Compatibility of ICRF antennas with W-coated limiters for different plasma geometries in ASDEX Upgrade

Since 2005/2006 all side limiters of ICRF antennas in ASDEX Upgrade are W-coated. Comparison of two plasma shapes with high and low triangularity δ shows that the high δ shape leads to the appearance of an intensified antenna–plasma interaction and to a significantly stronger W sputtering. In particular, for (0 π/2) antenna phasing W fluxes as high as 3 × 1019 m−2 s−1 were measured, a factor of 10 higher than W fluxes for dipole phasing while the difference between the phasings for the low δ discharges is small. Considering general evolution of local W fluxes and sputtering yields at the limiters, it takes ≈20 shots after boronization to increase W yields at the antenna limiters by a factor of 100. More than 100 shots pass until the yields at the limiters far from the antennas reach the same values as at the antenna limiters. W influxes and yields between ELMs show a dependence on RF power close to linear. In H-modes W yields can increase during ELMs by more than a factor of three.