Resonant X-ray emission spectroscopy using self-seeded hard X-ray pulses at PAL-XFEL.

Self-seeded hard X-ray pulses at PAL-XFEL were used to commission a resonant X-ray emission spectroscopy experiment with a von Hamos spectrometer. The self-seeded beam, generated through forward Bragg diffraction of the [202] peak in a 100 µm-thick diamond crystal, exhibited an average bandwidth of 0.54 eV at 11.223 keV. A coordinated scanning scheme of electron bunch energy, diamond crystal angle and silicon monochromator allowed us to map the Ir Lß2 X-ray emission lines of IrO2 powder across the Ir L3-absorption edge, from 11.212 to 11.242 keV with an energy step of 0.3 eV. This work provides a reference for hard X-ray emission spectroscopy experiments utilizing self-seeded pulses with a narrow bandwidth, eventually applicable for pump-probe studies in solid-state and diluted systems.

Optimized terahertz pulse generation with chirped pump pulses from an echelon-based tilted-pulse-front (TPF) scheme.

We successfully demonstrated the generation of single-cycle terahertz (THz) pulses through tilted-pulse-front (TPF) pumping using a reflective echelon in a lithium niobate crystal. By optimizing the pump pulse duration using a chirp, we achieved a maximum pump-to-THz conversion efficiency of 0.39%. However, we observed that the saturation behavior began at a relatively low pump energy (0.37 mJ), corresponding to a pump intensity of 22 GW/cm2. To elucidate this behavior, we measured the near- and far-field THz beam profiles and found variations in their beam characteristics, such as the beam size, location, and divergence angle in the plane of the tilted pulse direction, with the pump energy (intensity). This nonlinear behavior is attributed to the reduced effective interaction length, which ultimately leads to the saturation of THz generation. The results obtained from our study suggest that it is feasible to develop an effective THz source using echelon-based TPF pumping while also considering the impact of nonlinear saturation effects.

Statistical analysis of hard X-ray radiation at the PAL-XFEL facility performed by Hanbury Brown and Twiss interferometry.

A Hanbury Brown and Twiss interferometry experiment based on second-order correlations was performed at the PAL-XFEL facility. The statistical properties of the X-ray radiation were studied within this experiment. Measurements were performed at the NCI beamline at 10 keV photon energy under various operation conditions: self-amplified spontaneous emission (SASE), SASE with a monochromator, and self-seeding regimes at 120 pC, 180 pC and 200 pC electron bunch charge. Statistical analysis showed short average pulse duration from 6 fs to 9 fs depending on the operational conditions. A high spatial degree of coherence of about 70-80% was determined in the spatial domain for the SASE beams with the monochromator and self-seeding regime of operation. The obtained values describe the statistical properties of the beams generated at the PAL-XFEL facility.

Hard X-ray self-seeding commissioning at PAL-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.

FEL performance achieved at PAL-XFEL using a three-chicane bunch compression scheme.

PAL-XFEL utilizes a three-chicane bunch compression (3-BC) scheme (the very first of its kind in operation) for free-electron laser (FEL) operation. The addition of a third bunch compressor allows for more effective mitigation of coherent synchrotron radiation during bunch compression and an increased flexibility of system configuration. Start-to-end simulations of the effects of radiofrequency jitter on the electron beam performance show that using the 3-BC scheme leads to better performance compared with the two-chicane bunch compression scheme. Together with the high performance of the linac radiofrequency system, it enables reliable operation of PAL-XFEL with unprecedented stability in terms of arrival timing, pointing and intensity; an arrival timing jitter of better than 15 fs, a transverse position jitter of smaller than 10% of the photon beam size, and an FEL intensity jitter of smaller than 5% are consistently achieved.

Generation of time-synchronized two-color X-ray free-electron laser pulses using phase shifters.

To optimize the intensity of X-ray free-electron lasers (XFELs), phase shifters, oriented in phase with the phases of the XFEL pulse and electron beam, are typically installed at undulator lines. Although a π-offset between the phases (i.e., an "out-of-phase" configuration) can suppress the XFEL intensity at resonant frequencies, it can also generate a side-band spectrum, which results in a two-color XFEL pulse; the dynamics of such a pulse can be described using the spontaneous radiation or low gain theory. This attributes of this two-color XFEL pulse can be amplified (log-scale amplification) through an undulator line with out-of-phase phase shifters. In this study, the features of two-color XFEL pulses were evaluated through theory, simulations and experiments performed at Pohang Accelerator Laboratory X-ray Free Electron Laser. The XFEL gain slope and energy separation between the two-color spectral peaks were consistent through theoretical expectation, and the results of simulation and experiment. The experimentally determined two-color XFEL pulse energy was 250 µJ at a photon energy of 12.38 keV with a separation of 60 eV.

PAL-XFEL soft X-ray scientific instruments and X-ray optics: First commissioning results.

We report an overview of soft X-ray scientific instruments and X-ray optics at the free electron laser (FEL) of the Pohang Accelerator Laboratory, with selected first-commissioning results. The FEL exhibited a pulse energy of 200 µJ/pulse, a pulse width of <50 fs full width at half maximum, and an energy bandwidth of 0.44% at a photon energy of 850 eV. Monochromator resolving power of 10 500 was achieved. The estimated total time resolution between optical laser and X-ray pulses was <270 fs. A resonant inelastic X-ray scattering spectrometer was set up; its commissioning results are also reported.

Shock drive capabilities of a 30-Joule laser at the matter in extreme conditions hutch of the Linac Coherent Light Source.

We measure the shock drive capabilities of a 30 J, nanosecond, 527 nm laser system at the matter in extreme conditions hutch of the Linac Coherent Light Source. Using a velocity interferometer system for any reflector, we ascertain the maximum instantaneous ablation pressure and characterize its dependence on a drive laser spot size, spatial profile, and temporal profile. We also examine the effects of these parameters on shock spatial and temporal uniformity. Our analysis shows the drive laser capable of generating instantaneous ablation pressures exceeding 160 GPa while maintaining a 1D shock profile. We find that slope pulses provide higher instantaneous ablation pressures than plateau pulses. Our results show instantaneous ablation pressures comparable to those measured at the Omega Laser Facility in Rochester, NY under similar optical drive parameters. Finally, we analyze how optical laser ablation pressures are compare with known scaling relations, accounting for variable laser wavelengths.

Generating nearly single-cycle pulses with increased intensity and strongly asymmetric pulses of petawatt level.

Generation of petawatt-class pulses with a nearly single-cycle duration or with a strongly asymmetric longitudinal profile using a thin plasma layer are investigated via particle-in-cell simulations and the analytical flying mirror model. It is shown that the transmitted pulses having a duration as short as about 4 fs (1.2 laser cycles) or one-cycle front (tail) asymmetric pulses with peak intensity of about 10^{21}W/cm^{2} can be produced by optimizing system parameters. Here, a new effect is found for the shaping of linearly polarized laser pulses, owing to which the peak amplitude of the transmitted pulse becomes larger than that of the incoming pulse, and intense harmonics are generated. Characteristics of the transmitting window are then studied for different parameters of laser pulse and plasma layer. For a circular polarization, it is shown that the flying mirror model developed for shaping laser pulses with ultrathin foils can be successfully applied to plasma layers having a thickness of about the laser wavelength, which allows the shape of the transmitted pulse to be analytically predicted.