Simulation of the antenna-plasma coupling for the Low-field side ITER reflectometer system

High resolution edge density profile and fluctuation information is important for ITER operations and a Low-Field Side Reflectometer (LFSR) system will be installed to measure these quantities. For a reliable operation it is necessary that a significant amount of the transmitted microwave power is reflected back to the receiver antenna. The High-gain antennas used in the LFSR design give a good antenna-plasma coupling only when they are aligned with the plasma mid-plane. In ITER the plasma mid-plane can vary over vertically 0.5 m and in order to cover this range a vertical antenna array was proposed. Reflectometer simulations were performed to optimize the antenna geometry for optimal performance. The LFSR performance was studied with a 3D full-wave reflectometer code that include field-line aligned density fluctuations and relativistic effects. Parameter scans were done for various frequencies of the LFSR bandwidth (30-165 GHz), L- and H-mode profiles, density gradients, and fluctuation characteristics. The simulations show that a lay-out of 4 vertical antennas can cover the mid-plane variation and although density fluctuations reduce the coupling, sufficiently strong signals are reflected back to the receiver for reconstructing edge density profiles.