Velocity-space resolved measurements of fast-ion losses due to magnetohydrodynamic instabilities in the ASDEX Upgrade tokamak.

Abstract

The confinement of suprathermal ions in magnetically confined fusion plasmas is essential to ensure a good fusion performance. Auxiliary heating systems - and fusion reactions themselves - create fast-ion populations, which must be confined for long enough time to transfer their energy to the bulk of the plasma via Coulomb collisions. A good confinement of the fast-ions is needed to ensure a good plasma heating and current drive. Furthermore, if fast-ions are lost to the walls of the machine in a sufficiently intense and localized way, irreversible damage to plasma facing components can be provoked. Therefore, a deep understanding of the mechanisms leading to fast-ion transport and eventual losses is of paramount importance. The need to develop control tools to avoid these losses is now becoming a priority in the roadmap to future burning plasma experiments. In this sense, scintillator based fast-ion loss detectors (FILD) have been proven to be a powerful diagnostic to study the interaction between fast-ions and magnetohydrodynamic (MHD) instabilities, contributing to unravel the physics underlying the transport mechanisms. In this thesis the study of fast-ion losses in the presence of various MHD instabilities in the ASDEX Upgrade tokamak is presented. A comprehensive description of scintillator based FILDs response is given here for the first time, with a special focus on its velocity-space resolution. As any other instrument in physics, the resolution of the system is finite, in this case due to the size of the detector pinhole and the gyrophase distribution of the measured ions. The detector response is described in terms of a simple model based on a weight function formalism. The model allows to calculate synthetic FILD signals given a velocity-space distribution of fast-ions reaching the detector pinhole. This enables a direct comparison between simulations and experimental measurements, taking into account the response of the instrument. Velocity-space tomography techniques have been implemented, which allow to obtain the undistorted velocity-space distribution of fast-ions reaching the detector pinhole. The tool improves the velocity-space resolution of FILD measurements, which can potentially reveal additional details in the velocity-space dynamics of fast-ion losses. These improvements have been applied to the study of different MHD induced fast-ion losses. The first velocity-space resolved absolute measurement of fast-ion losses in the presence of a tearing mode in the ASDEX Upgrade tokamak is presented. An estimate of the different loss channels in absolute terms is given. These measurements, supported by simulations of fast-ion losses including the modelling of ICRF power deposition, suggest that MHDinduced fast-ion losses are responsible for the anomalously large heat load measured by the FILD detector, which is then damaged irreversibly. This case represents a perfect example of the potential consequences derived from a bad confinement of the fast-ion population. The velocity-space dynamics of fast-ion losses induced by edge localized modes (ELMs) are investigated. It is observed that, in low collisionallity discharges, a fastion population with energies well above the main neutral beam injection (NBI) - dubbed high-energy feature - is measured. The high-energy feature is correlated with the occurrence of ELMs. The pitch-angle structure of the high-energy feature is observed to change with the edge safety factor and the NBI source, which is found to be related with the topology of the orbits. The high-energy feature is also observed in mitigated ELM regimes, while not seen in ELM suppressed regimes. This observation is interpreted as the acceleration of beam ions during the ELM crash, when magnetic reconnection is believed to take place. A resonant interaction between the beam-ions and the parallel electric fields emerging during the ELM is proposed as a possible acceleration mechanism, and is observed to qualitatively agree with the main experimental results. The observation motivates a kinetic description of fast-ions in ELM models. Additionally, the finding might also be of interest to the astrophysics community, where acceleration of charged particles in plasmas is ubiquitous, in particular in solar flares, which show similarities with ELMs in tokamaks.