Scrape Off Layer (SOL) transport and filamentary characteristics in high density tokamak regimes

A detailed cross-device investigation on the role of filamentary dynamics in high density regimes has been performed within the EUROfusion framework comparing ASDEX Upgrade (AUG) and TCV tokamaks. Both devices have run density ramp experiments at different levels of plasma current, keeping toroidal field or q<sub>95</sub> constant in order to disentangle the role of parallel connection length and the current. During the scan at constant toroidal field, in both devices SOL profiles tend to develop a clear Scrape Off Layer (SOL) density shoulder at lower edge density whenever current is reduced. The different current behavior is substantially reconciled in terms of edge density normalized to Greenwald fraction. During the scan at constant q<sub>95</sub> AUG exhibits a similar behaviour whereas in TCV no signature of upstream profile modification has been observed at lower level of currents. Thelatter behaviour has been ascribed to the lack of target density roll-over. The relation between upstream density profile modification
 and detachment condition has been investigated. For both devices the relation between blob-size and SOL density e-folding
 length is found independent of the plasma current, with a clear increase of blob-size with edge density normalized to Greenwald fraction 
 observed. ASDEX Upgrade has also explored the filamentary behaviour in H-Mode. The experiments on AUG focused on the role of neutrals, performing discharges with and without the cryogenic pumps, highlighting how large neutral pressure not only in the divertor but at the midplane is needed in order to develop a H-Mode SOL profile shoulder in AUG. 

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Nuclear Fusion
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 Record created 2019-09-10, last modified 2020-04-20

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