Local bump method for measurement of transverse impedance of narrow-gap ID chambers in storage rings

A local bump method has been proposed to determine the transverse impedance of narrow-gap insertion-device (ID) vacuum chambers. As a stored bunch passes through an impedance section, the bunch receives a small transverse kick, which depends on the bunch total charge, the bunch length, and the transverse position. The kick can be measured as a function of orbit by applying a four-magnet bump and forcing closure by a running orbit correction. We measured the ID vacuum chambers with apertures of 5 mm and 8 mm, and a straight section with regular 42 mm aperture extrusion. Two types of local bumps are applied: a parallel and an angle bump, each of which emphasizes the impedance of different parts of the straight section.