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

KEKB是一台非对称电子/正电子对撞机，它保持了世界上最好的光度。我们得到了以下实验结果：(1)碰撞发生时有水平轨道偏移，避免了被称为“Egure”现象的不稳定性。(2)我们发现“Egure”现象是由束流和电子云效应在有限的交叉角下共同引起的。(3)我们获得了束流动态效应、发射度增长和β节拍的实验证据。我们发现了关于半整数共振的新现象。当倍他电子管的曲调更接近半整数时，我们观察到频谱中只有半整数的失真。这些观察到的现象可以通过假设束团中的一些非动量粒子在半整数共振中跳入不稳定区域来解释。需要包含误差的精确模拟来验证这一现象。我们发现了与碰撞有关的快离子效应。观察到在正常碰撞条件下，电子束的垂直调谐谱呈现出偏向高调的不对称分布。这些观测表明，光谱中的扭曲与电场有关，这是由于离子与向前放置在被测量的束团上的束团发生碰撞而产生的。时移测量支持离子的存在。基于这些发现，我们希望促进对KEKB束流动力学的更精确的研究。这一研究对于理解环形加速器中的束流动力学具有重要的参考价值。

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.