Magnetohydrodynamic stability analysis of the pedestals of ASDEX Upgrade plasmas

The high confinement mode (H-mode) dramatically improves the confinement properties of present tokamak plasmas and is therefore the scenario envisioned for future fusion reactors. The main characteristic of this scenario is the formation of a pedestal, a zone of steep temperature and density gradients, at the edge of the plasma, by means of a transport barrier. The height of the pedestal is limited by the onset of edge localised modes (ELMs), quasi-periodic explosive instabilities at the plasma edge which expel particles and energy on millisecond time-scales. While ELMs in present day machines pose no danger, when scaled to a fusion reactor device they are predicted to cause significant damage to the machine components. As such, the understanding and exploitation of alternative regimes with high confinement, but without ELMs, is of significant interest. The onset of an ELM can be described by magnetohydrodynamic (MHD) stability codes. The aim and project of the current thesis carried out at Max-Planck-Institut für Plasmaphysik (IPP) in Garching (Germany), involves the automation of a workflow which runs codes to test the pedestal MHD stability, such as MISHKA, starting from a standardised set of experimental information. In addition, the HELENA code is employed as a high resolution equilibrium solver through the calculation of the Grad-Shafranov equation for a toroidal axisymmetric plasma. Once the workflow is implemented, it is applied to a database of experimental data from the ASDEX Upgrade tokamak to study the properties of the pedestal through stability diagrams. It is particularly important to provide an estimate of the distance to the MHD stability boundary in the various ELM-free regimes to understand how robust these regimes are and the margin a given regime has before a large ELM is triggered. Various deuterium and helium plasma discharges are studied in this regard.