Abstract
A self-consistent theory of a free electron laser (FEL) with slowly varying beam and undulator parameters is developed using the WKB approximation. The theory is applied to study the performance of a self-amplified spontaneous emission (SASE) FEL when the electron beam energy varies along the undulator as would be caused by vacuum pipe wakefields and/or when the undulator strength parameter is tapered in the small signal regime before FEL saturation. We find that a small energy gain or an equivalent undulator taper slightly reduces the power gain length in the exponential growth regime and can increase the saturated SASE power by about a factor of 2. Power degradation away from the optimal performance can be estimated based upon knowledge of the SASE bandwidth. The analytical results, which agree with numerical simulations, are used to optimize the undulator taper and to evaluate wakefield effects.
Highlights
High-gain free electron lasers (FELs) are being developed as extremely bright x-ray sources of a next-generation radiation facility
The main effect of the undulator wakefield in a FEL slice is due to the energy change along the undulator distance and may be considered to be equivalent to that caused by tapering the undulator strength parameter
The classical treatment of a tapered undulator [3] has been focused on the FEL saturation regime where a significant energy loss induced through the FEL interaction can be offset by tapering the undulator parameter
Summary
High-gain free electron lasers (FELs) are being developed as extremely bright x-ray sources of a next-generation radiation facility. After integration over the length of the undulator, this correction can give rise to a noticeable change of the radiation power at the end of the undulator We apply this theory to study the SASE FEL under a linear energy variation along the undulator distance and find that a fractional energy gain of about 2 over the saturation distance or an equivalent undulator taper can slightly reduce the gain length in the exponential growth regime and improve the saturated power by about a factor of 2 as compared to a constant-parameter FEL. III, we ignore the transverse motion of electrons and the radiation diffraction to obtain the WKB solution for the one-dimensional (1D) FEL system We apply this solution to study the effect of a linear energy change on both seeded and SASE FELs. The results obtained in the 1D case are generalized to the threedimensional (3D) system in Sec. IV and are applied to study the effects of the LCLS undulator wakefields in. A general discussion of the WKB approximation using the matrix formalism and its application to the 3D FEL system is presented in Appendices A and B
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callMore From: Physical Review Special Topics - Accelerators and Beams
Disclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
