. A thorough understanding of the complex system of plasma turbulence, and any mechanisms connected to its moderation, is therefore needed. The geodesic acoustic mode (GAM) is a radially localised plasma flow oscillation observed in the edge region of tokamak plasmas, and it is an important part of the turbulent system which contributes to the reduction of turbulent transport through velocity shearing. This thesis investigates the fundamental behaviour of the GAM through a systematic experimental study of its properties in the ASDEX Upgrade tokamak. In particular, the role of the plasma geometry (e.g. the plasma boundary elongation, and whether a limiter or divertor configuration is used) on the scaling of the GAM frequency and amplitude, as well as the GAM radial structure are investigated in detail. The experimental data was obtained with the aid of microwave Doppler reflectometry, a diagnostic technique that can be used to measure plasma flow oscillations, such as the GAM, with high temporal and spatial resolution. The GAM frequency scaling is compared with multiple theoretical and empirical models. The expected fundamental scaling behaviour of GAM frequency f_GAM with sound velocity cs and major plasma radius R₀ (f_GAM ∝ c_s/R₀), is reproduced, however, none of the existing models give a satisfyingly accurate prediction. The GAM amplitude is studied in connection with damping rates predicted by models for collisional and collisionless Landau damping processes. It is found, that, for ASDEX Upgrade plasma parameters, finite orbit width effects need to be considered in the calculation of collisionless damping rates, and that, contrary to many theoretical and numerical attempts to model the GAM behaviour, collisional damping also cannot be neglected, and may in fact be dominant in the very edge plasma region. In studying the GAM radial structure, three distinct states are identified for different plasma conditions: a continuum GAM, where the local GAM frequency varies radially, f_GAM ∝ c_s(r); a single eigenmode GAM, where the GAM frequency is nearly constant across its region of existence; and multiple eigenmode GAMs, where two radially neighbouring regions with different locked GAM frequencies exist. Transitions between these states are observed under variations of the plasma geometry. Radial correlation measurements indicate that the GAM does not propagate radially in typical elongated divertor discharges, but that it propagates radially inwards in limiter discharges with low elongation. First results on the magnetic signature of the GAM at ASDEX Upgrade and its poloidal structure are also obtained. With this study, multiple shortcomings of the current theoretical models and numerical approaches, which are used to investigate the behaviour of GAMs and their role in the overall system of plasma turbulence, have now been clearly identified: the GAM frequency scaling is subject to more influences than is accounted for by any current model; collisional damping, which is often neglected, plays a similar role to collisionless damping and may even be dominant in the region of GAM activity; and the fundamental variation of GAM radial structure must also be properly accounted for in scaling predictions. The experimental results obtained for and presented in this thesis will be able to contribute to the development of future models for GAM behaviour.