Self-polarization in Storage Rings

The possibility of electrons becoming polarized in an uniform field was predicted in the early 60s by Loskutov, Korovina, Sokolov and Ternov (Sokolov-Ternov effect of radiative polarization). The very first experimental observation of beam self-polarization followed in 1968 at ACO in Orsay and therefore this year marks the fiftieth anniversary of that event. In particular at high energy, e+- polarization is not really for free and in addition experiments are mainly interested in longitudinal polarization. This is obtained through spin rotators which may lead to depolarization. The p/e+- high energy collider HERA in Hamburg was the first, and until now the only one, designed for delivering longitudinal e+- polarization to the experiments. In this paper, after a historical overview of the theory of radiative polarization in storage rings and its first observations, the unique case of HERAe will be highlighted. Measures needed for fostering high polarization will be illustrated with examples from HERAe operation.