Combining high-power heterogeneous RF sources for accelerator applications

Abstract

Facilities with a synchrotron light source at present utilize both vacuum-tube and solid-state RF sources to accelerate an electron beam, thanks to the rapid development of semiconductor technology. In NSRRC, a retired 500-MHz 100-kW klystron-based RF transmitter and a newly developed 80-kW solid-state RF transmitter became available for experimental study. We proposed the usage of a 3-dB waveguide hybrid coupler to combine these two heterogeneous RF sources to provide twice the RF-power of each alone. The intrinsic difference in RF properties of these two RF-power sources seen by the 3-dB hybrid coupler is compensated with an analogue low-level feedback system that locks the klystron side to the solid-state RF source side for a quasi-equal power level and a quadrature phase offset to obtain the greatest efficiency of the combination. In this manner, we demonstrated an efficiency up to 98.3% for combined RF-power 140 kW measured at the target port of a 3-dB waveguide hybrid coupler; the dummy load after the non-target port received negligible power within the full power range. The robustness to a power imbalance between these two RF-power sources was studied theoretically and experimentally; the efficiency of the RF-power combination was maintained greater than 90% for an available power ratio of these two RF sources up to 3 or phase error within ±45°. The results demonstrate an economical power-combination scheme for heterogeneous RF-power sources with high tolerance of imbalance for a symmetric power combiner. The usage of an asymmetric power combiner is foreseen to extend the imbalance tolerance.