Highlights 2013

Research news from the division Plasma Edge and Wall

High-accuracy radial electric field measurements at ASDEX Upgrade

The understanding of the physics relevant to the edge transport barrier (ETB) of an H-mode fusion plasma is of crucial importance as it leads to steep gradients at the plasma edge which implies a confinement gain at the boundary of the plasma. This improvement propagates into the plasma core, where a hot and dense plasma is required for fusion. The ETB is thought to be caused by a sheared plasma flow perpendicular to the magnetic field which is equivalent to a sheared radial electric field Er. In the present work this mechanism has been confirmed as the location of the steepest ion pressure gradient ∇pi was shown with unprecedented accuracy to match the position of the largest Er shear.

The installation of a new edge charge exchange recombination spectroscopy (CXRS) diagnostic at ASDEX Upgrade (AUG) enables high temporally and radially resolved measurements of the poloidal and toroidal rotation velocities, densities and temperatures of impurity ions. Thus, it provides all measurements for deriving Er from the radial force balance equation. The new CXRS measurements, combined with the unique edge diagnostic suite available at AUG, allowed for the high-accuracy localization (2–3 mm) of the Er profile based on an established alignment procedure. Using this technique it has been found that the maximum in the ExB shearing rate (ωExB) coincides with the steepest ∇pi and lies inside the position of the minimum of the Er well (see figure 1). This suggests that the negative shear region is the important region for the formation of the pedestal.

In the radial force balance of impurities the poloidal rotation contribution yields the dominant term in the evaluation of Er at the plasma edge. For the main ions, the Er minimum coincides with the maximum pressure gradient term ∇pi/eni supporting that the Er well is created by the main ion species. The fact that ∇pi/eni matches Er in the ETB is consistent with the main ion poloidal flow being at neoclassical levels. Quantitative comparisons between neoclassical predictions and experimental measurements of both impurity and main ion poloidal rotation show that the sign and the magnitude are in remarkably good agreement.

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