Longitudinal Oscillations with Batch Injection in the LHC

Abstract

Long lasting dipole oscillations have been observed at injection in the LHC, with 50 ns bunch spacing, 1.5 ns 4-sigma bunch length (0.5 eVs at SPS extraction), and 1.2E11 p/bunch. During the MD we have observed these oscillations with four different fillings, varying the total beam current, the number of bunches per batch, and the distance between batches. None of these parameters has a significant influence. The effect does not appear to be a multi-bunch instability. The MD has revealed that the amplitude of dipole oscillation initially grows for 5-10 min after injection, resulting in fast bunch lengthening. Then very slow damping takes place (~ 30 minutes time constant). A dependence on bunch length has been observed