Vertical-longitudinal coupling induced by wakefields at the Cornell Electron-Positron Storage Ring Test Accelerator

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.