Tungsten sputtering during ICRF in ASDEX upgrade

Since 2006, 85% of plasma facing components in ASDEX Upgrade are tungsten (W) coated, including all poloidal limiters of ICRF antennas. Sputtering of W in ICRF heated discharges and the increase of W concentration in the plasma are issues. In the discharges with higher heating power and smaller antenna-plasma distance, the parallel RF fields are more localized at the antenna region, but at the same time the absolute values of W influx and sputtering yield at the limiters are higher. To reduce the W sputtering yield, a low plasma temperature at the limiters is preferable, as shown in a set of discharges with a strong gas puff and a variation of antenna-plasma distance. Calculations of near-antenna parallel fields with High Frequency Structure Simulator code shows a significant contribution of the antenna box structure (including the limiters) to the parallel voltages on the long field lines in front of the antenna. This can partially explain the poor localization of RF parallel fields observed in the experiment and the fact of no clear experimental evidence of the reduction of the RF fields when magnetic field is aligned with the Faraday screen inclination angle.