Abstract

1. LHeC Spin Flipper* M. Bai, (BNL, Upton, NY 11973, USA) F. Zimmermann, R. Tomas (CERN, Switzerland) *Work supported in parts by the U.S. Department of Energy and RIKEN, Japan Abstract In addition to the on-going exciting physics program at LHC, there have been quests on expanding LHC to include an LHeC with electron-proton collisions. The potential of studying new physics in high precision QCD, substructure etc. at LHeC requires polarized electrons with spin aligned longitudinally at the collision point [1]. One option for the LHeC is based on an energy recovery linac, for which the electron beam can be generated with 80-90% polarization using a photocathode source. To avoid the polarization loss of the high energy electron beam, the spin vector will be aligned vertically during the acceleration in a re-circulating linac and then brought into the longitudinal direction for collision. This paper reports possible design choices for the LHeC spin rotator LHeC Layout • Spin Motion in an Accelerator • Thomas BMT Equation • here e, m and γ are the electric charge, mass and Lorentz • factor of the particle. G is the anomalous g-factor. For protons G=1.7928474, and for electrons, G=0.00115. and are the magnetic field perpendicular and parallel to the particle velocity direction, respectively. In the Thomas BMT equation, magnetic field is in the laboratory frame while the spin vector is in the particle’s rest frame. • Thomas BMT equation shows • a spin vector precessesGγ times its orbital bending in a • dipole. The amount of spin rotation is directly proportional • to the beam energy • The effect of a dipole field on spin motion is nearly • independent of beam energy • The effect of a solenoid field on spin motion is inversely • proportional to beam energy • LHeC Spin Rotor Options • a single low energy spin rotator in the injector of LHeC to rotate the spin vector • to a direction from which the spin precessions from each path in the arcs then • bring the spin vector to the longitudinal direction • pros: simple and economical • cons: difficult to keep high polarization due to the spread of spin vector in a • beam with a finite momentum spread • Helical dipole spin rotator • located upstream of final collision point • requires a low energy spin rotator(Wien Filter) to bring the spin vector to vertical direction for acceleration to high energy. This avoids the effective polarization loss due to the spread of spin vector in a beam Required magnetic field strength of the outer helices for rotating a vertical spin vector to a direction in the horizontal plane with an angle phi from longitudinal direction Corresponding magnetic field strength of the inner helices for a given outer helical magnetic field strength Schematic of spin-related IR layout with spin rotator, two polarimeters, and compensating bends Acknowledgement The authors would like to acknowledge fruitful discussion with Dr. Ptitsinand Dr. Roser at BNL Conclusion This paper presents two options of spin flipper for LHeC high energy electron beam. This design of using a group of helical dipoles next to the final collision point, similar to the spin flipper at RHIC, satisfies the requirement of high energy electron beam with longitudinally oriented high polarization. This option also provides the advantage of being compact and flexible. For this approach, low energy spin flipper like Wien Filter as part of the injector is also required to rotate the spin vector to the vertical direction for the acceleration. [1] F. Zimmermann, et al, 3rd CERN-ECFA-NuPECC Workshop on the LHeC, http://www.ep.ph.bham, ac,uk/exp/LHeC/, No. 12-13, 2010. [2] L. H. Thomas, Phil. Mag. 3, 1 (1927); V. Bargmann, L. Michel, V. L. Telegdi, Phys. Rev. Lett. 2, 435 (1959). [3] I. Alekseev, et al, Design Manual - Polarized Proton Collider at RHIC, Nucl. Inst. and Meth. A499, 392 (2003). [4] V. Ptitsin, Symmetric Designs for Helical Spin Rotators at RHIC, AGS/RHIC/SN No. 5 (1996). [5] M. Woods, The Scanning Compton polarimeter for the SLD experiment, SLAC-PUB-7319 (1996).