Development of a DC bias structure with reduced RF leakage for suppressing the multipacting effect in the high power coaxial couplers

Coaxial power couplers are being employed for RF power transmission to the low beta superconducting cavities used for proton beam acceleration in the 25 MeV superconducting linear accelerator (linac) demonstration facility of the China Initiative Accelerator Driven System (CiADS) project. However, the operation stability of the coaxial couplers was mainly limited by the multipacting effect, which can be suppressed by applying DC bias voltage between the inner and the outer conductors. In this paper, we found an RF leakage problem that the electromagnetic (EM) field inside the couplers can penetrate the bias capacitor and then propagate along the DC bias cable, thus disturbing the stability of the DC power supply and finally disabling the RF system. Therefore, a simple DC bias “T” transition part has been developed, where the series resistors are introduced to reduce the electromagnetic field propagating along the cable and make the structure intensely stable. The test results indicate that the blocking effect of the resistive solution is better than that of the conventional inductive method. The equivalent circuits of the resistive structure and the inductive structure are established and analyzed. The detailed test and analysis results of the new DC bias structure will be reported.