How the far scrape-off layer influences plasma confinement

Recent experiments [1] at ASDEX Upgrade have investigated the impact of the scrape-off layer (SOL) on the structure of the edge transport barrier (pedestal).

Stability diagrams of (top) a reference phase and (bottom) with nitrogen added (seeded) in the same discharge. The color scale indicates the growth rates of modes (blue = stable, green-red = increasing growth rate) as function of the edge plasma current density and the pressure at the pedestal top. The ideal MHD stability analysis shows that nitrogen seeding leads to an extension of the stable area to higher pressures consistent with the experimental data indicated by the symbols.  The addition of nitrogen leads to an inward shift of the density profile and a 25 % increase in the plasma pressure at the pedestal top.

Particular focus has been placed on poloidal fuelling asymmetries, such as those offered by the presence of a high density region localised in the SOL on the high-field-side of the plasma and generated by gas fuelling at moderate heating power.When this feature is present, it fuels the plasma close to the separatrix, effectively shifting the density profile in the pedestal outwards [2]. This outward shift has a negative effect on the pedestal stability, borne out by both predictive and interpretive pedestal modelling. In both experiment and modelling, a small shift of 5 mm reduces the allowed pedestal top pressure by 25 %, demonstrating how the far SOL can influence plasma confinement.
By puffing a radiating impurity (such as nitrogen, methane, or neon), the density in the high density region can be reduced or totally eliminated. This also removes its effect on plasma fuelling and allows the density profile to shift radially inwards, leading to a corresponding increase of the pedestal top pressure (see Figure). The understanding and control of this feature offers a new tool to influence pedestal stability and improves the modelling of confinement in future devices.

This work was presented at the EPS and IAEA conferences in 2016.

[1] M. Dunne et al., Plasma Physics and Controlled Fusion (accepted)
[2] M. Dunne et al., Plasma Physics and Controlled Fusion 59-1, 014017 (2017)

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