The E166 experiment: Development of an Undulator-Based Polarized Positron Source for the International Linear Collider

A longitudinal polarized positron beam is foreseen for the international linear collider (ILC). A proof-of-principle experiment has been performed in the final focus test beam at SLAC to demonstrate the production of polarized positrons for implementation at the ILC. The E166 experiment uses a 1 m long helical undulator in a 46.6 GeV electron beam to produce a few MeV photons with a high degree of circular polarization. These photons are then converted in a thin target to generate longitudinally polarized e+ and e-. The positron polarization is measured using a Compton transmission polarimeter. The data analysis has shown asymmetries in the expected vicinity of 3.4% and ∼1% for photons and positrons respectively and the expected positron longitudinal polarization is covering a range from 50% to 90%. The full exploitation of the physics potential of an international linear collider (ILC) will require the development of polarized positron beams. Having both e+ and e- beams polarized will provide new insight into structures of couplings and thus give access to physics beyond the standard model (1). The concept for a polarized positron source is based on circularly polarized photon sources. These photons are then converted to longitudinally polarized e+ and e- pairs. While in an experiment at KEK (1a), Compton backscattering is used (2), the E166 experiment uses a helical undulator to produce polarized photons. An undulator-based positron source for the ILC has been proposed in (3,4). The proposed scheme for an ILC positron source is illustrated in figure 1. In this scheme, a 150 GeV electron beam passes through a 120 m long helical undulator to produce an intense photon beam with a high degree of circular polarization. These photons are converted in a thin target to e+ e- pairs. The polarized positrons are then collected, pre-accelerated to the damping ring and injected to the main linac. The E166 experiment is dedicated to test this concept of a helical-undulator-based polarized positron source for the ILC. The experiment was carried out at SLAC where it benefitted from the high quality electron beam in the final focus test beam (FFTB).