The north arc of the SLC as a spin rotator

The 1992 running cycle of the Stanford Linear Collider (SLC) showed that the orientation of the electron polarization vector at the interaction point was very sensitive to the vertical orbit in the north collider arc. We point out the reason for this sensitivity-the spin tune is equal to the vertical betatron tune. We devise a scheme of closed vertical orbit bumps which provide arbitrary manipulations of the polarization vector at the IP. We have developed an analytical model for the tuning of this `arc spin rotator´ and have performed a detailed numerical analysis to show its effectiveness. We present experimental results