A self-seeded X-ray free-electron laser (XFEL) is a promising approach to realize bright, fully coherent free-electron laser (FEL) sources in the hard X-ray domain that have been a long-standing issue with longitudinal coherence remaining challenging. At the Pohang Accelerator Laboratory XFEL, we have demonstrated a hard X-ray self-seeded XFEL with a peak brightness of 3.2 × 10³⁵ photons s^–1 mm^–2 mrad^–2 0.1% bandwidth (BW)^–1 at 9.7 keV. The bandwidth (0.19 eV) is about 1/70 times as wide (close to the Fourier transform limit) and the peak spectral brightness is 40 times higher than in self-amplified spontaneous emission (SASE), with substantial improvements in the stability of self-seeding and noticeably suppressed pedestal effects. We could reach an excellent self-seeding performance at a photon energy of 3.5 keV (lowest) and 14.6 keV (highest) with the same stability as the 9.7 keV self-seeding. The bandwidth of the 14.6 keV seeded FEL was 0.32 eV, and the peak brightness was 1.3 × 10³⁵ photons s^–1 mm^–2 mrad^–2 0.1%BW^–1. We show that the use of seeded FEL pulses with higher reproducibility and a cleaner spectrum results in serial femtosecond crystallography data of superior quality compared with data collected using SASE mode.

自播种 X 射线自由电子激光器 (XFEL) 是一种很有前途的方法，可在硬 X 射线域中实现明亮、完全相干的自由电子激光 (FEL) 源，这一直是纵向相干的长期问题仍然具有挑战性。在浦项加速器实验室 XFEL，我们展示了一种硬 X 射线自晶 XFEL，其峰值亮度为 3.2 × 10 ³⁵光子 s ^–1 mm ^–2 mrad ^–2 0.1% 带宽 (BW) ^–1在 9.7 keV。带宽（0.19 eV）约为自放大自发发射（SASE）的 1/70 倍（接近傅里叶变换极限），峰值光谱亮度是自放大自发发射 (SASE) 的 40 倍，自发稳定性显着提高- 播种和明显抑制的基座效应。我们可以在 3.5 keV（最低）和 14.6 keV（最高）的光子能量下达到出色的自播种性能，具有与 9.7keV 自播种相同的稳定性。14.6 keV种子FEL的带宽为0.32 eV，峰值亮度为1.3 × 10 ³⁵ photons s ^–1 mm ^–2 mrad ^–2 0.1%BW ^–1. 我们表明，与使用 SASE 模式收集的数据相比，使用具有更高可重复性和更清晰光谱的种子 FEL 脉冲会产生更高质量的连续飞秒晶体学数据。