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arXiv · 1809.03206

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

N. R. Walkden·B. Labit·H. Reimerdes·J. Harrison·T. Farley·P. Innocente·F. Militello·the TCV Team·the MST1 Team
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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.

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N. R. Walkden, B. Labit, H. Reimerdes, J. Harrison, T. Farley, P. Innocente, F. Militello, the TCV Team, the MST1 Team. 2018-09-10. Fluctuation characteristics of the TCV snowflake divertor measured with high speed visible imaging. https://doi.org/10.1088/1361-6587/aae005

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