Asymmetrical Spectrum Observed at the KEKB High-Energy Electron Ring

It was observed that the vertical tune-spectrum of an electron bunch showed an asymmetrical distribution biased to a higher tune in normal colliding operations. The asymmetry depended on the structure of the fill pattern of bunches and the vacuum pressure in the electron ring. We found that a beam-beam interaction affected the asymmetry in the tune spectrum, although a measured bunch did not collide. 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.