Optics property analysis and a novel bandwidth control scheme for a terawatt FEL

Free Electron Laser (FEL) is a powerful source that generates high brightness radiation for scientific research. To facilitate single molecule imaging, higher brightness FEL is needed. One way to enhance brightness is to increase total photon number by tapering the undulator after saturation. Energy extraction efficiency can be further improved by a narrow bandwidth seed. This study develops a method to characterize radiation property in FEL and investigate a new scheme to generate narrow bandwidth seed for tapered FEL.;Mode decomposition method is applied to analyze transverse mode content in FEL radiation. Transverse electric field distribution is decomposed into a complete orthonormal modes by numerical integration. Laguerre Gaussian and Hermite Gaussian modes are used in this study. With this decomposition method, mode contents are analyzed along the undulator from linear regime to post saturation regime. Optics properties such as spot size and source location are obtained in different regime. A 'line source' model is also investigated and compared with the decomposition result in post saturation regime. Using this mode decomposition method and Genesis 1.3, eigenmodes are found in an FEL system.;The Improved Self-Amplified Spontaneous Radiation (iSASE) is a novel method to improve temporal coherence in a SASE FEL. Phase shifters (chicane) are used to provide additional relative slippage between the radiation field and electron bunch. Bandwidth reduction mechanism of iSASE is studied. It is demonstrated that the phase slippage is equivalent to a modulation function to FEL SASE power spectrum. Correlation function is introduced to describe the similarity between the shifted field and new grown field. We investigate several different iSASE configurations and their tolerance to the phase shifter error.