EXPERIMENTAL STUDY OF IMPEDANCES AND INSTABILITIES AT THE VEPP-4M STORAGE RING

Beam current at the VEPP-4M electron-positron collider is limited by transverse mode coupling (TMC) instability, occurred due to wake fields. Threshold current is decided mainly by transverse impedance of vacuum chamber. To study frequency dependence of the longitudinal and transverse impedances at the VEPP-4M, deviation of equilibrium RF phase and coherent betatron tune shift were measured in dependence of beam length. Impedance distribution along the ring was studied, using measurements of closed orbit in dependence of beam current. Instability evolution after injection was observed by a pickup using turn-by-turn measurement technique. Beam motion was also studied by computer simulation based on two particle model. Experimental results and results of the simulation are presented.