Conical Targets for Enhanced High-Current Positron Sources

Previous pair-production-driven positron source designs have assumed that the transverse dimension of the target is significantly greater than the secondary beam it generates. This paper explores the use of targets with different transverse profiles with the aim of enhancing positron production. The starting point of this research is the concept of wire targets, proposed by M. James et al. in 1991 for the former SLC positron source. Building on this foundation, this study takes this concept a step further by introducing conical-shaped targets, which can substantially improve the yield by reducing the reabsorption of positrons by the target--an issue that is worsened by the high-field solenoid lenses commonly used for positron capture. Using Geant4 simulations, we propose new conical targets adapted for the parameters of the future collider FCC-ee and its positron source test facility P-cubed (PSI Positron Production experiment) at the Paul Scherrer Institute. We find that conical targets can nearly double the positron production at the target and enhance the baseline positron yield of FCC-ee by around 60%. Additionally, we present the thermo-mechanical studies for the conical targets based on the FCC-ee primary beam power requirements and outline the mechanical implementation for a future proof-of-principle demonstration at the P-cubed facility.