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TY - UNPB AU - Tsai, C.-Y. ED - Schaa, Volker R.W. ED - Decking, Winfried ED - Sinn, Harald ED - Geloni, Gianluca ED - Schreiber, Siegfried ED - Marx, Michaela TI - Physics of Post-Saturation Tapered FEL Towards Single-Frequency Terawatt Output Power J2 - Proc. of FEL2019, Hamburg, Germany, 26-30 August 2019 CY - Hamburg, Germany T2 - Free Electron Laser Conference T3 - 39 LA - english AB - TW-level output power of a single-pass high-gain x-ray FEL has recently attracted much attention. GW-level power can be achieved out of beneficial FEL instability. The remaining factor of 50 before reaching TW poses great challenges, including sustaining power growth and retaining spectral purity in the post-saturation regime. Post-saturation FEL physics involves the sideband instability [1], radiation diffraction [2], undulator tapering [3] etc. In this talk I will introduce FEL sideband and diffraction effects, and give an estimate of individual effects to the overall performance. Undulator tapering is known an effective route to enhance power extraction efficiency. We recently proposed an efficiency-enhancement scheme based on preserving the longitudinal beam phase space, which ensures trapping of resonant particles in the ponderomotive bucket and meanwhile takes advantage of the increasing radiation amplitude to precipitate the particle deceleration process. Analysis shows that the optimal power efficiency based on the proposed scheme, together with a prebunched beam, can be greatly improved within a relative short taper length and the sideband effects effectively suppressed. PB - JACoW Publishing CP - Geneva, Switzerland ER -