Influence of Gas Injection Location and Magnetic Perturbations on ICRF Antenna Performance in ASDEX Upgrade

In ASDEX Upgrade H-modes with H-98 approximate to 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 7.5.10(21) 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-ICRF<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 Gamma(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 Gamma(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 Gamma(W) and Y-W at the antenna limiters.