Cumulative HOM Excitation and Transition Effects in LCLS-II

Abstract

LCLS-II is a proposed upgrade of the most powerful X-ray laser in the world built at SLAC National Accelerator Laboratory. When complete, multiple free electron laser (FEL) sections will be powered by a continuous wave superconducting radio frequency (SRF) electron linac. The vital parameters for FEL radiation quality are beam emittance and beam transverse position stability. Excitation of high order modes (HOM) in SRF niobium cavities leads to additional beam power dissipation through incoherent and coherent losses. Energy stored in HOM may cause cumulative effects — transverse and longitudinal position displacement of the center of bunches, which leads to transverse and longitudinal emittance dilution. Cumulative effects due to dipole HOM excitation in LCLS-II are analyzed. Transition HOM effects are caused by sudden changes (periodic or single case) in the beam bunch structure. Bunch center position disturbance due to transition HOM effects is estimated.