Abstract
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.
Highlights
Free-electron lasing at wavelengths shorter than the ultraviolet can be achieved with a single-pass, high-gain free electron laser (FEL) amplifier operating in the socalled self-amplified spontaneous emission (SASE) mode, where the amplification process starts from shot noise in the electron beam [1,2,3]
Otherwise this long-wavelength radiation pulse can be used for a cross-correlation measurement with a powerful pulse from a conventional laser [15]
When the SASE FEL operates in the exponential gain regime and just before the saturation, the average shape of the radiation pulse is imprinted in the electron bunch, i.e., it is repeated by the shape of beam energy loss
Summary
Free-electron lasing at wavelengths shorter than the ultraviolet can be achieved with a single-pass, high-gain free electron laser (FEL) amplifier operating in the socalled self-amplified spontaneous emission (SASE) mode, where the amplification process starts from shot noise in the electron beam [1,2,3]. Synchronization of pulses from an x-ray FEL with optical pulses from a conventional laser with femtosecond accuracy is not a trivial task due to a jitter in an electron bunch arrival time. If the optical pulse is sufficiently powerful, it can be directly used in a pump-probe experiment Otherwise this long-wavelength radiation pulse can be used for a cross-correlation measurement with a powerful pulse from a conventional laser (that is used in the pumpprobe experiment) [15]. This allows one to determine a relative delay between two pulses for every shot and to sort out experimental data using this information. Ered effect, namely on-line monitoring of x-ray pulse duration by translating its width and shape into optical range, i.e., making an ‘‘optical replica’’ of an x-ray pulse
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