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.
Highlights
Electron cloud buildup has been observed in many accelerators since the 1960s [1]
We report on the measurement of electron trapping in a quadrupole magnet over a 2.3 μs time interval between bunch train passages
Our measurements with a time-resolving electron detector located in a quadrupole magnetic field have provided comparisons of signals from 10- and 20-bunch trains of positrons which show clear evidence for electron trapping during the entire 2.3 μs time interval prior to the return of the bunch train
Summary
Electron cloud buildup has been observed in many accelerators since the 1960s [1]. Adverse consequences of electron cloud buildup include emittance growth, beam instabilities, and excess heat load to cryogenic systems. At LBNL, electrons were observed to be trapped in the fields of an ion beam and accelerator elements, and measurements of the time dependence of electron cloud buildup were carried out [11]. Estimates of long-lived electron cloud buildup at the LHC and consequences for vacuum chamber heat load have been presented in Ref. A principal goal of the Cornell Electron Storage Ring Test Accelerator program [16] is to investigate performance limitations in future high-energy low-emittance rings. These studies include measurements of electron cloud buildup caused by synchrotron-radiation-induced photoemission on the surface of the vacuum chamber. (c) electron detector cassette electron collection area (10 mm x 102 mm) copper collector (902 mm long)
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