Improved measurements of ICRF antenna coupling at ASDEX Upgrade and comparison with the ICRF coupling code TOPICA

Three methods of measuring antenna impedance on the transmission lines of the ASDEX Upgrade ICRF system have been compared on a 50 Ω test line – by using directional couplers, voltage probe arrays and voltage/current probe pairs. Under the best available conditions (straight 50 Ω line), the voltage/current probe pair consistently shows the highest accuracy, up to 1.2%, independent of VSWR. The current and voltage probes have been calibrated by measuring their coupling values on the 50 Ω line; the values for a 25 Ω line were obtained from the 50 Ω measured values by computing the difference in coupling in the two lines for both probes using simple circuit models. The new measurement system is implemented on ASDEX Upgrade antennas and will complement the already available measurements from directional couplers. In addition to improving the accuracy, it will also have the advantage of yielding two complex reflection coefficients per antenna (one for each strap) instead of one. The measurement is located significantly closer to the antenna input (∼3 m from the strap input), thereby reducing errors due to the geometrical complexity of the transmission lines.