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 powe r is ultimately limited only by the cavity losses. The analysis based upon the pendulum equation predicts two stable steady states. The lower steady s tate appears due to a new saturation mechanism caused by open phase-space t rajectories. The existence of 3 stable saturation powers leads to a bistabi lity in the output power of the pre-bunched FEL oscillator. In fact. when t he power in the oscillator reaches the lower steady state there is no furth er growth. A new technique is proposed to obtain the upper steady-state lev el. In this approach, a post-saturation dynamical readjustment of the FEL p arameters is used to reach the upper stable level of oscillator power emiss ion. (C) 2000 Published by Elsevier Science B.V. All rights reserved.