Abstract
A series of measurements were carried out to investigate the dynamic beam-beam effects at KEKB. The beam-beam kick and the coherent beam-beam tune shift were measured by comparing the beam parameters between a colliding bunch and a noncolliding one. We found that collisions occur with a horizontal orbit offset to obtain the maximum luminosity, and that this reproducible offset appears to be attributed to an electron cloud effect, which might play an important role in the beam-beam interaction. The horizontal beam size estimated from the beam-beam kick slightly decreased as the beam-beam parameter increased. The horizontal emittance estimated from the beam-beam tune shift was higher than an unperturbed emittance, and agreed with the calculated emittance using a higher-mode tune. The ratio of the square-root betatron function between two locations in the ring was measured as a function of the beam-beam parameter. These experimental results would provide useful information on dynamic beam-beam effects.
Highlights
KEKB [1,2] is a multibunch, high-current, electron/ positron collider for B-meson physics
The collider consists of two storage rings: the Low Energy Ring (LER) for a 3.5-GeV positron beam and the High Energy Ring (HER) for 8-GeV electrons
We understand that a positron bunch experiences a short-range wakefield due to an electron cloud produced by previous bunches in the case of a short bunch spacing [15]
Summary
KEKB [1,2] is a multibunch, high-current, electron/ positron collider for B-meson physics. When the horizontal strength of the iBump magnets was set below a limit, we observed a sudden decrease in the luminosity, called egure [5], accompanied by an increase of the vertical size of the positron beam. In order to investigate any dynamic beam-beam effects and mysterious phenomena in collision tuning, the beambeam effects were measured by comparing the beam parameters between collision and noncollision bunches. This type of monitor can detect the beam-beam effects, and is not necessary to install near the complicated IP region.
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