Modification of SOL profiles and fluctuations with line-average density and divertor flux expansion in TCV

Abstract

Abstract. A set of Ohmic density ramp experiments addressing the role of parallel connection length in modifying Scrape Off Layer (SOL) properties has been performed on the TCV tokamak. The parallel connection length has been modified by varying the poloidal flux expansion f x . It will be shown that this modification does not influence neither the detachment density threshold, nor the development of a flat Scrape Off Layer (SOL) density profile which instead depends strongly on the increase of the core line average density. The modification of the SOL upstream profile, with the appearance of what is generally called a density shoulder, has been related to the properties of filamentary blobs. Blob size increases with density, without any dependence pn the parallel connection length both in the near and far SOL. The increase of the density decay length, corresponding to a profile flattening, has been related to the variation of the divertor normalized collisionality Λ div [1,2], showing that in TCV the increase of Λ div is not sufficient to guarantee the SOL upstream profile flattening.

Scrape-off layer transport and filament characteristics in high-density tokamak regimes

“…In Figure 15 panels (b), (c) and (d), the different contributions are shown still as a function of edge density normalized to Greenwald fraction. We clearly see that the larger variations are observed for τ b , which increase with n e e /n G in agreement with the results reported in [9], with the increase of auto-correlation time observed in AUG [21] and with similar analyses based on LiB diagnostic on AUG as well [53]. It is worth noting that these results seems to contradict observations obtained using GPI on C-Mod [48] or probe measurements still in TCV [47] where the pulse shapes of the intermittent structures were insensitive to the changes in density.…”

“…The role of enhanced convective filamentary transport in the formation of SOL density shoulder has been already suggested [3,4,7,9], even though reduced parallel losses could also influence the process. The relation between profile evolution and blob-sizes has been investigated in the present scan using properly designed probes.…”

Investigation into the formation of the scrape-off layer density shoulder in JET ITER-like wall L-mode and H-mode plasmas

“…In summary of our judgment of shoulder formation mechanisms, we can agree with previous work [33,35] that parallel resistivity cannot be the sole mechanism for shoulder formation (may be necessary but not sufficient). However, unlike Carralero [35], the second mechanism appears related to divertor recycling as quantified through I-D a for attached plasmas.…”

“…There is broad agreement between our study and those on TCV [33] for L-mode discharges where increases in parallel resistivity (Λ div > 1) are a ÔnecessaryÕ, but not a ÔsufficientÕ condition for SOL density formation. This conclusion came from a set of discharges where Λ div >> 1 (near or far SOL) and yet, in some cases SOL density shoulders occurred, and in other discharges shoulders did not form [33]. In the case of our JET L-mode studies we reach the same conclusion but through different tests: In the case of N 2 seeding, changes in Λ div do not lead to increases in shoulder amplitude, with indications that shoulder amplitude can decrease.…”

“…What leads to the situation where Λ div rises strongly and no shoulder increase occurs? Is this consistent with Λ div being a necessary condition [33,35] for shoulder formation and/or growth?…”