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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.

Multibunch instabilities of a storage ring electron beam occur due to coherent particle oscillations generated through a bunch to bunch coupling via the impedances, deteriorating the beam quality. One cure for multibunch instabilities is Landau damping, i.e., introducing a spread in the oscillation frequencies among the particles of the individual bunches in order to destroy the coherence of the coupled multibunch oscillation. Measurements at ELETTRA have shown that the harmonic sextupole provides Landau damping capable of suppressing transverse multibunch instabilities. The damping is induced by the nonlinear tune spread with amplitude among the electrons within the individual bunches.

The ELETIRA storage ring is currently operated over a wide electron-energy range (0.75-1.5 GeV and 2-2.4 GeV).This paper reports on measurements performed in order to characterize the longitudinal beam stability when the machine is operated in the low-energy range in single-bunch filling mode.