Efficiency Enhancement Techniques for Free-Electron Lasers

Sammanfattning: The central question addressed in this thesis is how to make the free-electron laser (FEL) more efficient. In recent years, coherent diffraction imaging provides an important motivation for efficiency enhancement. This is because a more efficient FEL process enables converting a larger fraction of the electron beam's power into optical power. By increasing the average optical power to the terawatt level, an x-ray FEL will open the door to single-shot, single-molecule imaging with a sufficiently high signal-to-noise ratio.This thesis examines two techniques for efficiency enhancement, namely, undulator tapering and the phase jump method.For undulator tapering, the well-established analytical model by Kroll, Morton and Rosenbluth (KMR) is revisited and modified. With the aid of numerical simulations, it is demonstrated that the modified model results in a further enhancement in FEL efficiency beyond the original model.For the phase jump method, a new physics model is developed to describe the energy extraction mechanism in the longitudinal phase space. The model reveals the possibility to extract energy from electrons outside the ponderomotive bucket, as well as the potential to increase the spectral purity by suppressing the synchrotron sidebands.