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Spin Rotation at lower energy than the damping ring

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This note proposes a spin rotation scheme for the ILC that rotates the spin vector of the electron and positron beams into the direction normal to the plane of the damping ring (vertical) at a lower energy than the damping ring. Rotating the spin vector into the vertical direction at lower energy will reduce costs and performance requirements. Two options are discussed for the electron spin rotation. The first option has the beginning accelerator section for the electron beam at an angle of 23.33 degrees with respect to the main accelerator sections. The spin rotation uses the bend of 23.33 degrees and a solenoid of integrated field 8.908 Tesla-meters to rotate the spin to the vertical at 1.7 GeV. The second option uses a dedicated horizontal chicane containing a pre-accelerator increasing the energy of the electron beam from 140 keV to 400 MeV. The spin rotation is done with a bend of 99.146 degrees and a solenoid with integrated field of 2.096 Tesla-meters at 400 MeV. The transverse spin direction remains in the vertical during the remaining acceleration and transport to the damping ring (5 GeV). The positron spin rotation system is proposed to be done at 400 MeV directly after the positron pre-accelerator. Similar to the second option for electrons the rotation would be done with a bend of 99.146 degrees and a solenoid with integrated field of 2.096 Tesla-meters. For the positrons we also propose parallel spin rotation beam lines with suitable fast kicker magnets to rapidly switch between solenoids with opposite fields to allow rapid helicity switching for polarized positrons.

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