To create a FEL, a beam of
electrons is accelerated to almost the
speed of light. The beam passes through a periodic arrangement of
magnets with alternating
poles across the beam path, which creates a side to side
magnetic field. The direction of the beam is called the longitudinal direction, while the direction across the beam path is called transverse. This array of magnets is called an
undulator or a
wiggler, because due to the
Lorentz force of the field it forces the electrons in the beam to wiggle transversely, traveling along a
sinusoidal path about the axis of the undulator.
The transverse acceleration of the electrons across this path results in the release of
synchrotron radiation), which are monochromatic but still incoherent, because the electromagnetic waves from randomly distributed electrons interfere constructively and destructively in time. The resulting radiation power scales linearly with the number of electrons. Mirrors at each end of the undulator create an
optical cavity, causing the radiation to form
standing waves, or alternately an external excitation laser is provided. The synchrotron radiation becomes sufficiently strong that the transverse
electric field of the radiation beam interacts with the transverse electron current created by the sinusoidal wiggling motion, causing some electrons to gain and others to lose energy to the optical field via the
This energy modulation evolves into electron density (current) modulations with a period of one optical wavelength. The electrons are thus longitudinally clumped into microbunches, separated by one optical wavelength along the axis. Whereas an undulator alone would cause the electrons to radiate independently (incoherently), the radiation emitted by the bunched electrons is in phase, and the fields add together
The radiation intensity grows, causing additional microbunching of the electrons, which continue to radiate in phase with each other.
 This process continues until the electrons are completely microbunched and the radiation reaches a saturated power several orders of magnitude higher than that of the undulator radiation.
The wavelength of the radiation emitted can be readily tuned by adjusting the energy of the electron beam or the magnetic-field strength of the undulators.
FELs are relativistic machines. The wavelength of the emitted radiation, , is given by
or when the wiggler strength parameter K, discussed below, is small
where is the undulator wavelength (the spatial period of the magnetic field), is the relativistic
Lorentz factor and the proportionality constant depends on the undulator geometry and is of the order of 1.
This formula can be understood as a combination of two relativistic effects. Imagine you are sitting on an electron passing through the undulator. Due to
Lorentz contraction the undulator is shortened by a factor and the electron experiences much shorter undulator wavelength . However, the radiation emitted at this wavelength is observed in the laboratory frame of reference and the
relativistic Doppler effect brings the second factor to the above formula. Rigorous derivation from Maxwell's equations gives the divisor of 2 and the proportionality constant. In an X-ray FEL the typical undulator wavelength of 1 cm is transformed to X-ray wavelengths on the order of 1 nm by ≈ 2000, i.e. the electrons have to travel with the speed of 0.9999998c.
Wiggler strength parameter K
dimensionless parameter, tells the wiggler strength as the relationship between the length of a period and the radius of bend,
where is the bending radius, is the applied magnetic field, is the electron mass, and is the
Expressed in practical units, the dimensionless undulator parameter is .
In most cases, the theory of
classical electromagnetism adequately accounts for the behavior of free electron lasers.
 For sufficiently short wavelengths,
quantum effects of electron recoil and
shot noise may have to be considered.