Imaging neutral particle analyzer for fast-ion transport measurements in the ASDEX upgrade tokamak

Abstract

In future fusion reactors, suprathermal particles (fast ions, FI) will play a key role in the generation of fusion power as they are an important source of energy (heating) and momentum (current drive). A loss of their confinement will lead to a decrease in reactor performance, and, when localized and intense, to damage in the first wall components. Understanding the mechanisms behind the suprathermal particle transport and losses is capital for achieving a future fusion power plant. One of the main observed causes for the FI transport and eventual loss is their interaction with a wide range of electromagnetic fluctuations. An accurate understanding of the fast-ion behavior in the presence of magnetohydrodynamic fluctuations is required for achieving a good fast-ion confinement. To this end, new diagnostics are being developed to measure the fast-ion distribution over a broad region of the phase space with high resolution. In this PhD thesis, an Imaging Neutral Particle Analyzer (INPA) has been installed and operated at the ASDEX Upgrade (AUG) tokamak, located at the Max Planck Institute for Plasma Physics in Garching (Germany). INPA employs the operational principles of both fast-ion loss detectors (FILD) and neutral particle analyzers (NPA) to measure the fast-ion distribution in energy and radius. This diagnostic system analyses fast neutrals that emerge from charge exchange (CX) reactions between fast ions and neutral particles. These fast neutrals are ionized through an ultra-thin carbon foil located within the in-vessel optical head and are deflected towards a scintillator using the local magnetic field of the tokamak. From the impinging location of a particle on the INPA scintillator, its energy and velocity projection along the magnetic field lines can be deduced. The use of an active source of neutrals enables the direct correlation of this velocity projection with the radial position of the fast ion. The FILDSIM code, which facilitates the calculation of synthetic signals for the FILD diagnostic, has undergone a major upgrade to handle the INPA diagnostic. This upgrade includes a model for simulating the scattering and energy loss of fast neutrals within the carbon foil. Additionally, it encompasses a model for estimating the scintillator yield and the capacity to conduct tomographic reconstructions. This updated code has been benchmarked against experimental data during the 2021-2022 campaign, showing an excellent agreement between simulations and measurements. Tomographic inversions also agree with neoclassical calculations during MHD quiescent phases. Fast-ion acceleration during second harmonic ion cyclotron resonance heating has been characterized and compared to simulations. The agreement found serves as validation of these codes for their extrapolation to future machines. Fast-ion flows driven by Alfvén eigenmodes have been measured for the first time at ASDEX Upgrade. The observed flows align well with the theoretical models and with fullorbit simulations.