Energy load on first wall components in high density, small ELM regimes in ASDEX Upgrade
A. Redl, T. Eich, N. Vianello, P. David
Abstract
A set of dedicated shots with constant heating power combining NBI (Neutral Beam Injection) and ECRH (Electron Cyclotron Resonance Heating) have been executed at the ASDEX Upgrade (AUG) tokamak using a highly shaped small ELM magnetic geometry in order to investigate the differences on heat loads at the first wall of AUG by means of the revised cooling water calorimetry . The flattop fuelling has been varied on a shot-to-shot basis while keeping constant all the other parameters, exploring a large range of the density operational space of AUG. Reducing the fuelling rate allows a gradual transition from small ELM regime at high density into a Type-I ELMy H-mode. Additionally, the safety factor q has been increased/decreased by changing B tor , which leads also to variations in the heating scheme. With the lowest flattop fuelling, reaching a Type-I ELMy H-mode, a fraction of ∼ 6% of calorimetrically captured energy loads at the first wall normalized against the entire calorimetrically captured energy has been recorded, whereas in the highest fuelling case, being in a small ELM scenario, the first wall heat loads are increased up to 16%. Considering all cases reveals an almost linear trend between the fraction of energy deposited at the first wall and the turbulence control parameter α t . This observation is consistent with an enhancement of the filamentary activity at higher α t , when the plasma turbulence is dominated by resistive interchange modes at the separatrix.