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A simple and compact scheme that enhances the brightness of self‐amplified spontaneous‐emission (SASE) free‐electron lasers is presented. The method combines the high‐brightness SASE scheme and the optical klystron concept to increase the temporal coherence of the produced radiation and to reduce the required length of the undulator beamline at the same time. The scheme is very simple and only requires compact chicanes between the modules of the undulator beamline. Simulations show that, in comparison with SASE, the brightness can be improved by up to a factor of ten and the required length to achieve saturation can be reduced by 20% or more. [ABSTRACT FROM AUTHOR]

X‐ray free‐electron lasers (FELs) are cutting‐edge scientific tools able to generate transversely coherent radiation with very high power and ultra‐short pulse durations. The self‐seeding mechanism has been proven to increase the longitudinal coherence of the FEL radiation but its efficiency could be significantly improved, especially for soft X‐rays. This paper proposes the enhancement of the performance of self‐seeding by combining it with the harmonic generation mechanism. In particular, by starting the process with a subharmonic of the wavelength of interest, the coherence of the produced radiation is improved, the undulator beamline becomes more compact, and the monochromator realization is simplified. Numerical simulations for SwissFEL are presented showing that the method can be employed, within a given space, to increase the spectral brightness by one order of magnitude or more with respect to standard self‐seeding. This coherence enhancement will be fundamental for many photon science applications and techniques such as resonant inelastic X‐ray scattering. [ABSTRACT FROM AUTHOR]

With the successful operation of free-electron lasers (FELs) as user facilities there has been a growing demand for experiments with two photon pulses with variable photon energy and time separation. A configuration of an undulator with variable-gap control and a delaying chicane in the middle of the beamline is proposed. An injected electron beam with a transverse tilt will only yield FEL radiation for the parts which are close to the undulator axis. This allows, after re-aligning and delaying the electron beam, a different part of the bunch to be used to produce a second FEL pulse. This method offers independent control in photon energy and delay. For the parameters of the soft X-ray beamline Athos at the SwissFEL facility the photon energy tuning range is a factor of five with an adjustable delay between the two pulses from −50 to 950 fs. [ABSTRACT FROM AUTHOR]

A new and simple method to generate X-ray free-electron-laser radiation with unprecedented spectral bandwidth above the 10% level is presented. The broad bandwidth is achieved by sending a transversely tilted beam through a transverse-gradient undulator. The extent of the bandwidth can easily be controlled by variation of the beam tilt or the undulator gradient. Numerical simulations confirm the validity and feasibility of this method. [ABSTRACT FROM AUTHOR]

An optimization of the undulator layout of X-ray free-electron-laser (FEL) facilities based on placing small chicanes between the undulator modules is presented. The installation of magnetic chicanes offers the following benefits with respect to state-of-the-art FEL facilities: reduction of the required undulator length to achieve FEL saturation, improvement of the longitudinal coherence of the FEL pulses, and the ability to produce shorter FEL pulses with higher power levels. Numerical simulations performed for the soft X-ray beamline of the SwissFEL facility show that optimizing the advantages of the layout requires shorter undulator modules than the standard ones. This proposal allows a very compact undulator beamline that produces fully coherent FEL pulses and it makes possible new kinds of experiments that require very short and high-power FEL pulses. [ABSTRACT FROM AUTHOR]