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Description
A high-performance positron source is essential for FCC-ee to provide the high-intensity, low-emittance beams required for rapid injection, especially in the Z-pole operation mode. In the FCC-ee Feasibility Study, the reference positron source is based on a 2.86 GeV electron drive beam impinging on a 15 mm tungsten target, followed by an HTS matching device and a 2 GHz travelling-wave RF capture section embedded in a 0.5 T solenoidal channel. A chicane separates electrons from positrons, after which the positron linac, based on the same 2 GHz RF structures as the capture section, is divided into two focusing sections: a solenoid-focused section with a 0.5 T field, followed by a FODO-based section with quadrupole focusing between the RF structures.
This work investigates 3 GHz S-band RF structures as potential alternatives for the FCC-ee positron linacs, motivated by the maturity and broad availability of high-power S-band RF technology. Two 3 GHz configurations are studied in detail. The first, denoted v1, employs five 3 m long travelling-wave structures in the capture section, whereas the second, v2, uses twelve 1.2 m standing-wave structures developed for the P³ experiment. In both configurations, the capture structures are embedded in a solenoidal focusing channel. Following electron–positron separation in a chicane, the positron beam is matched into a quadrupole-focused linac based on large-aperture 3 GHz travelling-wave structures. Its first and second sections contain 16 and 48 structures, respectively. A downstream energy-compression system, comprising a magnetic chicane and four additional travelling-wave structures, together with a matching line, prepares the beam for injection into the damping ring. A 6D DR acceptance approach, benchmarked against direct particle tracking through the damping ring, is incorporated into the fast optimization workflow. Start-to-end simulations are performed using RF-Track, while the princpal machine parameters are optimized with Xopt. By retaining a common downstream layout, the study enables a consistent comparison of the two capture configurations, identifies their principal performance trade-offs and provides the basis for establishing the current 3 GHz baseline design for the FCC-ee positron source.