Design and high power processing of RFQ input power couplers

A RF power coupling system has been developed for future upgrade of input coupling of the RFQ in the SNS linac. The design employs two coaxial loop couplers for 402.5 MHz operation. Each loop is fed through a coaxial ceramic window that is connected to an output of a magic-T waveguide hybrid through a coaxial to waveguide transition. The coaxial loop couplers are designed, manufactured, and high power processed. Two couplers will be used in parallel to power the accelerating structure with up to total 800 kW peak power at 6% duty cycle. RF and mechanical properties of the couplers are discussed. Result of high power RF conditioning that is performed in the RF test facility of the SNS is presented.

INTRODUCTION

Various RF coupling methods were used on RFQ structures [1][2]. A coaxial power coupling system for 402.5 MHz RFQ has been developed at SNS for full power operation in the future. The new coupling system developed and shown in this paper employs two loop couplers through a hybrid power splitter to feed the structure in parallel. Figure 1 shows the connection using a waveguide magic-T to deliver equal power to the loops with good isolation between the ports. The coaxial alumina ceramic window employed in this design is identical to the 81 coupler windows presently being used on the SNS SRF cavities operating at 805 MHz. [3][4]. The SRF couplers with electrically coupling antenna were successfully conditioned and tested for forward power of 650 kW peak, 8% duty cycle at 805 MHz. Operation of linac with the windows has shown excellent RF power handling capability and mechanical reliability. Figure 1: RF power coupling with two coaxial loop couplers to an RFQ structure. Magic-T waveguide power splitter is used to have isolation between the two ports.