XFEL

XFEL stands for X-ray Free Electron Laser, a type of light source that produces extremely bright, ultrashort X-ray pulses by passing a high-energy electron beam through a periodic magnetic structure called an undulator. Unlike conventional synchrotron X-ray sources, XFELs generate highly coherent X-rays with peak brightness far exceeding other sources, enabling time-resolved studies on femtosecond to attosecond scales.

The radiation is produced when relativistic electrons interact with the magnetic field of the undulator, emitting X-ray photons. The emitted waves reinforce as they travel with the electron beam, a process known as self-amplified spontaneous emission (SASE) in many machines. Resulting pulses typically last tens to a few hundred femtoseconds, with photon energies spanning soft to hard X-rays. Some facilities use seeding techniques to improve temporal coherence and stability. Electron energies are typically several GeV to tens of GeV, and repetition rates vary from tens of hertz to kilohertz, depending on the facility and configuration.

XFELs enable experiments in areas such as femtosecond time-resolved crystallography, pump-probe spectroscopy, single-particle imaging, and nonlinear X-ray science. They allow observation of ultrafast chemical reactions, phase transitions, and delicate biomolecules that are difficult to study with traditional sources.

Notable facilities include the Linac Coherent Light Source (LCLS) in the United States; the European XFEL near Hamburg; SACLA in Japan; SwissFEL at the Paul Scherrer Institute; and PAL-XFEL in Korea. The construction and operation of XFELs involve large, complex accelerator complexes, long undulator lines, and advanced detectors and control systems.