Stochasticity and resonances in the two beam accelerator

Abstract

The two beam accelerator is a proposed linear e+e− machine employing 2×500 tapered free electron lasers (FELs) as a source of microwave power. A high current, low energy (20 MeV) beam derives the FELs, producing 1 cm microwaves. These microwaves are used to accelerate a low current beam to energies 1 TeV. High efficiencies are obtained by re‐accelerating the low energy beam after passing through each FEL. Restoring the 2 MeV/particle loss on passing through each FEL section (using an induction accelerator) avoids wasting the 18 MeV/particle left in the beam. The periodic nature of the acceleration and deceleration of the low energy beam can lead to stochastic motion. We exhibit design criteria that ensure that detrapping of the low energy beam by this stochasticity is minimal, preserving the high efficiences inherent in a single tapered FEL. Numerical integrations of a one dimensional model for the FELs are presented showing various degrees of detrapping. We also explore the effects of islands produced by resonances between the periodic acceleration and the trapped particle motion. These islands represent coherent motion of large numbers of trapped particles, leading to oscillations in the power output of the FELs.