3D Characterisation of Turbulent Plasma Filaments Using Gas Puff Imaging in the Tokamak à Configuration Variable

A key challenge for the development of fusion reactors based on magnetic confinement, such as tokamaks and stellarators, is the control of the turbulent processes. The most prominent feature of turbulence in the Scrape-Off Layer (SOL), the volume between the main core plasma and the vessel wall, are filaments, also known as blobs. Besides influencing particle and energy confinement, filaments pose a severe threat to plasma-facing components in the reactor. The mechanisms governing SOL filaments, particularly their dependence on SOL conditions and geometry, are not yet fully understood, and extrapolations to future devices are challenging. To study plasma turbulence and filaments in the SOL of the Tokamak à Configuration Variable (TCV) at EPFL, in collaboration with MIT-PSFC, we designed, commissioned, and employed a Gas Puff Imaging (GPI) diagnostic. Here, we present this new diagnostic to explore the cross-field dynamics of outboard midplane filaments. We describe the light detection and the innovative control systems for D2 and He gas injection, which is currently also being adopted as the default gas injection scheme at TCV. Furthermore, we present and compare different analysis techniques to measure the filament properties, such as size, velocity, and appearance frequency, and discuss the scenarios in which these techniques are best applied.With GPI, we characterise the poloidal and parallel properties of turbulent filaments in both attached and detached divertor conditions across a wide range of plasma core densities (for Greenwald fractions ranging from 0.09 to 0.66) in diverted L-mode plasma configurations. Filament radial velocities and sizes increase with increasing core density (from 390m/s to 800m/s and from 8.5mm to 13.4mm). Interpreting the filament behaviour in the context of the two-region model by Myra et al., Phys. Plasmas, 2006, they are found to populate the ideal-interchange regime (Ci) in discharges at very low densities (fG ≲0.2) and the resistive X-point regime (RX) for all other discharges. The measured size and velocity scalings confirm this interpretation. Correlating the signal from various diagnostics, precisely aligned along the magnetic field in the SOL (GPI, wall-embedded Langmuir probes, and a reciprocating divertor probe), we study the parallel extension of filaments. In agreement with the theory, we find that filaments in the Ci and RX regimes extend from the midplane into the divertor region in the far SOL. However, in the near SOL, unlike RX filaments, Ci filaments are found to be disconnected at the X-point. This effect is ascribed to magnetic shear having a stronger impact on the smaller and hotter filaments in the C i regime, compared to the larger and colder ones in the RX regime. Following these findings, we explore how filamentary turbulence is affected by magnetic geometry. We present initial experiments in which the effect of alternative divertor geometries on upstream filaments is investigated. These indicate that the position of a secondary X-point in snowflake and X-point target divertor configurations can affect filament cross-field size and velocity, as long as the parallel connection length is significantly altered. Also linked to strong variations in parallel connection length, almost full suppression of filaments is observed in plasmas with strongly negative core shapes. This has potentially important implications for the prospects of negative triangularity as a reactor solution.