Study of voltage handling capability of hybrid combiner for ITER ion cyclotron radio frequency source

The ion cyclotron radio frequency (RF) source for the ITER tokamak will employ two amplifier chains combined at the end stage using a wideband quadrature hybrid combiner. The combiner response has been simulated for the matched & mismatched load condition (3MW/VSWR 1.5/ all reflection phases) of the RF source. The fabricated prototype combiner has been validated for the RF performance in combiner mode up to ~5 kW level and up to 1.5 MW in power splitter mode. This work discusses the study performed to validate the voltage handling capability of the prototype combiner, which, further provided inputs to improve the combiner design for the RF source operating conditions. The voltage handling test setup for the combiner was prepared by shorting two diagonally opposite ports through variable short circuit coaxial stubs. The RF simulations of the test setup provided a return loss better than -26dB at 60MHz with peak voltage ~18 kV (E-field 2.26 kV/mm) near the target voltage probes with the input power of 1.7MW. During the high-power test, the input power was increased up to 2.26 MW which provided a measured voltage level up to ~21.4 kV where the voltage breakdown occurred inside the combiner. Investigation through simulation suggested an E-field level of 2.61 kV/mm for the given breakdown condition. Finally, based on these experimental findings, the design of the combiner was updated to limit the E-field level below 1.45 kV/mm for the load power of 3MW at VSWR 1.5 thus providing a two-fold margin from the breakdown limit.