The wavelength of the light emitted can be readily tuned by adjusting the energy of the electron beam or the magnetic field strength of the undulators. Whereas conventional undulators would cause the electrons to radiate independently, instabilities in the electron beam resulting from the interactions of the oscillations of electrons in the undulators and the radiation they emit leads to a bunching of the electrons, which continue to radiate in phase with each other. Since the electron motion is in phase with the field of the light already emitted, the fields add together coherently. The acceleration of the electrons along this path results in the release of photons (synchrotron radiation). This array of magnets is commonly configured as an undulator, or a wiggler, because it forces the electrons in the beam to follow a sinusoidal path. The beam passes through the FEL oscillator, a periodic transverse magnetic field produced by an arrangement of magnets with alternating poles within an optical cavity along the beam path.
To create a FEL, a beam of electrons is accelerated to almost the speed of light.
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