Polarization Reversal and Luminosity Gain by Spin Reversal of Stored Proton Beams

In conventional polarized beam experiments, the projectile is incident upon a target external to the accelerator. In this case, the direction of the polarization vector of the beam can be reversed at the polarized ion source without any loss in luminosity. In the Cooler, however, the polarization of the stored beam has been given by the polarization direction during injection. Reversal of the polarization requires dumping the beam and waiting until the ring is filled again with protons of the opposite spin. Refilling the ring periodically in order to invert the spin results in a loss in time-averaged luminosity. Considerably increased luminosity results if it is possible to flip the spin of the stored beam without discarding the stored particles. Here we describe the successful operation of a spin flipper of high flip efficiency in the Cooler. While successful tests of the spin flipper have been reported previously,1 the emphasis here is on improvements that lead to very reliable and robust operation during long data-taking runs, as well as the associated increase in time-averaged luminosity. When a depolarizing resonance is crossed, the polarization direction of the stored beam is either conserved or reversed, depending on whether the crossing occurs rapidly or slowly.2 In a storage ring, a depolarizing resonance occurs when the spin tune is such that non-vertical magnetic field components are encountered in phase on subsequent revolutions. In other words, small rotations of the polarization vector of the stored beam add up during many turns around the ring. It has been shown that a depolarizing resonance can be introduced by an RF solenoid whose magnetic field oscillates along the beam axis.3 A depolarizing resonance is introduced if the frequency of the oscillating longitudinal field, fsol, is given by