Reconstruction of visible emission in ASDEX Upgrade divertor

A tomographic algorithm for obtaining the 2D emission of D_α and D_γ in the divertor region of ASDEX Upgrade is developed. It allows for characterizing the time and space evolution of the D emission in the visible range, in particular when the electron density increases during the discharge. At higher Greenwald fraction, the density profile in the scrape-off layer (SOL) becomes flatter, with the formation of a shoulder. With this phenomenon, a change in the SOL transport is observed, with a modification in the behavior of turbulent blobs, which become larger. At the same time, the tomography shows that visible emission, which is localized in the inner divertor target at lower n_e, extends in the whole divertor region when density increases, and this effect can be due to a change in n_e, T_e and neutral profile density. From the measurements of D_α and D_γ emission it is possible to estimate these quantities, and thus studying the role of the divertor neutrals on the shoulder formation. In fact they could lead to density shoulder formation by reducing the flow of ions parallel to the field and by changing the radial electric field affecting SOL turbulence and blobs characteristics [1].

[1] A.Wynn et al., 2018 Nucl. Fusion 58 056001