Abstract
Energy extraction efficiency of a free electron laser (FEL) can be greatly increased using a tapered undulator and self-seeding. However, the extraction rate is limited by various effects that eventually lead to saturation of the peak intensity and power. To better understand these effects, we develop a model extending the Kroll-Morton-Rosenbluth, one-dimensional theory to include the physics of diffraction, optical guiding, and radially resolved particle trapping. The predictions of the model agree well with that of the GENESIS single-frequency numerical simulations. In particular, we discuss the evolution of the electron-radiation interaction along the tapered undulator and show that the decreasing of refractive guiding is the major cause of the efficiency reduction, particle detrapping, and then saturation of the radiation power. With this understanding, we develop a multidimensional optimization scheme based on GENESIS simulations to increase the energy extraction efficiency via an improved taper profile and variation in electron beam radius. We present optimization results for hard x-ray tapered FELs, and the dependence of the maximum extractable radiation power on various parameters of the initial electron beam, radiation field, and the undulator system. We also study the effect of the sideband growth in a tapered FEL. Such growth induces increased particle detrapping and thus decreased refractive guiding that together strongly limit the overall energy extraction efficiency.
Highlights
Recent results on single pulse coherent diffraction imaging of proteins [1] and viruses [2] using an x-ray free electron laser (FEL) show that the resolution can be improved by both increasing the number of the coherent photons and simultaneously reducing the pulse duration to about 10 femtoseconds or less, requiring a peak power of one terawatt (TW) or larger compared to the present values of 20 to 50 GW available at saturation from the selfamplified spontaneous emission (SASE) mode
Theoretical work done at DESY [3] and SLAC [4] shows that one way to increase the peak radiation power of a SASE x-ray FEL to the TW level is to use a tapered undulator, following a concept initially proposed by Kroll, Morton, and Rosenbluth (KMR) [5], together with the self-seeding option [6]
IV, we introduce another important phenomenon that leads to premature saturation of the radiation power in a tapered FEL, i.e., the sidebands excited by the SASE components that originate from the shot noise on the MODELING AND MULTIDIMENSIONAL OPTIMIZATION OF . . . Phys
Summary
Recent results on single pulse coherent diffraction imaging of proteins [1] and viruses [2] using an x-ray free electron laser (FEL) show that the resolution can be improved by both increasing the number of the coherent photons and simultaneously reducing the pulse duration to about 10 femtoseconds (fs) or less, requiring a peak power of one terawatt (TW) or larger compared to the present values of 20 to 50 GW available at saturation from the selfamplified spontaneous emission (SASE) mode. To improve the overall energy extraction efficiency requires sophisticated control of the decrease of refractive guiding and particle detrapping along the undulator Gaussian-profile approximation.—It is assumed that both the radiation and electron beam transverse profiles approximately follow Gaussian distributions throughout the undulator; i.e., we neglect higher order modes in the radiation field that can develop when rs starts to significantly exceed rb
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