Abstract

Transverse vertical wakefields can increase vertical emittance and distort the phase space of a bunch in a storage ring. Here we report recent measurements and simulations of these effects from wakefields from movable scrapers at the Cornell electron-positron storage ring test accelerator. Charge-dependent vertical beam size growth was observed with a single scraper inserted through the top of the storage ring vacuum chamber. No change in the beam size was observed with top and bottom scrapers inserted symmetrically. The apparent growth in the vertical beam size was due in large part to the $yz$ coupling (vertical crabbing) induced by the transverse monopole wake of the asymmetric scraper configuration. We explored this $yz$ coupling and thus the orientation at the observation point by varying the vertical betatron phase advance between the vertical beam size monitor and the scrapers. In addition, we found that existing residual, current-independent $yz$ coupling, perhaps due to nonzero vertical dispersion in the rf cavities, could be compensated by the scraper wake. Predictions from tracking simulations are in good agreement with the measurements. Moreover, the vertical beam size as a function of vertical displacement at narrow-gap chambers (closed scrapers and undulator chamber) was measured. We found the transverse wakefield produced by the off-axis beam could also introduce a $yz$ tilt as well as dilute the beam emittance.

Highlights

  • Charged particles moving in vacuum chambers can generate electromagnetic fields called wakefields

  • The measured beam size was obtained by fitting the 32 pixel data from the x-ray beam size monitor (xBSM) using a Gaussian function [20]. This fitting method catches the core part of a non-Gapussffiffiiffiffiaffiffinffiffi profile

  • Due to the non-Gaussian profile, this method tends to overestimate the vertical beam size compared to the Gaussian fitting method

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Summary

INTRODUCTION

Charged particles moving in vacuum chambers can generate electromagnetic fields called wakefields. In 2014, we observed a current-dependent growth in vertical beam size of a single electron bunch at low energy (2.1 GeV) upon inserting a single vertical scraper into the vacuum chamber [7]. The growth in beam size is due to the short-range transverse monopole wake associated with the asymmetric scraper configuration [8]. The good agreement between the simulations and measurements shows that the charge-dependent vertical beam size growth can be largely explained by the vertical-longitudinal (yz) coupling induced by the transverse monopole wake. The vertical beam size is measured as a function of the vertical displacement of the beam at both the scraper and the narrow-gap undulator chamber. A discussion and conclusion are provided in the last two sections

VERTICAL-LONGITUDINAL TILT
LATTICE AND COUPLING BUMP
Transverse monopole wake
Tracking simulation
Procedure
Beam size versus current
Phase difference
Preexisting tilt
BEAM SIZE VERSUS DISPLACEMENT
DISCUSSION
VIII. CONCLUSION

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