Experimental Research for Complex Beam-Beam Effects under Asymmetric Collision with a Finite Angle

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KEKB is an asymmetric electron/positron collider, which keeps the best luminosity in the world.The beam-beam effects were investigated with a gated-bunch method at KEKB. We obtained the following experimental results :(1)Collisions occur with a horizontal orbit offset to avoid the instability called "Egure" phenomena.(2)We found that the "Egure" phenomena are caused by a combination of the beam-beam and electron cloud effects under a finite crossing angle.(3)We obtained the experimental evidence of the dynamic beam-beam effects ; the emittance growth and the beta beat.We found new phenomena about a half-integer resonance. When a betatron tune more closely approached a half-integer, we observed a distortion just on a half-integer in the spectrum. The observed phenomena could be explained by supposing that some off-momentum particles in a bunch jumped into an unstable region in the half-integer resonance. A precise simulation including errors would be required to verify the phenomena.We found a fast ion effect related to collisions. It was observed that the vertical tune-spectrum of an electron bunch showed an asymmetrical distribution biased to a higher tune under normal colliding operations. The observations imply that the distortion in the spectrum is related to the field due to ions produced by colliding bunches placed forwards to a measured bunch. A tune-shift measurement supported the presence of ions.Based on these findings, we would like to promote more precise investigation about the beam dynamics of KEKB. The study would be useful and valuable for understanding beam dynamics in the ring accelerators.