RF optimization of the ITER ICRF antenna plug including its broad banding by a service stub

RF optimization of the ITER ICRF antenna has been performed on one strap triplet (1/8th of the antenna) using the semi-analytic coupling code Antiter II, the 3D electromagnetic code CST Microwave Studio® and transmission line theory. The front end of the antenna and the 4-port junction are optimized in order to maximize the power coupled to the plasma for a given maximum voltage in the ICRF system. Attention is brought to the subsequent increase of current in the antenna. The 4-port junction characteristics are brought as close as possible to those of an ideal transmission line 4-port junction. The implementation of a service stub inside the antenna is considered and it is shown that advantage can be taken from its perturbation to significantly improve the response of the system over the whole frequency band. The overall optimization leads to a substantial improvement of the performance of the whole system: about 98% at mid-band and from 26% to 165% at the band edges with respect to the reference design of October 2007. A preliminary comparison of simulation results with measurements carried out on a dedicated reduced-scale mock-up is briefly discussed.