Measurement of Electron Trapping in the CESR Storage Ring

TL;DR

This study measures long-lived electron clouds trapped in a quadrupole magnet in a positron storage ring, revealing their persistence, dependence on bunch spacing, and mitigation via witness bunches, impacting accelerator stability.

Contribution

It provides the first measurements and modeling of long-lived electron trapping in a quadrupole magnet, including evidence of cloud clearing by witness bunches.

Findings

Approximately 7% of electrons survive longer than 2.3 μs in the trap.

Trapping effect varies non-monotonically with bunch spacing.

Witness bunches effectively clear trapped electron clouds.

Abstract

The buildup of low-energy electrons has been shown to affect the performance of a wide variety of particle accelerators. Of particular concern is the persistence of the cloud between beam bunch passages, which can impose limitations on the stability of operation at high beam current. We have obtained measurements of long-lived electron clouds trapped in the field of a quadrupole magnet in a positron storage ring, with lifetimes much longer than the revolution period. Based on modeling, we estimate that about 7% of the electrons in the cloud generated by a 20-bunch train of 5.3 GeV positrons with 16-ns spacing and $1.3x10^{11}$ population survive longer than 2.3 $\mu$s in a quadrupole field of gradient 7.4 T/m. We have observed a non-monotonic dependence of the trapping effect on the bunch spacing. The effect of a witness bunch on the measured signal provides direct evidence for the existence of trapped electrons. The witness bunch is also observed to clear the cloud, demonstrating its effectiveness as a mitigation technique.