An FEL, or free electron laser, is a type of laser that produces light using a beam of electrons moving at nearly the speed of light through a series of magnets. Unlike conventional lasers, which generate light from electrons trapped inside a crystal, gas, or liquid, an FEL uses “free” electrons that have been stripped from atoms entirely. This distinction gives FELs a remarkable ability: they can be tuned to produce light across a huge range of wavelengths, from infrared all the way down to powerful X-rays.
How a Free Electron Laser Works
A conventional laser works by exciting electrons that are bound to atoms inside some kind of material, called a gain medium. When those electrons release energy, they emit light at a fixed wavelength determined by the material itself. A ruby laser, for instance, always produces red light. An FEL takes a completely different approach. It starts with an electron gun that strips electrons from atoms, then feeds those electrons into a particle accelerator that brings them up to relativistic speeds, meaning a significant fraction of the speed of light.
Once accelerated, the electrons enter a long device called an undulator. This is a row of magnets with alternating poles arranged in a line, creating a periodic magnetic field. As the electrons pass through, the magnets force them into a wiggling, sinusoidal path. That constant change in direction counts as acceleration, and accelerating charged particles emit electromagnetic radiation.
At first, each electron radiates independently, producing weak, incoherent light. But as the electrons interact with the radiation field they’ve created, something interesting happens. The light nudges electrons into tighter clusters, called micro-bunches, spaced exactly one wavelength apart. Once bunched this way, the electrons begin radiating in unison, and the light amplifies exponentially in a single pass through the undulator. This process is called self-amplified spontaneous emission, or SASE, and it’s how X-ray FELs generate their extraordinary power without needing mirrors to bounce light back and forth.
What Makes FELs Different From Regular Lasers
The most important difference is tunability. Because the wavelength of light an FEL produces depends on the electron energy and the spacing of the undulator magnets rather than on a fixed material, operators can adjust the output across a wide swath of the electromagnetic spectrum. A single facility can shift from producing hard X-rays at 5 to 15 keV to extreme ultraviolet or even infrared light, depending on its configuration.
FELs also reach peak brightness levels that no conventional laser can match in the X-ray range. The European XFEL in Germany, for example, delivers pulses as short as 25 femtoseconds (a femtosecond is one quadrillionth of a second) carrying more than a trillion photons each, at a rate of up to 27,000 pulses per second. When focused down to a spot just a few micrometers wide, those pulses can reach power densities above 1017 watts per square centimeter. For context, that concentration of X-ray energy is millions of times greater than what a hospital X-ray machine produces, compressed into a flash so brief it can freeze atomic motion in place.
Conventional lasers can’t operate at these short X-ray wavelengths because mirrors don’t reflect X-rays well enough to form the optical cavities that traditional laser designs require. The SASE approach sidesteps this entirely by building up all the light power in a single trip through the undulator.
What FELs Are Used For
The primary use of X-ray FELs today is scientific research, particularly in structural biology and materials science. A technique called serial femtosecond crystallography uses XFEL pulses to determine the three-dimensional structure of proteins. The pulses are so fast that they capture an image of a molecule before the X-ray energy has time to destroy it. Researchers have used this “diffraction before destruction” approach to observe changes in protein backbones, chemical bonds, and molecular structures that were previously impossible to see.
Beyond static snapshots, FELs can create what amount to molecular movies. Because the pulses are measured in femtoseconds, scientists can take sequential images of chemical reactions as they happen, watching atoms rearrange in real time. This has opened up entirely new fields of study in chemistry and physics, including research into how materials behave under extreme pressure and temperature, conditions relevant to everything from fusion energy to planetary science.
In one notable experiment, researchers used intense XFEL light to create a population inversion in neon atoms, essentially turning a gas into an X-ray amplifier. Two-color pulse operations, where an FEL produces two different X-ray wavelengths simultaneously, have enabled pump-probe experiments that trigger a reaction with one pulse and photograph it with the second.
Lasers in Medicine: Where FELs Fit
Lasers in general have become essential surgical tools. Carbon dioxide lasers vaporize soft tissue by targeting water content, and laser thermal therapy guided by MRI is used in neurosurgery to treat brain tumors that can’t be removed conventionally, as well as epilepsy cases that don’t respond to medication. These medical lasers are compact, relatively affordable, and designed for clinical settings.
FELs, by contrast, have seen limited medical use. Infrared FELs were explored in the 1990s and 2000s for soft tissue surgery because their tunable wavelength could be optimized to cut specific tissue types with minimal damage to surrounding areas. But the sheer size and cost of FEL facilities have kept them out of hospitals. Their primary contribution to medicine is indirect: by revealing the atomic structure of disease-related proteins, XFELs help researchers design better drugs and understand disease mechanisms at the molecular level.
Where FEL Facilities Exist
Only a handful of X-ray FEL facilities operate worldwide, and access is competitive. The Linac Coherent Light Source (LCLS) at SLAC National Accelerator Laboratory in California was the first hard X-ray FEL when it came online in 2009. The European XFEL near Hamburg, Germany, is currently the most powerful, stretching 3.4 kilometers underground and serving researchers from dozens of countries. Japan’s SACLA facility produces hard X-ray pulses in the 5 to 15 keV range from a more compact design. The FERMI facility in Italy specializes in extreme ultraviolet and soft X-ray experiments.
These machines are enormous and expensive. Even a scaled-down XFEL design proposed recently would run about 500 meters long and cost around 7.5 million euros to build, which its designers described as a fraction of the cost of existing facilities. Full-scale XFELs cost hundreds of millions to billions of dollars. That price tag is why they remain accessible mainly to international research consortia and wealthy nations, with beam time allocated through competitive application processes where scientists propose experiments months or years in advance.
Why FELs Matter
FELs fill a gap that no other light source can. Synchrotrons, the ring-shaped particle accelerators that preceded XFELs as the go-to X-ray source for research, produce light that is billions of times dimmer at peak brightness and thousands of times longer in pulse duration. That difference is what separates a blurry image of a molecule from a sharp one, or a time-averaged view of a chemical reaction from a frame-by-frame sequence. For any experiment that requires seeing atoms move on their natural timescale, or delivering enough X-ray photons to get a clear picture from a tiny or fragile sample, FELs are the only tool that works.