Positron focusing in the Advanced Photon Source (APS) linear accelerator

Positrons are created by a bremsstrahlung shower process, and are therefore produced with broad divergence and a large energy spread. The e⁺/e^- conversion ratio is on the order of 1/200, so the positron focusing system is critical to ensure good positron capture and transport efficiency. The positron focusing system is described, and functions of the different magnetic elements are discussed. Some improvements to the focusing system are suggested, although the linac´s design positron intensity of 8 mA has already been achieved