Free-electron laser oscillator in a symmetrical, confocal resonator

A tapered wiggler is used in a free-electron laser (FEL) oscillator to improve the saturation efficiency. During signal buildup the tapered wiggler does not provide optimum phase synchronism between the electron beam and the electromagnetic wave, resulting in an appreciable loss in small-signal gain. If the taper is too large, the decrease in gain during buildup may preclude the onset of oscillation. This problem can be ameliorated with a multicomponent wiggler, which is a combination of a uniform wiggler and a tapered section. During buildup gain is primarily contributed by the linear element, and at high power levels the gain and efficiency are enhanced by the taper. Ideally, one would like to have a uniform wiggler at small-signal levels and then be able to substitute a taper at saturation. Placing the FEL in a symmetrical confocal resonator approaches this desired effect automatically.