First observations of a fast beam-ion instability at the ALS

The authors report the results of experiments on a fast beam-ion instability at the Advanced Light Source (ALS). This ion instability, which can arise even when the ions are not trapped over multiple beam passages, will likely be important for many future accelerators. In the experiments, the authors filled the ALS storage ring with helium gas, raising the pressure approximately two orders of magnitude above the nominal pressure. With gaps in the bunch train large enough to avoid conventional (multi-turn) ion trapping, they observed a factor 2--3 increase in the vertical beam size along with coherent beam oscillations which increased along the bunch train