The measurements of the coupler's RF parameters at low power were conducted adopting a rigorous calibration procedure. Directivity is one of the key parameters of a directional coupler. It characterizes how well the coupler isolates two travelling signals in opposite directions at the coupled port. A remarkable 48dB directivity value was measured for the prototype coupler, thus enabling a precise measurement of RF power over the transmission line. This value largely exceeds that of imported couplers, which have a specified directivity of 32dB.

In the recent high-power tests, the prototype directional coupler was assembled with four waveguide E-bends and one straight waveguide previously developed by the same team as well as a variable waveguide short circuit. RF power of 200 kW in continuous-wave (CW) mode was subsequently sent through the entire transmission line by using the BEPCII 500 MHz klystron. A power variation of only ±1% was measured at the prototype directional coupler when shifting the phase of the reverse wave by over half of the guide wavelength, while the imported coupler recorded a value of ±7%. Finally, the phase of the reverse wave was varied to move the maximum magnetic field to the directional coupler to examine its high-power capabilities. After more than six hours of conditioning at CW 200 kW, the coupler showed excellent RF and thermal performance. The specifications for HEPS had been fulfilled.

The successful development of the high-performance waveguide directional coupler and waveguide bends indicates that the technology has been fully mastered by the HEPS RF team and can readily be extended to future development of other high-power RF components for accelerator applications.