Abstract
We propose a simple method to significantly enhance the temporal coherence and spectral brightness of a self-amplified spontaneous emission (SASE) free-electron laser (FEL). In this purified SASE (pSASE) FEL, a few undulator sections (called slippage-boosted sections) resonant at a subharmonic of the FEL radiation are used in the middle stage of the exponential growth regime to amplify the radiation while simultaneously reducing the FEL bandwidth. In this slippage-boosted section, the average longitudinal velocity of electrons is reduced, which effectively increases the FEL slippage length that allows the radiation fields initially far apart to create a phase relation, leading to $n$ times increase in FEL cooperation length, where $n$ is the ratio of the resonant wavelength of the slippage-boosted section to that of the original FEL radiation. The purified radiation, as a seed with improved temporal coherence, is further amplified to saturation in the undulator sections tuned to the FEL wavelength. Using the linac coherent light source II (LCLS-II) parameters as an example, we show that with the proposed configuration the temporal coherence and spectral brightness of a SASE FEL can be significantly enhanced. This scheme may be applied to many SASE FEL light sources to enhance the FEL performance.
Highlights
High-gain free-electron lasers (FELs) working in the self-amplified spontaneous emission (SASE) mode [1,2] have been successfully operated in the x-ray wavelengths [3,4,5], which marked the beginning of a new era of x-ray science
Using the linac coherent light source II (LCLS-II) parameters as an example, we show that even with conservative parameter sets, the FEL bandwidth can be reduced by a factor of 5 with the proposed purified SASE (pSASE) scheme
We have studied a simple scheme to significantly enhance the temporal coherence and spectral brightness of a SASE FEL
Summary
High-gain free-electron lasers (FELs) working in the self-amplified spontaneous emission (SASE) mode [1,2] have been successfully operated in the x-ray wavelengths [3,4,5], which marked the beginning of a new era of x-ray science. The spectrum of a SASE FEL is noisy with $N spikes, each having a frequency spread c=lb and the overall frequency spread of the FEL pulse is approximately c=2lc, where c is the speed of light This spiky output is a result of the fact that in an FEL the radiation only propagates through a fraction of the electron bunch such that radiation fields with distance larger than 2lc evolve independently and are uncorrelated in phase. Using the LCLS-II parameters as an example, we show that even with conservative parameter sets, the FEL bandwidth can be reduced by a factor of 5 with the proposed pSASE scheme This method will enable FEL single spike operation [17,18,19] (namely reducing the number of spikes to one) with a relatively long bunch. We believe this pSASE configuration can be used in many future SASE FEL light sources to enhance the FEL performance
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