Study of turbulent electron temperature fluctuations and their cross-phase angles with electron density fluctuations at the ASDEX Upgrade tokamak

Abstract

Turbulence is generally the dominant mechanism for energy transport in tokamak fusion reactors and has thus far prevented tokamaks from producing net energy. The measurement of plasma turbulence is important for improving our understanding of turbulent transport, for the validation of physics-based transport models, and ultimately for guiding the design of future fusion reactors. This Thesis presents an investigation of turbulent electron temperature fluctuations, their cross-phase angles with electron density fluctuations, and the impact of turbulent transport on plasma confinement at the ASDEX Upgrade (AUG) tokamak. This work spans three different confinement regimes - the low (L), improved (I), and high (H) confinement modes - as well as the physics of both core and edge turbulence.

As part of this Thesis, a new Correlation Reflectometer Radiometer (CRR) diagnostic was designed and installed on AUG. Novel measurements in the edge of an H-mode plasma across a transition from small Edge Localized Modes (ELMs) to ELM-free operation show that the cross-phase angle between turbulent electron density and temperature fluctuations changes from approximately 90 degrees to 120 degrees, indicating changes in the physics driving transport. Of note is that this relatively large change in the cross-phase angle occurred despite only small changes in the edge profiles and gradients, indicating that the CRR diagnostic and the measured turbulence are particularly sensitive to small variations in plasma conditions that yield changes in ELM stability.

Experiments and turbulence measurements using the Correlation Electron Cyclotron Emission (CECE) diagnostic were also conducted across the Linear Ohmic Confinement to Saturated Ohmic Confinement (LOC-SOC) transition in an ohmically-heated L-mode plasma at AUG. A multi-regime confinement database was then built to include I-mode and H-mode plasmas. A comparative analysis of these plasmas was performed. The results demonstrate for the first time that these three distinct confinement regimes, distinguished primarily by their different edge characteristics, nevertheless exhibit similar trends in energy confinement and density peaking as the core plasma effective collisionality increases in conditions with strong electron heating. One possible explanation for this result is that similar changes in core turbulence drive are occurring in all three confinement regimes. This study also found that two additional effects, including the strength of the neutral beam injection (NBI) particle source relative to transport effects as well as fast ion activity, are correlated with the observed trends. Therefore, multiple mechanisms could be playing a role in the underlying physics.

Novel methods were also developed to improve the spatial and temporal resolution of turbulence diagnostics. In addition, real-time applications of turbulence diagnostics relevant to tokamak operation were explored. Overall, the results of this thesis research contribute to our understanding of turbulence and transport across multiple confinement regimes and detail new methods for measuring turbulent fluctuation amplitudes and cross-phase angles.