Polarization issues in circular electron–positron super-colliders

We study a possibility of obtaining transversely polarized electron/positron beams in the CEPC collider. Also, original results are presented for another project, the FCCee collider, which allows us to identify some features and differences of these similar machines. Important is the comparison of the data obtained with the expected and measured polarizations at LEP. In estimation of the depolarizing effect of field errors, in addition to the usual nonresonant spin diffusion, an attempt was made to take into account the resonant diffusion, which is due to a large spin tune spread. We consider a possibility to obtain polarization by accelerating polarized particles in the CEPC booster and then injecting them into the main ring. This option can be crucial for obtaining longitudinal polarization. We propose a scheme in which long-term preservation of such polarization is possible.