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

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

Fluctuation Characteristics of the TCV snowflake divertor measured with high speed visible imaging 150 150 Mathew
UKAEA-CCFE-PR(18)34

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

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 apparent flow observed in the post-processed movie shows two distinct regions of the camera frame exhibiting differing flow patterns. One flow 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 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 between the two nulls. These filaments have a spread of velocities, with some capable of reaching the proximity of the outer X-point, providing a possible way for power and particles to spread into the un-connected divertor legs of the snowflake. In all cases the inner X-point is quiescent, consistent with a hypothesis that when the second X-point lies on a flux-surface that passes through this quiescent region filaments from upstream are inhibited in their connection into the divertor and therefore local production of filamentary structures can occur.

Collection:
Journals
Journal:
Plasma Physics and Controlled Fusion
Publisher:
IOP
Published date:
28/09/2018