Study of the q-profile determination by pellet ablation at ASDEX Upgrade

This report presents a diploma-thesis investigating the possibility of q-profile determination at ASDEX Upgrade by observation of pellet ablation by use of a high speed framing camera. The q-profile is correlated to the tokamak's current density profile. Knowledge and control of the current density profile is of great interest for fusion research, as the profile governes plasma stability and confinement. It is possible to determine the current profile from measurements of the magnetic field. Pellet injection can be applied to visualize the magnetic field lines. The ablated material is ionized. The ionized particles stick to the magnetic field lines, as will be shown in this report. In the early stages the pellet is surrounded by a cold, dense plasma cloud. This cloud contains a small fraction of neutral particles, undergoing collisional excitation and emitting photons. Therefore it is possible to observe this plasma cloud. The inclination angle of the cloud with respect to the torus midplane shows the ratio Bvertical/Btoroidal. Only Bvertical is a function of the current density profile in the plasma, while Btoroidal is well known. For the observation of the pellet ablation with high temporal resolution a fast-framing camera was used. This camera was installed and brought into operation during the first period of this work. It was possible to observe the pellet ablation with high temporal (4μs) and spatial resolution for a large number (63) of frames. Special software for the analyses of the data has been developed. Different methods of data analysis have been applied in order to detect possible systematic errors in the procedures used. A detailed analysis of the errors will be presented. The results of the pellet ablation method will be compared to the results of other measurement techniques