Calculation of the magnetic field and its modal analysis in the port plug cavity of the ITER ICRF antenna

The ITER Ion Cyclotron Range of Frequencies (ICRF) antenna plug and the vacuum vessel port form an electromagnetic cavity that can exhibit resonances of the coaxial type within the range of frequencies relevant for ion cyclotron heating (between 40 and 55 MHz). At resonance (~47.5 MHz), and for specific phasing, large electric fields and Radio Frequency (RF) losses can occur. This work focuses on the calculation of the magnetic field and its modal analysis in the port-plug cavity, an important step in the assessment of the resonant effects. The approach consists of several steps. First, a detailed model of the antenna is imported into the TOPICA (TOrino Polytechnic Ion Cyclotron Antenna) code [1], meshed, and run with several plasma density profiles. Subsequently, the electric surface current is reconstructed from the TOPICA code outputs on the inner conductor of the antenna plug, and the magnetic field in the gap around the plug is calculated from the values of the surface current. Furthermore, the cavity model is simulated using electromagnetic software CST Microwave Studio (MWS) [2] to calculate the cavity eigenmodes. The magnetic field is then expanded into a series of cavity eigenmodes previously obtained and expansion coefficients are determined. In the next step, the modal expansion of the cavity fields at the reference plane can be used to excite the lossy cavity with the corresponding forward wave spectrum, enabling the subsequent evaluation of magnetic fields and RF losses in the cavity [3].