Transverse coupling impedance of the VEPP-4M collider: measurements and simulations

The main contribution into the total impedance of the VEPP-4M is made by places of non-uniformity of vacuum chamber, such as sharp change of cross section, ceramic inserts, vertical electrostatic separators and by HOMs of cavities. The computer code GdfidL is used for the estimate of impedances in some of above structures. A new method for measurement of transverse coupling impedance has been used. The method is based on measuring of the closed orbit deviation caused by localized impedance. Transverse impedance acts on the beam as a defocusing quadrupole, strength of which depends on the beam current. If a local bump of closed orbit has been created at the impedance location, then the orbit deviation occurs while varying the beam current. The local impedance can be evaluated using the orbit deviation measured. The results of the measurements and numerical calculations are presented and compared