SPS Beams for LHC: RF Beam Control to Minimize Rephasing in the SPS

After acceleration to 450 GeV/c in the SPS, the proton beams for the LHC must be rephased in order to be injected into the correct place in the collider. If the position before rephasing is random, th e beam must be rotated by up to 1/14th of an SPS turn. This procedure will take about 330 ms on the flat top. Besides lengthening the cycle, the beam may deteriorate if instabilities develop during th is long process. We present a beam control system that reduces the rotation angle needed and which has been successfully tested in the SPS.