Design and experimental research of a compact directional coupler for X-band relativistic triaxial klystron amplifier

To realize online measurement of power, frequency, and phase for a modularized relativistic triaxial klystron amplifier in a narrow space, a compact directional coupler with high directivity, high power handling capacity, and large bandwidth has been investigated numerically and experimentally. The method with coupling holes distributed in both the longitudinal and lateral directions is proposed, and the simulation results indicate that when two holes are employed, directivity larger than 20 dB is achieved over a 300 MHz bandwidth in the X-band and the maximal electric field strength is controlled to be 198.5 kV/cm with 1 GW power injection, while the whole coupler longitude is optimized to be as short as 50 mm. High power experiments have been performed with a 10 GHz relativistic triaxial klystron amplifier, and the results show that the measured power, frequency, and phase shift of the directional coupler have good agreement with the results obtained in the far-field.