Ion instability experiments on the ALS

We report the results of experiments to study ion effects at the Advanced Light Source (ALS) in two pressure regimes. At the nominal pressure of 0.25 nTorr, we observe small vertical coherent beam oscillations for the nominal filling pattern (2.5% gap) that correlate with the expected ion frequency for nitrogen or carbon monoxide. The signals disappear for larger gaps in the filling pattern. We observe little increase in vertical beam size. We have also made experiments to look for unconventional ion effects 12 at elevated gas pressures that may be important for future accelerators. In these experiments, we introduce a single gas species (helium) into the storage ring, raising the pressure approximately two orders of magnitude above the nominal pressure. For filling patterns with gaps in the bunch train large enough that conventional ion trapping should not play a role, we observe roughly a doubling of the vertical beam size along with coherent beam oscillations. We compare the results of the experiments with the predictions of one possible mechanism: the fast beam-ion instability.