Effect of Crab cavity HOMs on the coupled-bunch stability of HL-LHC

High frequency high order modes can significantly affect the coupled-bunch stability in a circular accelerator. With a large enough shunt impedance they may drive a coupled-bunch transverse instability. We have developed an analytical model and implemented it in the nested head-tail Vlasov solver. The results show that the instability is characterized by the excitation of many azimuthal intrabunch modes, which have similar growth rates, that make the traditional remedies such as a flat resistive feedback and chromaticity inefficient in suppressing it. For the High Luminosity Large Hadron Collider this could result in a significant increase of the stabilizing Landau octupole current, up to ∼100 A (∼20% of the maximum available current). In order to limit the increase below 10 A (∼2%), the transverse shunt impedance has to be kept below 1 MΩ/m.