Development of a 500-MHz waveguide directional coupler with high directivity for HEPS

Purpose In order to achieve a high-precision measurement of the incident and the reflected power, a WR1800 rectangular waveguide directional coupler with high directivity and high power level has been in-house developed. Multiple couplers will be installed in the 500-MHz high-power radio-frequency transmission lines delivering 200-kW continuous-wave power for the High Energy Photon Source (HEPS). Methods The directional coupler adopts the design scheme of primary and secondary transmission lines and coaxial coupling-head structure. The shape and dimensional parameters of the coupling head were carefully optimized by using microwave simulation codes. An optimum directivity of 64 dB was achieved in simulations. Results and conclusions A prototype coupler was subsequently manufactured, and its directivity was measured to be 48.2 dB following a rigorous calibration procedure, largely exceeding the design goal and the commercial product. The coupler was then connected to an existing 500-MHz klystron system, and a high-power test with short-circuit termination was conducted. The high directivity of the coupler was confirmed up to 200 kW. During the 6 hours of power aging with continuous-wave 200 kW in a standing-wave setup, no performance degradation was observed on the coupler. The coupler temperature was measured to be 20 degrees C above the ambient environment. The design requirements were comfortably fulfilled. These constitute the first in-house development of a large-size waveguide directional coupler with high directivity and high power level for HEPS. The design, fabrication, and performance tests of the directional coupler are presented.