Variations of betatron tune spectrum due to electron cloud observed in KEKB

In order to investigate the characteristics of electron clouds, the wake effects were measured at KEKB using a test bunch placed behind a bunch-train, where there was a rapid decay in the electron cloud density. The current- dependent tune-shift of the test bunch exhibited nonlinear behaviour in the vertical plane [1]. By observing the tune spectrum, we found that the spectrum width expanded and this was accompanied with a large negative tune slope at a low cloud density and at a low bunch current. However, as the cloud density increased, the spectrum width shrunk and this was accompanied with a positive tune slope. These experimental results suggested that a high electron cloud density caused an anti-damping effect in the tune spectrum. We believe that the variations in the tune slope and spectrum width might be related to the wake field in the resonator model, where the wavelength is comparable to the bunch length.