Abstract
In the design of the vacuum chamber of a storage ring, special care has to be taken to keep the impedance as low as possible, in order to not degrade the quality of the circulating beam. Since the start of the commissioning of the Italian synchrotron light source ELETTRA in 1993, several low gap insertion vacuum chambers with internal aperture of 14 mm have been installed into the storage ring. Monitoring of their individual impedances and effects has been carried out with measurements on the electron beam after each new installation. The subsequent evolutions of the total longitudinal and transverse impedances are presented in this paper. Furthermore, in contradiction with the foreseen single-bunch effect, noticeable excitations of particular narrow band longitudinal coupled multibunch instabilities as a function of the position of the blades of the vertical scraper have been observed. In order to achieve a deeper insight of the phenomenon, an exhaustive characterization of both the longitudinal and transverse impedances of the scraper has been carried out.
Highlights
ELETTRA is the Italian third generation synchrotron light source generating photons, mainly provided by insertion devices, in the very ultraviolet to the soft x-ray regions
As for all third generation light sources, the high brilliance and quality of the photons are guaranteed by the stringent requirements on the electron beam parameters, its high stability, and the maximum achievable beam current
The impedance status is reflected greatly by the maximum achievable stored current and during the first year of commissioning it was possible to store more than 60 mA in a single bunch and 700 mA in a multibunch with an 80% filling, exceeding by a large extent the design values in both cases
Summary
ELETTRA is the Italian third generation synchrotron light source generating photons, mainly provided by insertion devices, in the very ultraviolet to the soft x-ray regions. With a slightly negative chromaticity y ÿ0:5 the mode m 0 threshold was found at 22 mA above which the beam blew up vertically and oscillated This threshold was theoretically expected since using the measured impedance data the necessary Landau damping frequency full spread should have been at about 400 Hz, whereas the ‘‘natural’’ tune spread available = is 6 10ÿ6 yielding a frequency spread of 357 Hz. Because of the scarce interest of users in single bunch, the lack of special instrumentation, the low impedance, the reliability of the model, and no serious instability problems, no beam excitation measurements to distinguish the real and imaginary parts of the impedances or other additional measurements were performed at that time. A series of experiments is performed to understand the impedance behavior of this object, very important for radiation safe operation, and at the same time to ensure that it does not deteriorate the beam quality
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