Siberian Snakes and Spin-Flipping in Storage Rings

A Siberian snake, which is a 180° spin rotator, can be used to preserve electrons', protons' or light ions' beam polarization during acceleration by forcing the spin tune to be constant and equal to 1/2, independent of the beam energy. A compact helical dipole snake, which creates only a small orbit excursion inside the snake, could be used in EPIC. Frequent polarization reversals, or spin-flips, of a stored polarized high-energy beam could greatly reduce the systematic errors of spin asymmetry measurements in a scattering asymmetry experiment. Such polarization reversals can be done by ramping an rf-dipole or an rf-solenoid magnet's frequency through an rf-induced depolarizing resonance. The strength of an rf-solenoid decreases with the beam's energy due to the Lorenz contraction of the solenoid's longitudinal fB-dl, while the strength of an rf-dipole remains almost unchanged. Thus, it is more practical to use an rf-dipole spin-flipper in EPIC's electron and ion rings. INTRODUCTION In a circular accelerator ring, each proton's spin precesses around the vertical magnetic fields of the ring's bending dipoles, with a frequency, called the spin precession frequency /s, that is related to the proton's circulation frequency fc by: