Stark split Dα spectra of BEP diagnostics

New concept for measuring the edge magnetic field line angle

A new edge diagnostic for the measurement of the magnetic field line angle has been installed on the ASDEX Upgrade tokamak. The new system relies onthe motional Stark effect and is based on the simultaneous measurement ofthe polarization direction of the linearly polarized π (parallel to the electricfield) and σ (perpendicular to the electric field) lines of the Balmer spectralline Dα. In this setup, we use 3 independent observations of the same spot with polarizer angles α ≈ 0°, 45° and 90° (see figure), where 0° serves as a reference for the π lines, 90° for the σ lines, and 45° measures a combination of both σ and π lines. In order to extract the magnetic field line angle a forward model for the Stark split Dα spectra has been developed.

More details on this work can be found in:
[1] E. Viezzer et al., RSI 87 11E528 (2016)
[2] R. Dux et al., 42nd EPS Conference, P1.121, Lisbon, 2015


Poloidal asymmetries of parallel and poloidal flows (upper plot), measured impurity density at the LFS and HFS (bottom plot)

Pedestal Poloidal Impurity Asymmetries

The interplay between plasma flows and transport is a key ingredient of the physics that governs the edge transport barrier (ETB) of an H-mode fusion plasma. Impurity transport is of particular importance as it is crucial for understanding and controlling the impurity content in the plasma. In the ETB, the impurity particle transporth as been observed to be determined by neoclassical theory and the question arises whether the particle transportlevel is affected if poloidal impurity asymmetries are present in the plasma edge. Using the high-resolution edge charge exchange diagnostic suite at ASDEX Upgrade we could study asymmetries on the flux surfaces at the plasma edge.
The measurements reveal that in the ETB the flow structure is asymmetric on the flux surfaces. The asymmetry in the flow pattern can be explained by an excess of impurity density at the HFS following the condition of divergence-free flows on a flux surface, which is based on the general continuity equation. Comparison of the measured flows to theoretical predictions based on the parallel momentum balance reveals the nature of theparallel impurity dynamics. The key features of the experimental data including the shape of the rotation profilesand the poloidal impurity density asymmetry are reproduced quantitatively for the first time.

More details on this work can be found in:
[1] E. Viezzer et al., NF 55 123002 (2015)
[2] E. Viezzer et al., PPCF 55 124037 (2013)
[3] T. Pütterich et al., NF 52 083013 (2012)
[4] R. M. Churchill et al., NF 53 122002 (2013)


Measured radial electric field and comparison to neoclassical theory

High-resolution measurements of the edge radial electric field

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. We have confirmed this mechanism as the location of the steepest ion pressure gradient ∇pi was shown with unprecedented accuracy to match the position of the largest Er shear. These measurements were performed using the high-resolution edge CXRS diagnostic suite available at ASDEX Upgrade.
We have found that, 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 themagnitude are in remarkably good agreement.

More details on this work can be found in:
[1] E. Viezzer et al., NF Letter 54 012003 (2014)
[2] E. Viezzer et al., NF 53 053005 (2013)
[3] E. Viezzer et al., PPCF 56 075018 (2014)