Once the electron beam reaches just the right speed, it enters the wiggler. The wiggler directs the electron beam back and forth, or "wiggles" the beam, to make it produce electromagnetic radiation. "It's just like surfing, like catching a wave," says Henry Freund, a long-time free electron laser scientist and vice president at Science Applications International Corporation.
Inside the wiggler, very strong, oppositely charged magnets are placed side-by-side. As the electron beam passes through the magnets, it is first attracted to the positive pole of a magnet. But then the negative magnetic pole right next to it kicks it to the other side of the wiggler. This back and forth motion of the electron beam causes it to emit electromagnetic radiation. The wavelength of that radiation--the laser beam--depends on the size and spacing of the magnets in the wiggler.