Divertor turbulence characterisation using Gas Puff Imaging in the TCV tokamak

The performance of magnetic confinement fusion devices, such as tokamaks, is strongly correlated to the phenomena that occur in the boundary region of the plasma core that faces the wall of the device. The dominant cross-field transport mechanisms from the confined region into the Scrape-Off-Layer (SOL) is turbulent. In the SOL, about half of this turbulent transport occurs in the form of field-aligned filaments moving radially outwards. To reliably predict the heat and particle fluxes at the target plates and extrapolate to future fusion reactors, a comprehensive understanding of turbulence throughout the entire SOL is imperative. This thesis aims to characterise fluctuations around the divertor region and to assess their role on cross-field transport. The TCV tokamak is an excellent candidate for turbulence studies, but the diagnostic coverage around the X-point, similarly to most present-day devices, was limited prior to this thesis. Therefore, two new Gas Puff Imaging (GPI) systems are designed and commissioned. This suite of GPIs enables a unique SOL coverage for 2D turbulence measurements with high temporal and spatial resolutions, in the vicinity of the X-point and along the divertor legs. To evaluate the capabilities of such an X-point GPI prior to construction, a neutral fluid model, called "zero-T model", is developed and utilised. The gas injection system is generalised for all 10 fuelling and seeding valves on TCV. These hardware developments are complemented by a number of extensive GPI analysis routines. A detailed study in L-mode discharges shows that statistical properties of SOL fluctuations have little poloidal variations, while vast differences are present in the 2D behaviour of filaments. Strongly elongated filaments, above the X-point, in the divertor far-SOL show a good consistency with field-line tracing from filaments at the outboard midplane (OMP), highlighting their connection as Upstream-Connected Filaments (UCFs). In the near-SOL of the outer divertor leg, short-lived, high frequency and more circular filaments are observed. These Divertor-Localised Filaments (DLFs) are examined in detail and a significant contribution of DLFs on cross-field transport is found. This thesis further examines divertor turbulence across a broad parameter range on the TCV. DLFs are identified as a common feature. Their relative fluctuation amplitudes and particle flux are found to significantly increase with plasma density and collisionality. The trends in sizes, velocities and fluctuation amplitudes of DLFs broadly align with UCFs and with filament properties at the OMP but are absent at detachment onset or at sufficiently high edge safety factor. Possible reasons are discussed and the relation with fall-off lengths of heat flux and density at the outer target are examined. The analysis is also extended to H-mode by comparing the filamentary transport in the SOL in type-I ELMy and small ELM H-mode scenarios. Finally, SOL filament properti