ITER ICRH antenna grounding options

To avoid the excitation of modes in the 20 mm clearance gap between the ITER ICRH antenna plug and the vacuum vessel in the ITER frequency band it was proposed to perform galvanic grounding at 1 m from the antenna plug front. Experimental S-matrix and gap voltage measurements on a reduced-scale mock-up of the ITER ICRH antenna as well as numerical simulations show indeed that the frequency response of the antenna array is perturbed by a TE_0,1 mode resonance in the ITER frequency band (around 45 MHz) in absence of any additional grounding and may lead to large voltages in the clearance gap. They also confirm the efficiency of the proposed grounding solution. Grounding all around the antenna plug periphery ("full" grounding) is compared to grounding with discrete contacts ("partial" grounding). The influence of the position of the grounding is also studied. Eigen modes analysis allows identifying the modes experimentally observed. Finally a practical solution considered for implementation on ITER is validated both by numerical simulations and experimental measurements.