Experiments on the Fast Beam-Ion Instability at the ALS

We report on the first observation of the fast beam–ion instability (FBII), at the Advanced Light Source (ALS). The FBII [1, 2] is a novel single-pass instability, which is similar to the classical beam break up instability in a linac. Unlike the classical trapped-ion instability, the FBII cannot be cured by a clearing gap in the bunch train, and it is predicted to be a potential limitation for many multi-bunch small-emittance storage rings and linacs. In order to induce the FBII in the ALS, we added helium gas to the vacuum system so as to increase the vacuum pressure by two orders of magnitude above its normal value. At an elevated pressure of about 50–100 nTorr, we observed a variety of effects, including an increase of the vertical beam size by a factor 2–4, self-excited betatron sidebands, and a growth of the betatron motion along the bunch train. The onset of the vertical beam-size increase (as a function of bunch number) occured close to the theoretically predicted instability threshold.