A method is proposed to generate an isolated attosecond X-ray pulse in freeelectron lasers, using irregularly spaced current peaks induced in an electron beam through interaction with an intense short-pulse optical laser. In comparison with a similar scheme proposed in a previous paper, the irregular arrangement of current peaks significantly improves the contrast between the main and satellite pulses, enhances the attainable peak power and simplifies the accelerator layout. Three different methods are proposed for this purpose and achievable performances are computed under realistic conditions. Numerical simulations carried out with the best configuration show that an isolated 7.7 keV X-ray pulse with a peak power of 1.7 TW and pulse length of 70 as can be generated. In this particular example, the contrast is improved by two orders of magnitude and the peak power is enhanced by a factor of three, when compared with the previous scheme.

We demonstrate a hard-X-ray self-seeded (HXRSS) free-electron laser (FEL) at Pohang Accelerator Laboratory with an unprecedented peak brightness (3.2 × 10 35 photons/(s•mm 2 •mrad 2 •0.1%BW)). The self-seeded FEL generates hard X-ray pulses with improved spectral purity; the average pulse energy was 0.85 mJ at 9.7 keV, almost as high as in SASE mode; the bandwidth (0.19 eV) is about 1/70 as wide, the peak spectral brightness is 40 times higher than in self-ampli ed spontaneous emission (SASE) mode, and the stability is excellent with > 94% of shots exceeding the average SASE intensity. Using this self-seeded XFEL, we conducted serial femtosecond crystallography (SFX) experiments to map the structure of lysozyme protein; data-quality metrics such as R split , multiplicity, and signal-to-noise ratio for the SFX were substantially increased. We precisely map out the structure of lysozyme protein with substantially better statistics for the diffraction data and signi cantly sharper electron density maps compared to maps obtained using SASE mode.

Isolated terawatt (TW) attosecond (as) hard X-ray pulse is greatly desired for four-dimensional investigations of natural phenomena with picometer spatial and attosecond temporal resolutions. Since the demand for such sources is continuously increasing, the possibility of generating such pulse by a single current spike without the use of optical or electron delay units in an undulator line is addressed. The conditions of a current spike (width and height) and a modulation laser pulse (wavelength and power) is also discussed. We demonstrate that an isolated TW-level as a hard X-ray can be produced by a properly chosen single current spike in an electron bunch with simulation results. By using realistic specifications of an electron bunch of the Pohang Accelerator Laboratory X-ray Free-Electron Laser (PAL-XFEL), we show that an isolated, >1.0 TW and ~36 as X-ray pulse at 12.4 keV can be generated in an optimized-tapered undulator line. This result opens a new vista for current XFEL operation: the attosecond XFEL.

A wake monochromator based on a large‐area diamond single crystal for hard X‐ray self‐seeding has been successfully installed and commissioned in the hard X‐ray free‐electron laser (FEL) at the Pohang Accelerator Laboratory with international collaboration. For this commissioning, the self‐seeding was demonstrated with a low bunch charge (40 pC) and the nominal bunch charge (180 pC) of self‐amplified spontaneous emission (SASE) operation. The FEL pulse lengths were estimated as 7 fs and 29.5 fs, respectively. In both cases, the average spectral brightness increased by more than three times compared with the SASE mode. The self‐seeding experiment was demonstrated for the first time using a crystal with a thickness of 30 µm, and a narrow bandwidth of 0.22 eV (full width at half‐maximum) was obtained at 8.3 keV, which confirmed the functionality of a crystal with such a small thickness. In the nominal bunch‐charge self‐seeding experiment, the histogram of the intensity integrated over a 1 eV bandwidth showed a well defined Gaussian profile, which is evidence of the saturated FEL and a minimal electron‐energy jitter (∼1.2 × 10−4) effect. The corresponding low photon‐energy jitter (∼2.4 × 10−4) of the SASE FEL pulse, which is two times lower than the Pierce parameter, enabled the seeding power to be maximized by maintaining the spectral overlap between SASE FEL gain and the monochromator.

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