Power, bistability and post-saturation optimization in a pre-bunched free electron laser

Prebunching of the electron beam in a free-electron laser enables very high output powers and energy conversion efficiencies ( > 90%). The output power is ultimately limited only by the cavity losses. The analysis based upon the pendulum equation predicts two stable steady states. The lower steady state appears due to a new saturation mechanism caused by open phase-space trajectories. The existence of 2 stable saturation powers leads to a bistability in the output power of the pre-bunched FEL oscillator. In fact, when the power in the oscillator reaches the lower steady state there is no further growth. A new technique is proposed to obtain the upper steady-state level. In this approach, a post-saturation dynamical readjustment of the FEL parameters is used to reach the upper stable level of oscillator power emission.