Abstract
It is well known that seeding can be used to produce narrow-bandwidth and fully coherent x-ray free-electron lasers (FELs). Self-seeding, which uses an extra undulator to generate the seed pulse, is perhaps one of the most promising methods to accomplish this. In the hard x-ray regime with high-energy electrons, this method requires a large magnetic chicane to match the path-length delay of the x-ray monochromator that selects a narrow bandwidth of radiation. Such a chicane not only takes a large footprint to build, but also may degrade the electron-beam qualities through incoherent and coherent synchrotron radiation. In this paper, we present an alternative two-bunch self-seeding scheme. The two bunches are precisely separated to match the x-ray delay of the monochromator and eliminate the need for a long, complex magnetic chicane. The spectrally filtered self-amplified spontaneous emission x-ray pulse produced by the first bunch is combined with the second electron bunch at the entrance of the second undulator and then amplified to the saturation level. We present start-to-end simulation results based on the linac coherent light source hard x-ray FEL and show that this method can produce a nearly fully coherent x-ray pulse at a few GW power level.
Highlights
The successful commissioning and operation of the linac coherent light source (LCLS) [1] has demonstrated that the x-ray free-electron laser (FEL) has come of age; these types of x-ray sources are poised to revolutionize the ultrafast x-ray sciences
The LCLS and other hard x-ray freeelectron lasers (FELs) under construction are based on the principle of selfamplified spontaneous emission (SASE) [2,3,4,5])
Instead of seeding with an external laser, the self-seeding approach [9] makes use of an x-ray monochromator to spectrally filter SASE light produced from a first undulator, which can be used to seed an unmodulated electron bunch passing through a second undulator to produce an amplified narrow-bandwidth x-ray FEL pulse
Summary
The successful commissioning and operation of the linac coherent light source (LCLS) [1] has demonstrated that the x-ray free-electron laser (FEL) has come of age; these types of x-ray sources are poised to revolutionize the ultrafast x-ray sciences. A more efficient harmonic generation process called echo-enabled harmonic generation has been proposed [7,8] These two laser seeding schemes are expected to be used in extreme ultraviolet and soft x-ray FEL facilities. Other methods to achieve full longitudinal coherence include the use of x-ray cavities and multiple bunches of electrons to form either an x-ray regenerative amplifier [12] or an x-ray FEL oscillator [13] Among these seeding schemes, self-seeding is relatively straightforward to implement for a single-pass high-gain FEL. Self-seeding is relatively straightforward to implement for a single-pass high-gain FEL It consists of two sequential undulators, and an x-ray monochromator and electron bypass chicane located between them. III, we use an idealized bunch and a more realistic LCLS bunch to demonstrate the feasibility of the proposed scheme.
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