An X-ray Free-Electron Laser, or XFEL, is an advanced scientific instrument that generates extremely powerful and ultrashort pulses of X-rays. Unlike conventional X-ray sources, XFELs produce laser-like beams, making them exceptionally bright and coherent. These unique properties allow scientists to observe processes at the atomic and molecular scales with unprecedented detail and speed. XFELs function as a type of super microscope, enabling researchers to capture “molecular movies” of materials, chemistry, and biology in action.
Understanding X-ray Free-Electron Lasers
One defining characteristic is their extreme brightness. An XFEL pulse can be approximately 100 times more intense than all the sunlight hitting the Earth’s surface focused onto a thumbnail. This intense light allows for the study of phenomena that are otherwise difficult or impossible to probe.
Another distinguishing feature is their ultrashort pulse duration, measured in femtoseconds—a millionth of a billionth of a second. These incredibly brief flashes “freeze” the motion of atoms and molecules, allowing scientists to capture snapshots of fleeting processes. Stringing these snapshots together reveals how chemical reactions unfold or how materials change at an atomic level.
XFELs also exhibit high coherence. This property, where all the X-ray waves are in phase with each other, enables techniques like coherent diffraction imaging, which is used to resolve the structures of tiny protein crystals and even large virus particles. The combination of extreme brightness, ultrashort pulses, and high coherence makes XFELs powerful tools for examining matter at its most fundamental level.
How XFELs Produce Light
The operation of an XFEL begins with an electron accelerator that propels bunches of electrons to nearly the speed of light. These electrons are generated by striking a metal plate with a pulse of ultraviolet light. They then gain energy as they travel through resonators, where oscillating microwaves transfer energy to them.
After acceleration, these high-energy electron bunches enter an undulator. An undulator is a series of precisely arranged magnets with alternating north and south poles. As the electrons pass through the undulator, the magnetic fields force them to follow a “slalom course,” wiggling back and forth. This wiggling motion causes the electrons to emit X-ray radiation.
The emitted X-rays then interact with the electron bunches as they continue through the undulator, leading to a process known as Self-Amplified Spontaneous Emission (SASE). The radiation overtakes the electrons, causing some to speed up and others to slow down, which gradually organizes the electrons into thin disks or micro-bunches. This “bunching” of electrons enhances the X-ray emission, making it more coherent and significantly amplifying the power of the X-ray beam.
Applications in Science and Technology
XFELs have opened new avenues for scientific research, allowing scientists to investigate matter on ultrafast timescales. A significant application is the imaging of biological molecules, such as proteins and viruses, without the need for crystallization. This capability, known as serial femtosecond crystallography (SFX) and single-particle imaging (SPI), provides high-resolution 3D structures, even for difficult-to-crystallize proteins like membrane proteins.
XFELs are also used to study chemical reactions in real-time, capturing the fleeting steps as they occur. For example, researchers have observed processes like photosynthesis, allowing for a deeper understanding of bond formation and breakage.
XFELs also enable the investigation of material properties under extreme conditions, such as those found inside planets. Scientists can measure stress, strain, and failure in advanced materials and study how matter behaves under immense pressures. These insights contribute to the development of new materials and technologies. XFELs also contribute to probing quantum phenomena, important for the future of ultrafast computing and communications.
Key XFEL Facilities Around the World
Several major XFEL facilities are operational globally. The Linac Coherent Light Source (LCLS) at SLAC National Accelerator Laboratory in the USA was the world’s first XFEL to produce hard X-rays, beginning operations in 2009. An upgrade, LCLS-II, is underway to increase its X-ray pulse repetition rate significantly, from 120 pulses per second to 1 million pulses per second.
The European XFEL, located in Germany, is currently the largest XFEL facility, extending 3.4 kilometers. It generates 27,000 X-ray flashes per second, with a brilliance a billion times higher than conventional X-ray sources. Its superconducting accelerator technology allows for a particularly high-quality electron beam, enabling more simultaneous experiments.
Japan hosts SACLA (SPring-8 Angstrom Compact free electron LAser), which became operational in 2011 as the second XFEL in the world. It produces bright X-ray pulses at a repetition rate between 30 and 60 Hz. The Pohang Accelerator Laboratory X-ray Free Electron Laser (PAL-XFEL) is located in Pohang, South Korea, and began commissioning in 2016. Switzerland also has the SwissFEL at the Paul Scherrer Institute, which was inaugurated in December 2016.