Measurement of Wakefield Effects Caused by Electron Cloud at Kekb

In order to study the wakefield effects of an electron cloud, a test bunch was placed behind a bunch train, where the cloud density rapidly varied. A currentdependent tune shift and the tune spread of the test bunch were measured as a function of the bunch current while varying the bucket position of the test bunch. The vertical tune shift indicated a strong defocusing force together with a widened tune spread in a region of relatively low cloud density and low bunch current. However, the vertical defocusing disposition changed to a focusing force at a high cloud density and a high bunch current. The horizontal and vertical tune spreads also demonstrated nonlinear, as a function of the bunch current. The current at the maximum horizontal tune spread was approximately equal to the threshold current of the vertical size blowup. The variation in the horizontal tune spread suggested a new index for the vertical instability threshold caused by the electron cloud.