where "e" is the particle charge, "B" is the magnetic field, "formula_3", "formula_4" is the electron rest mass and "c" is the speed of light, characterizes the nature of the electron motion. For formula_5 the oscillation amplitude of the motion is small and the radiation displays interference patterns which lead to narrow energy bands. If formula_6 the oscillation amplitude is bigger and the radiation contributions from each field period sum up independently, leading to a broad energy spectrum. When K is much bigger than 1, the device is no longer called an "undulator"; it is called a wiggler.