Dispersion and energy compensation in high-gradient linacs for lepton colliders

The shape of an rf pulse is distorted due to dispersion encountered in acceleration through traveling-wave linear accelerator structures. Simulations are made to ascertain the severity of this distortion in cavities designed to operate at various group velocities. The pulse suffers maximum degradation when propagated through accelerating cavities with a phase advance per cell in the vicinity of π, where the group velocity reaches its minimum value. Several cavities are simulated to study the pulse distortion and compared with experiments performed on a high phase advance structure H60VG3, which has a phase advance of 5π/6 per cell. A circuit model and a mode matching code are used to perform these simulations on accelerating structures consisting of multiple cells. Beam loading is taken into account and the implications on energy dispersion are also analyzed. Means of mitigating for this energy deviation are discussed.