PSI, Villigen
The coupled system of radiation interacting with a co-propagating electron beam within an undulator of an FEL exhibits many degrees of freedom. Only in an idealized and simplified model can the FEL equations be solved analytically and a more complete description requires numerical methods. Therefore numerical codes have been developed along with the advances in FEL theory, starting from a simple 1 D model to today's fully time-dependent 3D simulations, utilizing large scale parallel computers. This presentation gives a brief history of FEL simulation and addresses the remaining challenges in FEL modeling which we hope to solve in the near future.
DESY, Hamburg
Start-up of the amplification process in x-ray FELs from the shot noise in the electron beam defines a specific behavior of longitudinal and transverse coherence properties of the radiation. Particularly important is the case of an x-ray FEL optimized for maximum gain of the fundamental radiation mode. Applying similarity techniques to the results of numerical simulations allowed us to find universal scaling relations for the main characteristics of an optimized X-ray FEL operating in the saturation regime: efficiency, coherence time and degree of transverse coherence. We find that with an appropriate normalization of these quantities, they are functions of only the ratio of the geometrical emittance of the electron beam to the radiation wavelength. Statistical and coherence properties of the higher harmonics of the radiation are highlighted as well.
University of Tel-Aviv, Faculty of Engineering, Tel-Aviv
The cooperative spontaneous emission from a bunch of particulate dipole radiators (e.g. excited atoms) can be enhanced (super-radiance) or suppressed (sub-radiance) by proper phasing of the radiators[1] Analysis and 3-D simulation of collective interaction micro-dynamics in a drifting e-beam, reveal a process of homogenization of the particles distribution and suppression of the beam current shot-noise[2]. Consequently, such a beam would exhibit Dicke’s kind of spontaneous emission sub-radiance when injected into a FEL. With present state of the art technology, suppression and control of FEL SASE power can be attained at optical frequencies up to the UV. A theory will be presented on the implications of this beam noise control on the attainable coherence level of seed injected FELs. It is shown that when the beam current shot-noise is suppressed, the coherence of the FEL is limited by the beam energy spread. The fundamental theoretical limit of FEL coherence, analogously to conventional lasers[3], is found to be, the quantum noise limit. This would be attainable only if the beam energy spread can be reduced below the level of the photon emission energy.
[1] R. H. Dicke, Phys. Rev. 93, 99 (1954).
[2] A.Gover, E.Dyunin, Phys. Rev.Lett., {10}2, 154801, (2009).
[3] A.L.Schawlow and C.H.Townes, Phys. Rev., 112, 1940
USTC/NSRL, Hefei, Anhui
The harmonics field effect of planar undulator on Free-Electron Laser (FEL) harmonic generation has been analyzed. For both the linear and the nonlinear harmonic generation, the harmonic generation fraction can be charactered by the coupling coefficients. The modification of coupling coefficients is given when third harmonics field component exist, thus the enhancement of the harmonic radiation can be predicted. With the third harmonics magnet field being 30 percent of the fundament, for both the small signal gain and the nonlinear harmonic generation in high gain, the intensity of third-harmonic radiation can maximally be doubled.