Proof-of-principle $e^{+}$ source for future colliders

The PSI Positron Production ($P^3$ or P-cubed) experiment is a $e^{+}$ source and capture system with the potential to increase by an order of magnitude the state-of-the-art $e^{+}$ yield normalized to the drive linac energy, a highly desirable goal for future colliders. The experiment is framed in the FCC-ee injector study and will be hosted at SwissFEL, located at the Paul Scherrer Institute in Switzerland. This paper presents the $P^3$ project at an advanced stage, with an emphasis on a capture system featuring a novel $e^{+}$ matching device based on high-temperature superconducting solenoids, followed by two large aperture rf cavities surrounded by normal-conducting solenoids. The diagnostics design is also introduced, including monitors of charge, energy spectrum, and bunch-by-bunch longitudinal profile simultaneously for secondary $e^{+}$ and $e^{-}$. The last chapter of the text overviews the currently ongoing installation at SwissFEL, including the beam transfer line, rf network, radiation protection, and other relevant activities toward the operation with $e^{+}$ in the coming years.