Effects of drifts on scrape-off layer transport in W7-X

Abstract

Drifts affect particle, momentum, and energy transport in the scrape-off layer (SOL) of tokamaks and stellarators, altering plasma flows and creating asymmetries between divertors. To understand how drifts affect SOL transport in the W7-X island divertor, an experiment was performed to compare plasmas with matched core parameters but opposite magnetic field directions, and therefore opposite drift transport directions. Parallel flow measurements made with coherence imaging spectroscopy are interpreted with the aid of a diagnostic forward model and a 1D simple SOL model that includes the E × B drift. In low-density plasmas (n e < 2 × 1019 m−3), the poloidal E × B drift induces a large poloidal density asymmetry within the island SOL, as measured by divertor Langmuir probes. This in turn causes the parallel flow stagnation point to shift from the position halfway between targets to the X-point in the drift direction, leading to near-unidirectional flow throughout the SOL. As density increases, the effects of the poloidal E × B drift decrease substantially, resulting in a smaller density asymmetry and the development of a counter-streaming flow pattern. For the entire density range probed in this experiment (n e = 1.5–6 × 1019 m−3), the experimental observations are more consistent with the effects of the poloidal E × B drift than the radial E × B drift.

On the interaction between the island divertor heat fluxes, the scrape-off layer radial electric field and the edge turbulence in Wendelstein 7-X plasmas

“…The effect of magnetic configuration on E r,SOL formation must also be clarified before a quantitative comparison of our results can be carried out with drift experiments previously discussed, as they are typically carried out in low iota configuration. However, the qualitative trend relating connection length and T e gradients at the target is also consistent in those studies: the density over which the counter-streaming flows abruptly appear (indicating the reduction of the E r -induced stagnation point shift) is around 3 × 10 19 m −3 [63], substantially lower than the values over which the E r,SOL is reduced in the present work. This could be similarly explained by the longer connection lengths found in the low iota configuration when compared to the standard, which would lead to increased SOL width and lower values of n e and P ECH for a given value of E r,SOL (as is the case between high iota and standard configuration in [18]).…”

“…This is consistent with the reduction of E r,SOL in figure 8(a). More recent studies [62,63] have investigated the impact of drifts on parallel flows, as observed by coherence spectroscopy. Their results point towards a rather similar picture: for low density plasmas, strong poloidal drifts cause a strong deviation of the parallel stagnation points with respect to the X point, where they would be expected if the effect of drifts is not taken into account (as is the case of EMC3-EIRENE simulations discussed in those works).…”

A study of turbulent filaments in the edge plasma of the Wendelstein 7-X stellarator

“…The paper also claims that sheared flows in the edge caused by the sign flip of E r can cause a moderate reduction in turbulence. Poloidal non-uniformity of density and temperature profiles have also been reported [34,35]. Based on the findings cited here, it is reasonable to expect that as magnetic geometry changes, so do edge profiles.…”

Multi-machine benchmark of the self-consistent 1D scrape-off layer model DIV1D from stagnation point to target with SOLPS-ITER

“…In terms of general 1D modeling efforts, many of the recommendations in [7] still hold, including drifts [24,39], molecules [10], and better impurity emission rates [40]. Additionally, one could estimate peak fluxes by superimposing skewed Gaussians or distributions from EMC3-Lite [41].…”