Fast ion instability at the Cornell Electron Storage Ring Test Accelerator

Fast ion instability can lead to deterioration of an electron beam (increasing emittance and instability of a train of bunches) in storage rings and linacs. We study this at the Cornell electron storage ring test accelerator using a 2.1 GeV low emittance beam. As the source of ions is residual gas, our measurements are conducted at various pressures, including nominal vacuum as well as injected gas (Ar, Kr). We experiment with mitigation techniques, including changing the bunch pattern to have mini-trains instead of one long train, as well as increasing the initial vertical emittance of the beam. We also check to ensure that ion-trapping is not a substantial effect in our measurements. We measure turn-by-turn vertical bunch size and position, as well as the multibunch power spectrum. Our measurements confirm fast ion instability under all vacuum conditions. A detailed simulation is then used to compare theory with observations.