X-ray Self-amplified Spontaneous Emission Research Articles

High spatial coherence and short pulse duration revealed by the Hanbury Brown and Twiss interferometry at the European XFEL.

Second-order intensity interferometry was employed to study the spatial and temporal properties of the European X-ray Free-Electron Laser (EuXFEL). Measurements were performed at the soft x-ray Self-Amplified Spontaneous Emission (SASE3) undulator beamline at a photon energy of 1.2 keV in the Self-Amplified Spontaneous Emission (SASE) mode. Two high-power regimes of the SASE3 undulator settings, i.e., linear and quadratic undulator tapering at saturation, were studied in detail and compared with the linear gain regime. The statistical analysis showed an exceptionally high degree of spatial coherence up to 90% for the linear undulator tapering. Analysis of the measured data in spectral and spatial domains provided an average pulse duration of about 10 fs in our measurements. The obtained results will be valuable for the experiments requiring and exploiting short pulse duration and utilizing high coherence properties of the EuXFEL.

A hard X-ray self-amplified spontaneous emission free-electron laser optimizationusing evolutionary algorithms for dedicated user applications

Accelerator-based fourth-generation light sources are utilized in a wide range of interdisciplinary applications such as nanotechnology, materials science, biosciences, and medicine. A hard X-ray free-electron laser (FEL), as a state-of-the-art light source, was optimized using evolutionary algorithms for dedicated user applications such as X-ray Raman scattering (XRS), resonant inelastic X-ray scattering (RIXS), and X-ray emission spectroscopies (XES). Optimal parameter sets were obtained for an in-vacuum planar undulator driven by an 8 GeV electron beam. Performance parameters of self-amplified spontaneous emission (SASE) operation (i.e. optimized SASE performance parameters through evolutionary algorithms) were found to be consistent with operating X-ray FEL facilities around the world. It is shown that FEL characteristics for specific user experiments can be optimized by finding several evolutionary algorithm solutions within the range of 5 keV to 10 keV.

Diffraction properties of a strongly bent diamond crystal used as a dispersive spectrometer for XFEL pulses

Self-amplified spontaneous emission (SASE) enables X-ray free-electron lasers (XFELs) to generate hard X-ray pulses of sub-100fs duration. However, due to the stochastic nature of SASE, the energy spectrum fluctuates from pulse to pulse. Many experiments that employ XFEL radiation require the resolution of the spectrum of each pulse. The work presented here investigates the capacity of a thin strongly bent diamond crystal to resolve the energy spectra of hard X-ray SASE pulses by studying its diffraction properties. Rocking curves of the symmetric C*(440) reflection have been measured for different bending radii. The experimental data match the theoretical modelling based on the Takagi-Taupin equations of dynamical diffraction. A uniform strain gradient has proven to be a valid model of strain deformations in the crystal.

Optical afterburner for an x-ray free electron laser as a tool for pump-probe experiments

We propose a new scheme for two-color operation of an x-ray self-amplified spontaneous emission free electron laser (SASE FEL). The scheme is based on an intrinsic feature of such a device: chaotic modulations of electron beam energy and energy spread on the scale of FEL coherence length are converted into large density modulations on the same scale with the help of a dispersion section, installed behind the x-ray undulator. Powerful radiation is then generated with the help of a dedicated radiator (like an undulator that selects a narrow spectral line), or one can simply use, for instance, broadband edge radiation. A typical radiation wavelength can be as short as a FEL coherence length, and can be redshifted by increasing the dispersion section strength. In practice it means the wavelength ranges from vacuum ultraviolet to infrared. The long-wavelength radiation pulse is naturally synchronized with the x-ray pulse and can be either directly used in pump-probe experiments or cross correlated with a high-power pulse from a conventional laser system. In this way experimenters overcome jitter problems and can perform pump-probe experiments with femtosecond resolution. Additional possibilities like on-line monitoring of x-ray pulse duration (making ``optical replica'' of an x-ray pulse) are also discussed in the paper. The proposed scheme is very simple, cheap, and robust, and therefore can be easily realized in facilities like FLASH, European XFEL, LCLS, and SCSS.

