Effects of a harmonic cavity at the ESRF

The installation of a harmonic cavity to provide bunch lengthening and thereby increase the Touschek lifetime is considered for the ESRF storage ring. Simulations with a particle tracking code have shown that for few bunch operation-where the current per bunch is large-a strong lengthening already results from the coupling with the broadband impedance of the vacuum chamber. Therefore the net gain with an additional harmonic cavity is only moderate. For multibunch operation, however, where the current per bunch is small, a harmonic cavity is expected to increase the bunch length by up to a factor six, with a corresponding increase in Touschek lifetime. The Landau damping provided by the additional nonlinearity of the RF voltage has also been studied in detail. As an important and somewhat surprising result, it turns out that the thresholds for longitudinal coupled bunch instabilities may even be reduced when the operation parameters of the harmonic cavity are optimised to provide a maximum bunch lengthening.