Characterization of the ion pedestal in low and high collisionality plasmas

Abstract

The high confinement mode (H-mode) is a very important regime for future fusion devices. In this regime, the global confinement is increased and a pedestal structure is developed in the profiles. However, a complete understanding of how it is formed is still missing. In this regime, the toroidal impurity velocity profile exhibits a local minimum close to the separatrix and, under certain conditions, the impurities at the plasma edge can rotate in the opposite direction compared to the plasma core. A pedestal database was compiled with data from ASDEX Upgrade to try to progress in understanding the pedestal physics. This thesis is focussed on the study of the ion temperature and toroidal impurity velocity profiles, obtained with the charge exchange recombination spectroscopy system, at low and high collisionality. A correlation between the characteristics of the pedestal with the minimum in the toroidal impurity velocity was studied. It has been observed that the minimum in the toroidal impurity velocity reaches negative values in low collisionality discharges, while it is positive in high collisionality discharges. Moreover, the position of the minimum in the toroidal impurity velocity is correlated with the position of the ion temperature and density pedestal tops in high collisionality discharges, while it is only correlated with the position of the ion temperature pedestal top in low collisionality discharges.