An improved method to measure the antenna resistance and radiated power of ICRF-antennas using current probes

The coupling properties of ICRF antennas depend on the plasma boundary characteristics, which are modified for example by H-Mode or ELMs. The impedance and the power into the antenna feeding line are measured with a pair of directional couplers near the double stub tuner matching system. Those measurements include the losses of the transmission line and the antenna. An approach to separate the radiated power from the losses is to measure the current in the antenna loops. To measure the current in the short of the antenna loop two current probes have been installed near the short of the antenna loops in one of the ICRF antennas during the 2009/2010 shutdown of ASDEX Upgrade. These signals allow a new approach in calculating the antenna resistance and the launched power and can be used to separate the launched power from the losses in the matching system and transmission lines up to the antenna. Although the calibration of the signal amplitude from the current probes needs further investigation, the amplitude is proportional to the current in the antenna loop. With those probes and a new real-time DSP-based system, the radiated (as opposed to the total power i.e. including loss power) power can be feedback controlled.