Abstract
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.
Highlights
The residual gas in the accelerator vacuum system can be ionized by a circulating electron beam
In a storage ring, having a long charge-free gap at the end of the train prevents multiturn ion trapping. This cannot prevent ions from accumulating during a single passage of the bunch train, a phenomenon referred to as fast ion instability (FII) [1]
Since the only known multibunch instability that is affected by increasing vacuum pressure is FII, we can infer that the observed sidebands are a consequence of beam-ion coupling
Summary
The residual gas in the accelerator vacuum system can be ionized by a circulating electron beam. The resulting instability limits the total charge in each bunch and the number of bunches in the train It is observed in linear accelerators as well as electron storage rings. In a storage ring, having a long charge-free gap at the end of the train prevents multiturn ion trapping. This cannot prevent ions from accumulating during a single passage of the bunch train, a phenomenon referred to as fast ion instability (FII) [1]. FII has been qualitatively observed at many accelerator facilities: by injecting gas or turning off vacuum pumps [3,4,5], or by reducing the beam emittance to increase the trapping potential under nominal vacuum [6].
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