Assessment of compatibility of ICRF antenna operation with full W wall in ASDEX Upgrade

The compatibility of ICRF (ion cyclotron range of frequencies) antenna operation with high-Z plasma facing components is assessed in ASDEX Upgrade (AUG) with its tungsten (W) first wall. The mechanism of ICRF-related W sputtering was studied by various diagnostics including the local spectroscopic measurements of W sputtering yield Y_W on antenna limiters. Modification of one antenna with triangular shields, which cover the locations where long magnetic field lines pass only one out of two (0π)-phased antenna straps, did not influence the locally measured Y_W values markedly. In the experiments with antennas powered individually, poloidal profiles of Y _W on limiters of powered antennas show high Y_W close to the equatorial plane and at the very edge of the antenna top. The Y _W-profile on an unpowered antenna limiter peaks at the location projecting to the top of the powered antenna. An interpretation of the Y _W measurements is presented, assuming a direct link between the W sputtering and the sheath driving RF voltages deduced from parallel electric near-field (E_||) calculations and this suggests a strong E _|| at the antenna limiters. However, uncertainties are too large to describe the Y_W poloidal profiles. In order to reduce ICRF-related rise in W concentration C_W, an operational approach and an approach based on calculations of parallel electric fields with new antenna designs are considered. In the operation, a noticeable reduction in Y_W and C _W in the plasma during ICRF operation with W wall can be achieved by (a) increasing plasma-antenna clearance; (b) strong gas puffing; (c) decreasing the intrinsic light impurity content (mainly oxygen and carbon in AUG). In calculations, which take into account a realistic antenna geometry, the high E_|| fields at the antenna limiters are reduced in several ways: (a) by extending the antenna box and the surrounding structures parallel to the magnetic field; (b) by increasing the average strap-box distance, e.g. by increasing the number of toroidally distributed straps; (c) by a better balance of (0π)-phased contributions to RF image currents.