Abstract
Effective head-on collision and improvement in specific luminosity were established by the installation of crab cavities in KEKB. Tune spectra of a colliding bunch in a crabbing collision were observed by using a spectrum analyzer. The beam-beam spectrum showed strong nonlinear resonance. By taking into account this nonlinearity, the coherent beam-beam tune shift was measured as a function of the bunch current. The vertical beam-beam parameter estimated from the coherent beam-beam tune shift was found to agree with that observed by using a bunch-by-bunch luminosity monitor. The estimated vertical beam-beam parameter is saturated at approximately 0.05; this value is the beam-beam limit. Further, the bunch current corresponding to the beam-beam limit was found to be considerably lower than that used in usual operations. The horizontal beam-beam parameter was not saturated over 0.15. The horizontal beam-beam parameter estimated from the coherent beam-beam tune shift approximately agrees with a calculated beam-beam parameter considering the dynamic beam-beam effect.
Highlights
KEKB [1] is a multibunch, high-current, electron/positron collider for producing B mesons
The collider consists of two storage rings: a low energy ring (LER) for a 3.5GeV positron beam and a high energy ring (HER) for 8GeV electrons, respectively
The electron and positron beams collide at an interaction point (IP) with a horizontal crossing angle of 22 mrad
Summary
KEKB [1] is a multibunch, high-current, electron/positron collider for producing B mesons. The collider consists of two storage rings: a low energy ring (LER) for a 3.5GeV positron beam and a high energy ring (HER) for 8GeV electrons, respectively Both rings can store more than 1500 bunches, where the harmonic number is 5120 with an rf frequency of 509 MHz. The bunches are stored in two rings with a three-bucket (6 ns) or four-bucket (8 ns) spacing, forming a single bunch train followed by empty buckets that occupy approximately 5% of the circumference. Crab cavities that were installed at KEKB in 2007 can provide horizontal deflections operating at the rf frequency to bunches without changing the central orbit [2]. These crab cavities enable the realization of an effective head-on collision, while maintaining the crossing orbit at the IP. The section describes the relationship between the tune shifts and the beam-beam parameters
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