Fluctuation characteristics of the TCV snowflake divertor measured with high speed visible imaging

TL;DR

This study analyzes the fluctuation characteristics in the snowflake divertor of TCV using high-speed imaging, revealing how X-point proximity affects filament motion, fluctuation coherence, and cross-field transport.

Contribution

It provides new insights into how the X-point gap influences filament dynamics, fluctuation coherence, and transport in the snowflake divertor configuration.

Findings

Fluctuations are spatially structured according to magnetic field lines.

Close X-points lead to decorrelated, locally generated filamentary structures.

Filament motion shifts from poloidal to radial as X-point gap decreases.

Abstract

Tangentially viewing fast camera footage of the low-field side snowflake minus divertor in TCV is analysed across a four point scan in which the proximity of the two X-points is varied systematically. The motion of structures observed in the post- processed movie shows two distinct regions of the camera frame exhibiting differing patterns. One type of motion in the outer scrape-off layer remains present throughout the scan whilst the other, apparent in the inner scrape-off layer between the two nulls, becomes increasingly significant as the X-points contract towards one another. The spatial structure of the fluctuations in both regions is shown to conform to the equilibrium magnetic field. When the X-point gap is wide the fluctuations measured in the region between the X-points show a similar structure to the fluctuations observed above the null region, remaining coherent for multiple toroidal turns of the magnetic field and indicating a physical connectivity of the fluctuations between the upstream and downstream regions. When the X-point gap is small the fluctuations in the inner scrape-off layer between the nulls are decorrelated from fluctuations upstream, indicating local production of filamentary structures. The motion of filaments in the inter-null region differs, with filaments showing a dominantly poloidal motion along magnetic flux surfaces when the X-point gap is large, compared to a dominantly radial motion across flux-surfaces when the gap is small. This demonstrates an enhancement to cross-field tranport between the nulls of the TCV low-field-side snowflake minus when the gap between the nulls is small.