Development of an RF-input coupler with a multi-loop antenna for the RFQ linac in IFMIF project

An radio-frequency (RF)-input coupler with a multi-loop antenna using co-axial waveguides has been developed for the radio-frequency quadrupole (RFQ) linac on International Fusion Materials Irradiation Facility (IFMIF). In the case of using a multi-loop antenna, the antenna structure affects the phase difference and the power balance in each cavity. A mock-up loop antenna using a pipe diameter of 10 mm has been fabricated, and phase differences and power balances were measured by a low power test. It was found that the installed depth has to be shortened by 3 cm to make phase differences small. But, due to voltage limits, it is difficult to couple multi-MW using a single coupler. Coupling using two-loop or four-loop antenna is better than one-loop antenna. Phase differences and power balances were also measured in both cases. When a four-loop antenna was installed into the RFQ, a good average phase difference of 180° and a good RF power balance within 8% variation was obtained. The electric field profiles in the beam bore were also calculated by MW-Studio. It was found that distortion of electric field is negligible small in the case of using four-loop antenna. These results showed the feasibility of using as an RF-input coupler comprising a four-loop antenna in the IFMIF 175 MHz RFQ.