ICRF antenna-plasma interactions and its influence on W sputtering in ASDEX upgrade

Analysis of the W concentration during ICRF over AUG experimental campaigns confirms the critical role of W antenna limiters for the W content in plasma, though other structures connected to antennas along magnetic field lines cannot be neglected as W sources. Abrupt changes of spectroscopically measured W sputtering patterns are observed which correlate with step-wise changes of connection lengths at antenna limiters. Analysis of discharges with the reversed direction of toroidal magnetic field shows less W release compared to identical discharges with the normal direction. The lower W release is accompanied by lower intensity of fluctuations of reflected ICRF power in the 1-60 kHz range. The observations suggest that local magnetic geometry and density convection at the antennas are at least as important for the W sputtering as the distribution of RF near-fields at the antenna. Measurements of DC currents flowing through the antenna limiters show that the limiters at the active antenna collect predominantly negative DC currents whereas those distant from the active antenna collect predominantly positive DC currents. The latter decrease and become more negative when the intensity of the RF pickup measured at the limiters increases. The mutual compensation between the positive and negative currents can lead to lower values of the DC current than those expected from simplified theoretical models of the RF/DC circuit.