Experimental observation of a mode-1 instability driven by Landau cavities in a storage ring

Landau cavities used to lengthen the bunches in storage rings necessarily constitute a significant impedance. Because of the particular phase of the field required for bunch lengthening, they are often detuned quite considerably from resonance, more so than the main cavities. As a result, their impedance can excite the first coupled-bunch mode such that it becomes unstable. This phenomenon has previously been predicted [M. Venturini, Phys. Rev. Accel. Beams 21, 114404 (2018)] and characterized in simulations [T. He, Phys. Rev. Accel. Beams 25, 024401 (2022)] but experimental observation is yet to be documented. In this paper, the experimental observation of coupled-bunch modes-$\pm 1$ excited by the Landau and main cavities in a fourth-generation light-source storage ring is presented. Features of the instability such as amplitude and coherent frequency at saturation have been measured and its dependency on the main rf voltage has been explored. The impact of a parked main cavity has also been investigated.