Almost stationary direction of polarisation in an electron storage ring

It is shown that, in presence of synchrotron and betatron oscillations, there exists a quasi-stationary direction of spin with respect to spin precession, along which polarisation is built-up by the Sokolov-Ternov effect. The variation of this quasi-stationary direction, under quantum fluctuations, give the spin-orbit coupling in an equivalent way to the ultimate deviation of spin after damping