MEASUREMENT OF WAKE EFFECTS BY MEANS OF TUNE SHIFT IN THE KEKB LOW-ENERGY RING

Electron clouds produced by a positron beam induce both single-bunch and coupled-bunch instabilities. The effects of electron clouds limit the performance of KEKB. An effect of the transverse dipole wake-field due to the electron cloud including the impedance effects was tried to measure at the KEKB LER using a tune-shift method. A test bunch was placed behind a bunch-train of a positron beam. The tune shift as a function of the current of a test bunch was measured under a constant traincurrent, while changing the bucket position of the test bunch. The horizontal tune shift did not quite change as a function of the bucket position. Compared to that, the vertical tune shift indicated a strong defocusing field and changed to a focusing field when a test bunch greatly approached a train. The measurements also indicated an asymmetric contribution of the solenoids between the horizontal and vertical planes.