Speaker
Description
Toroidal symmetry is one of the basic design principles of a Tokamak. ASDEX Upgrade (AUG) has recently undergone a major hardware upgrade [Herrmann FED 2017,Herrmann FED 2019,Teschke FED 2019] in order to study alternative divertor configurations [Lunt NME 2017,Pan PPCF 2018]. Apart from a pair of in-vessel coils, a new inner and outer upper target and a new cryo-pump a large set of new edge diagnostics as well as new gas valves located at several toroidal locations have also been installed. Due to the fact that the total flux of ions to all plasma facing components is typically larger than the injected deuterium flux by more than an order of magnitude, the local gas puff is typically regarded as a small perturbation and its exact position to be rather irrelevant. However, in the 2025 AUG campaign substantial toroidal asymmetries were observed in the divertor shunt current measurements [Giannone EPS 2025]. Puffing deuterium with a gas valve located near the strike line in segment 14 the currents in segments 2, 6 and 10 are approximately equal at -10 A per tile, while the one in segment 14 shows a dramatic increase to about +40 A. The phenomenon is explained by a toroidally localized detachment of the plasma in segment 14 accompanied by a reversal of the thermo-currents. The toroidal asymmetry in the thermo currents remains stable for several hundred milliseconds up to the point where the nitrogen seeding is switched-on and the plasma undergoes a transition to a globally detached state with symmetric currents of the order of +10 A per tile. A breaking of the toroidal asymmetry is also observed when going to a low-field side snowflake minus configuration where the gas is puffed into the secondary X-point, i.e. into a region with long connection lengths to the strike lines. If the gas is injected by another valve at the same radial position but in segment 2, the current asymmetry also shifts to segment 2 as expected, confirming the robustness of the observations. In this contribution we will make an attempt to describe the observations by the 3D transport code EMC3-EIRENE and study the possibility of using the localized gas puff system as a re-attachment avoidance system.