Coherence properties of the European XFEL

The European x-ray free-electron laser (XFEL) provides x-ray self-amplified spontaneous emission (SASE) FEL radiation in the wavelength range from 0.1 to 3 nm using three undulator systems. The SASE mode of operation at the European XFEL defines specific behavior of longitudinal and transverse coherence properties. In this paper, we describe the evolution of the temporal and transverse correlation functions along the undulator length, and we extract the corresponding evolution of coherence time and degree of transverse coherence as typical figures of merit. Generation of coherent radiation inside the FEL undulators is followed by beam transport to the experiments. During transport, the total number of coherent modes is preserved, but the wavefront can be disturbed, and we analyze the conditions under which this occurs. It is emphasized that the development of experimental observables for the degree of coherence and wavefront properties will be important for experiments using coherent x-ray radiation.

Scheme for stabilization of output power of an x-ray self-amplified spontaneous emission free-electron laser

Output stability of an XFEL setup is naturally linked to the stability of the linac rf system through the bunch compression process. This fact leads to very tight tolerances for rf amplitudes and phases. We propose a feedback scheme using an optically modulated electron beam where the compression factor is self-stabilized due to the action of longitudinal space charge fields. Our scheme allows loosening rf tolerances by an order of magnitude. Alternatively, for the same variations of rf parameters, stability of the FEL pulse energy could be improved by an order of magnitude.

Design formulas for short-wavelength FELs

Simple formulas for optimization of vacuum ultraviolet (VUV) and X-ray self-amplified spontaneous emission (SASE) free electron lasers (FELs) are presented. The FEL gain length and the optimal β-function are explicitly expressed in terms of the electron beam and undulator parameters. The FEL saturation length is estimated taking into account energy diffusion due to quantum fluctuations of the undulator radiation. Examples of the FEL optimization are given. Parameters of a SASE FEL, operating at the Compton wavelength, are suggested.

X-ray FEL with a meV bandwidth

A new design for a single pass X-ray Self-Amplified Spontaneous Emission (SASE) FEL was proposed by Feldhaus et al. (Opt. Commun. 140 (1997) 341) and named “two-stage SASE FEL”. The scheme consists of two undulators and an X-ray monochromator located between them. For the Angström wavelength range the monochromator can be realized using Bragg reflections from crystals. We propose a scheme of monochromator with a bandwidth of 20meV for the 14.4keV X-ray SASE FEL being developed in the framework of the TESLA linear collider project. The spectral bandwidth of the radiation from the two-stage SASE FEL (20meV) is determined by the finite duration of the electron pulse. The shot-to-shot fluctuations of energy spectral density are dramatically reduced in comparison with the 100% fluctuations in a SASE FEL. The peak and average brilliance are three orders of magnitude higher than the values which could be reached by a conventional X-ray SASE FEL.

Possible application of X-ray optical elements for reducing the spectral bandwidth of an X-ray SASE FEL

A new design for a single pass X-ray Self-Amplified Spontaneous Emission (SASE) FEL is proposed. The scheme consists of two undulators and an X-ray monochromator located between them. The first stage of the FEL amplifier operates in the SASE linear regime. After the exit of the first undulator the electron bunch is guided through a non-isochronous bypass and the X-ray beam enters the monochromator. The main function of the bypass is to suppress the modulation of the electron beam induced in the first undulator. This is possible because of the finite value of the natural energy spread in the beam. At the entrance to the second undulator the radiation power from the monochromator dominates significantly over the shot noise and the residual electron bunching. As a result the second stage of the FEL amplifier operates in the steady-state regime when the input signal bandwidth is small with respect to that of the FEL amplifier. Integral losses of the radiation power in the monochromator are relatively small because grazing incidence optics can be used. The proposed scheme is illustrated for the example of the 6 nm option SASE FEL at the TESLA Test Facility under construction at DESY. As shown in this paper the spectral bandwidth of such a two-stage SASE FEL (Δλ/λ⋍ 5 × 10−5) is close to the limit defined by the finite duration of the radiation pulse. The average brilliance is equal to 7 × 1024photons/(s × mrad2 × mm2 × 0.1% bandw.) which is by two orders of magnitude higher than the value which could be reached by the conventional SASE FEL. The monochromatization of the radiation is performed at a low level of radiation power (about 500 times less than the saturation level) which allows one to use conventional X-ray optical elements (grazing incidence grating and mirrors) for the monochromator design.