Edge poloidal impurity asymmetry studies using gas puff based charge exchange recombination spectroscopy at the ASDEX Upgrade tokamak

Description

Nuclear fusion, the energy source of the stars, is expected to be a clean and abundant source of energy on Earth in the near future. However, fusion reactors have to face different challenges due to the high temperatures required to reach ignition. One of the major concerns is the integrity of the first wall of the machine. If enough heat is transported from the fuel, which is in plasma state, to the wall of the reactor, the integrity of the machine could be at stake. The injection of impurities in the plasma has shown to reduce the heat loads in the reactor walls via radiation. Hence, understanding the physics of these impurities and their distribution along the plasma edge is a requirement for future fusion devices. Furthermore, some impurity species have been shown to be beneficial for the plasma confinement and thus, for fusion performance. The work presented in this thesis is based on the Charge eXchange Recombination Spectroscopy (CXRS) technique. This technique exploits the light emitted after the charge transfer between injected neutrals and ionized impurities of the plasma. The emitted light gives information on the temperature, rotation and density of the observed species. Normally, the CXRS measurements are taken at the Low Field Side (LFS). However, this work is focussed on the study of the impurity properties at two different poloidal locations. For this purpose, the High Field Side (HFS) gas puff based CXRS system of the ASDEX Upgrade tokamak has been upgraded. This diagnostic injects thermal neutrals into the plasma to produce the charge exchange reactions. A new piezo driven gas valve provides higher signal to noise ratio and a better characterization of the background light. The upgraded optical heads have 16 lines of sight (LOS) each, covering around 7 cm of the HFS edge region. While impurity temperature and rotation can be obtained directly from the emitted light, the evaluation of the impurity density requires information on the injected neutral density. In order to enable impurity density measurements from gas puff based CXRS diagnostics, a new gas puff module has been included in the FIDASIM code. This module simulates the injection of neutrals by a gas puff system and provides the generated neutral population. Several experiments have been carried out at ASDEX Upgrade to study edge poloidal impurity asymmetries in different scenarios. In H-mode, asymmetries in the toroidal and poloidal impurity rotations have been found. Impurity densities obtained with the new gas puff module show poloidal asymmetries between HFS and LFS. Close to the separatrix, the HFS impurity density exceeds the LFS values. These density asymmetries are in agreement with the poloidal impurity flow structure. The impact of the heating scheme on edge poloidal impurity asymmetries is addressed. The comparison between neutral beam heating and wave heating results in stronger impurity density asymmetries when wave heating is applied. In L-mode, no asymmetries in the toroidal rotation, poloidal rotation, density and temperature of the impurities have been found. In I-mode, asymmetries in the toroidal impurity rotation have been measured. In summary, the existence of impurity density asymmetries is linked to strong poloidal impurity flows at the edge, as it occurs in H-mode plasmas. In general, the edge impurity properties measured at the LFS should not be understood as global parameters. The edge poloidal impurity asymmetries observed in this work should be taken into account in studies where impurities play an important role.

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