Longitudinal Beam Dynamics with a Higher-Harmonic Cavity for Bunch Lengthening

We discuss the longitudinal beam dynamics in storage rings in the presence of a higher-harmonic cavity (HHC) system for bunch lengthening. We first review the general conditions for HHC operations, either in active or passive mode, assuming the stability of the system. For uniform filling patterns, a distinction is made between operations with a normal-conducting HHC, where optimal conditions for bunch lengthening can be satisfied, and operations with super-conducting HHC, where optimal conditions can be met only approximately. The option to operate the NSLS-II storage ring with a passive, super-conducting third harmonic cavity (3HC) system is discussed next. The stability and performance of the system in the presence of a gap in the uniform filling, which corresponds to the present mode of operation of the NSLS-II storage ring, is investigated with self-consistent Vlasov-Fokker-Planck simulations performed with the code SPACE [1].