First observations of a ''fast beam-ion instability''

We report the results of observations of a new regime of ion instabilities at the Advanced Light Source (ALS). With artificially increased pressure and gaps in the bunch train large enough to avoid multiturn ion trapping, we observed a factor of 2-3 increase in the vertical beam size along with coherent beam oscillations which increased along the bunch train. The observations are qualitatively consistent with the ''fast beam-ion instability'' [T. O. Raubenheimer and F. Zimmermann, Phys. Rev. E 52, 5487 (1995); G. V. Stupakov et al., Phys. Rev. E 52, 5499 (1995)], which can arise even when the ions are not trapped over multiple beam passages. This effect may be important for many future accelerators.