Abstract
We report on extensive measurements at the Cornell Electron-Positron Storage Ring of electron-cloud-induced betatron tune shifts for trains of positron bunches at 2.1 and 5.3 GeV with bunch populations ranging between $0.64\ifmmode\times\else\texttimes\fi{}{10}^{10}$ and $9.6\ifmmode\times\else\texttimes\fi{}{10}^{10}$. Measurements using a witness bunch with variable distance from the end of the train and variable bunch population provide information on cloud decay and cloud pinching during the bunch passage. We employ Monte Carlo simulations of the reflection and absorption of synchrotron radiation photons to determine the pattern of absorption sites around the circumference of the storage ring. The Geant4 simulation toolkit is used to model the interactions of the photons with the beampipe wall and determine the production energy and location distributions of the photoelectrons which seed the electron cloud. An electron cloud buildup model based on fitted ring-averaged secondary-yield properties of the vacuum chamber predicts tune shifts in good agreement with the measurements.
Highlights
The buildup of low-energy electron densities in the vacuum chamber of a positron storage ring can result in betatron tune shifts, instabilities and emittance growth
The modeling of electron cloud effects on beam dynamics proceeds in four steps: (1) 3D calculation of the pattern of absorbed synchrotron radiation around the ring including the effects of photon reflections [11], (2) simulation of the interactions of absorbed photons with the vacuum chamber wall which lead to the emission of electrons [12,17,18], (3) a time-sliced weak-strong model [19,20] for electron cloud development along a train of positron bunches, including a phenomenological model for secondary-electron yield (SEY) from the beampipe walls, and (4) calculations of betatron tune shifts using the space-charge electric field gradients derived from the cloud buildup model [4,5,6,21]
We have obtained improved measurements of coherent betatron tune shifts along trains of positron bunches in the horizontal and vertical planes for bunch populations ranging from 0.64 × 1010 to 9.6 × 1010 at 2.1 and 5.3 GeV, enabling advances in the predictive power of electron cloud buildup modeling
Summary
The buildup of low-energy electron densities in the vacuum chamber of a positron storage ring can result in betatron tune shifts, instabilities and emittance growth. We describe techniques to measure electron-cloud-induced tune shifts, and to use the measurements to constrain predictive numerical models of electron cloud phenomena. Analytic and numerical treatments of electron cloud (EC) contributions to coherent tune shifts were originally presented in Ref. The Cornell Electron-Positron Storage Ring (CESR) was reconfigured as a test accelerator in 2008 [3]. A comprehensive summary of the project, which included electroncloud buildup and low-emittance lattice studies, can be found in the CESRTA Phase I Report [4]. The results reported here concern three lattice configurations of the CESR ring: the test accelerator configurations at 2.1 GeV and at 5.3 GeV, and the 6.0 GeV upgrade to be commissioned in 2019.
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