Symmetries and Asymmetries in the Divertor Detachment in ASDEX Upgrade

Divertor detachment is a prerequisite for large burning plasma fusion devices. The understanding of the processes leading to divertor detachment is currently incomplete and a reliable prediction for future large scale devices out of reach. In the full-metal ASDEX Upgrade divertor detachment has been studied for Ohmic and L-mode density ramp discharges with deuterium, hydrogen and helium as a fuelling species and both toroidal field directions. For hydrogenic fuelling species the integrated ion flux at the inner target reaches its peak value at an only ∼ 10 - 20% lower ne than for the outer divertor target. However, the maximum of the integrated ion flux to the inner target remains well below what is observed at the outer target.
The numerical transport code package SOLPS5.0 is applied. Under common assumptions for the model it is found that the roll over of the ion flux at the inner and outer targets occurs at a similar ncsep. Contrary to experimental findings the simulated peak values remain comparable for both target plates and no strong reduction of the ion flux density is seen for the inner divertor. An experimental observation of high ne in the far SOL during a fluctuating phase indicates that plasma is transported outward into the far SOL while radiation increases over a large volume. Based on these observations various levels of perpendicular transport in the X-point region of the inner divertor are assumed in the model and impurities are included. The simulated inner target peak ion flux is reduced by a factor of 3 - 4.