Free-electron laser

A free-electron laser produces coherent radiation when a relativistic electron beam passes through a periodic magnetic field. Its tunable output arises from unbound electrons rather than transitions between atomic energy levels.

Undulator: A periodic magnetic structure that makes charged particles oscillate and emit radiation. Its alternating field drives the electron oscillations that generate the laser’s radiation.
Linear accelerator: A device that accelerates charged particles along a straight path using electromagnetic fields. Many free-electron lasers use linacs to give electrons the energy needed for short-wavelength radiation.
X-ray free-electron laser: A free-electron laser that produces intense, coherent X-ray pulses. It extends FEL methods to atomic-scale imaging and ultrafast X-ray experiments.
Optical laser: A device that generates coherent optical radiation through stimulated emission in a gain medium. Its bound-state gain medium distinguishes it from the free electrons driving an FEL.
SASE free-electron laser: A free-electron laser mode that builds coherent radiation from electron shot noise without an external seed. Self-amplified spontaneous emission explains how many X-ray free-electron lasers start coherent output.
Synchrotron radiation: Electromagnetic radiation emitted by charged particles accelerated along curved paths. It is the broader radiation phenomenon that includes emission from undulators.
Linac Coherent Light Source: A user facility at SLAC National Accelerator Laboratory that produces hard X-ray free-electron laser pulses. This facility exemplifies how accelerator-based FELs support experiments across many sciences.
Synchrotron light source: A facility that uses relativistic charged particles in storage rings to produce intense electromagnetic radiation. Storage-ring sources offer broad access to synchrotron radiation, while FELs can deliver much brighter pulses.
Electron beam: A directed stream of electrons, often accelerated to high energies for scientific and industrial uses. Its energy, density, and quality shape the radiation the undulator can produce.
Wiggler: A periodic magnetic device that makes charged particles follow a strongly oscillating path and emit radiation. Its stronger deflection typically produces less narrowly concentrated radiation than an undulator.

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