Spin flipping with an rf dipole and a full Siberian snake

Abstract

We recently used an rf dipole magnet to study the spin flipping of a 120 MeV horizontally polarized proton beam stored in the presence of a nearly full Siberian snake in the Indiana University Cyclotron Facility Cooler Ring. We flipped the spin by ramping the rf dipole's frequency through an rf-induced depolarizing resonance. After optimizing the frequency ramp parameters, we used multiple spin flips to measure a spin-flip efficiency of 86.5 6 0.5%. The spin-flip efficiency was apparently limited by the field strength in the rf dipole. This result indicates that spin flipping a stored polarized proton beam should be possible in high energy rings such as the Brookhaven Relativistic Heavy Ion Collider and HERA where Siberian snakes are certainly needed and only dipole rf-flipper magnets are practical. PACS numbers: 29.27.Bd, 29.27.Hj, 41.75.Ak Polarized beam experiments are now a major component of the programs in storage rings such as the Indiana University Cyclotron Facility (IUCF) Cooler Ring [1], the MIT-Bates Storage Ring [2], the Brookhaven Relativistic Heavy Ion Collider [3], and HERA at DESY [4]. Frequent reversals of the beam polarization direction can significantly reduce the systematic errors in an experiment's spin asymmetry measurements. An rf solenoid was used earlier to spin flip a horizontally polarized proton beam stored in the Cooler Ring containing a Siberian snake [5] with 97 6 1% spin-flip efficiency [6]. However, a solenoid's spin rotation decreases linearly with energy because of the Lorentz contraction of its R B dl; thus, a solenoid is impractical for spin flipping in high energy rings. Fortunately, a dipole's spin rotation is energy independent. Therefore, we recently used an rf dipole to spin flip a 120 MeV horizontally polarized proton beam stored in the IUCF Cooler Ring with a nearly full Siberian snake.In any flat circular accelerator or storage ring with no horizontal magnetic fields, each proton's spin precesses around the vertical fields of the ring's dipole magnets. The spin tune n s , which is the number of spin precessions during one turn around the ring, is proportional to the proton's energywhere G ͑g 2 2͒͞2 1.792 847 is the proton's gyromagnetic anomaly and g is its Lorentz energy factor. This vertical spin precession can be perturbed by the horizontal rf magnetic field from either an rf solenoid or an rf dipole. This perturbation can induce an rf depolarizing