Influence of gas injection location and magnetic perturbations on ICRF antenna performance in ASDEX Upgrade

In ASDEX Upgrade H-modes with H₉₈≈0.95, similar effect of the ICRF antenna loading improvement by local gas injection was observed as previously in L-modes. The antenna loading resistance R_a between and during ELMs can increase by more than 25% after a switch-over from a deuterium rate of 7.5⋅10²¹ D/s injected from a toroidally remote location to the same amount of deuterium injected close to an antenna. However, in contrast to L-mode, this effect is small in H-mode when the valve downstream w.r.t. parallel plasma flows is used. In L-mode, a non-linearity of R_a at P_ICRP<30 kW is observed when using the gas valve integrated in antenna. Application of magnetic perturbations (MPs) in H-mode discharges leads to an increase of R_a>30% with no effect of spectrum and phase of MPs on R_a found so far. In the case ELMs are fully mitigated, the antenna loading is higher and steadier. In the case ELMs are not fully mitigated, the value of R_a between ELMs is increased. Looking at the W source modification for the improved loading, the local gas injection is accompanied by decreased values of tungsten (W) influx Γ_W from the limiters and its effective sputtering yield Y_w, with the exception of the locations directly at the antenna gas valve. Application of MPs leads to increase of Γ_W and Y_w for some of the MP phases. With nitrogen seeding in the divertor, ICRF is routinely used to avoid impurity accumulation and that despite enhanced Γ_W and Y_W at the antenna limiters.