Impact of shape and global β on ASDEX Upgrade pedestal structure

Abstract

The stability and confinement of the pedestal, the outermost region of the confined plasma in a tokamak, are crucial for its efficient operation and performance. This work describes ASDEX Upgrade experiments designed to analyse the pedestal structure under varying conditions of normalized poloidal pressure (ωpol) and plasma shaping. The individual treatment of temperature, density, and pressure for ion and electron pedestals is emphasized. We show that the ion temperature (Ti) increases with ωpol, whereas the electron temperature (Te) shows only a slight increase and the electron density (ne) remains relatively unaffected. The changes in shape influence ne, making its pedestal higher and wider, whereas Ti remains unchanged despite a lower heating power required to keep the same ωpol at high shaping. The findings highlight the importance of distinguishing between different channels when predicting and controlling the pedestal. The stabilising influence of the radial electric field Er, and its correlation with different pedestal top positions, is explored. The roles of ballooning modes and local magnetic shear are emphasized, and the conditions for access to second stability in different pedestal regions are presented. The global MHD stability sets the overall limit, but the radial composition of electron density and electron and ion temperature can strongly vary. The results show that the width of the electron pressure pedestal is determined by the equilibrium via the local magnetic shear. The strongest correlation of the ion pressure pedestal top position is found with the gradient of Er. We found that the second stability access requires both a highly shaped boundary and a q profile modification due to higher pressure gradients. The results contribute to understanding the mechanisms governing the pedestal behaviour, offering insights for optimizing plasma performance and stability.