A Coaxial Subharmonic Cavity Design for Direct Injection at the Advanced Photon Source

Coaxial subharmonic cavity designs are being investigated at the Advanced Photon Source to improve injector reliability by injecting beam directly from the linac to the booster in storage ring top-up mode. The subharmonic system must operate jointly with the present 352-MHz booster to accelerate the beam to 7 GeV with minimal beam degradation. Design considerations must be made to ensure that bunch purity is maintained and that a large percentage of the linac macropulse is captured. An analysis of rf cavity designs using electromagnetic simulation software has been conducted at 29 MHz and 117 MHz. Higher-order modes are evaluated as well as the total power loss and peak surface fields produced at the required gap voltage.