Measurements of the scattering matrix of the W7-X prototype ICRH antenna and capacitor units

Low-power measurements of the W7-X prototype ICRH antenna and variable capacitor units have been performed at FZJ Jülich. The antenna scattering matrices, with and without the carbon limiter tiles, have been de-embedded from network analyzer measurements using custom-built RF adapters. The results have been compared to the matrices calculated with CST Microwave Studio 2017 using a model based on the original CAD files. A very good agreement is seen, since a) the difference in magnitude is less than 0.02 in the whole frequency range, and ∼ 0.015 or less in the range of interest (between 20 and 40 MHz) and b) the difference in phase is less than 5^◦ in the whole frequency range, and ∼ 2^◦ or less between 20 and 40 MHz. The S-parameters of the tuning capacitors have also been measured, to determine the relationship between the internal DC capacitance C_DC of each unit, and the actual”equivalent” capacitance C_eq seen by the antenna at the ports. It is seen that the latter, 36-475 pF, is substantially different from the nominal DC range, which is 15-200 pF. Nonetheless this is not expected to pose a problem for antenna operation. The S-parameters of other key components of the ICRF system, such as the matching unit and line sections containing directional couplers, were also measured and, where applicable, compared to models or used to extract calibration data.