Dynamic desynchronization of BFEL optical resonator

The start-up of a free electron laser driven by short electron bunches is lethargic when the cavity roundtrip time is perfectly synchronized to the electron bunch repetition rate. In order to restore the gain, it is necessary to slightly desynchronize the resonator, which is usually done by reducing the resonator length. At the desynchronism resulting in the largest small-signal gain per pass during the build-up stage, the largest laser power output cannot be obtained simultaneously. Dynamic desynchronization of a FEL optical resonator by linear ramping of the operating frequency is not only to restore the gain, but also to optimize the laser output of a FEL oscillator. We use an arbitrary variation of desynchronism instead of the linear variation at FELIX in the experiment of the Beijing FEL to optimize its performance. The fast electronic phase shifter in the RF system is used to adjust the desynchronism of the optical resonator of the Beijing FEL oscillator. In the paper, the basic principle of operation is briefly introduced first. Then, the schematic diagram of the control system is presented. Finally, typical experimental results are given to illustrate the effectiveness of the method.