Frequency of Positron Helicity Reversal

In compliance with the RDR [2], the baseline configuration for the positron source is based on a helical undulator passed by an electron beam of 150GeV. About 500m downstream the resulting photon beam hits a thin target, creates electron-positron pairs; the emerging positrons are collected and accelerated. The photon beam is circularly polarized, the spectrum is shown in Fig. 1 (left plot). The circularly polarized photons create longitudinal polarized positrons whose spectrum is also presented in Fig. 1 (right plot). Taking into account the damping ring acceptance, the average positron polarization of about 30% after the target will increase up to almost 40% [3]. Further increase of polarization can be achieved by collimation of the photon beam which cuts lower energetic photons with lower polarization but then the intensity loss has to be compensated, e.g. with a longer undulator. In order