ICRF Antenna Operation with Full W-wall in ASDEX Upgrade

Abstract. The compatibility of ICRF (Ion Cyclotron Range of Frequencies) antenna operation with high-Z plasma facing components (PFCs), needs improvement to keep ICRF as a good candidate for heating and current drive system in a fusion reactor. ASDEX Upgrade (AUG) with its tungsten (W) first wall and ICRF system allows to study ways to do this. A noticeable improvement of the ICRF operation with W-wall can be achieved by forcing low plasma temperature conditions at the PFCs. These conditions can be fulfilled by increasing plasma-antenna clearance and by strong gas puffing, thus approaching the conditions ITER ICRF antenna plans to operate at. W sputtering during ICRF can be significantly decreased when the intrinsic light impurity content is decreased. However, an additional improvement is required for further reduction of the high-Z impurity sputtering during ICRF in the present and for the future devices. The improved theoretical modelling of ICRF antenna near-fields shows that the RF voltages along the magnetic field lines may originate from RF currents on the antenna box to a large extent, and not directly from antenna straps and their RF magnetic flux. Experimental results in AUG corroborating this picture are described. The calculations for future antenna design show that a reduction of the antenna box contribution can be achieved by extending the antenna box parallel to the magnetic field and increasing the number of toroidally distributed straps with (0π...π0) or (0π...0π) phasing.