Electron Polarization

This chapter focuses on the introduction and discussion of electron polarization. In addition to the gyromagnetic ratio, the most different character of electrons compared to protons is that electrons radiate electromagnetic energy in a circular accelerator. A very small correction has to be applied to the electron spin flip to account for the synchrotron radiation. The different instantaneous spin flip probabilities, up to down and down to up, can build up the electron beam polarization state. However, mostly synchrotron radiation tends to disturb the electron orbital motion that is eventually balanced by the radiation damping along an equilibrium orbit. The electron spin motion is described by the modified Thomas-BMT equation with the radiative spin transition term included. Detail of the electron (de)polarization phenomena is described in this chapter. The lecture is extracted from various early theoretical papers, lectures, thesis and presentations (Lee, Accelerator Physics. World Scientific Publishing, 1999; Buon and Koutchouk, Polarization of Electron and Proton Beams. CERN-SL-94-80-AP, 1994; Montague, Phys. Rep. 113(1):1–96, 1984; Lee, Spin Dynamics and Snakes in Synchrotrons. World Scientific Publishing, 1997; Barber and Ripken, Handbook of Accelerator Physics and Engineering, 1st edn. World Scientific Publishing, 2006; Barber, An Introduction to Spin Polarisation in Accelerators and Storage Rings. Cockcroft Institute Academic Training Winter Term, 2014; Mane, Nucl. Instr. Methods Phys. Res. A 292:52–74, 1990; Berglund, Spin-Orbit Maps and Electron Spin Dynamics for the Luminosity Upgrade Project at HERA. DESY-THESIS-2001-044, 2001; Electron-Ion Collider Conceptual Design Report, 2020).