Compound wiggler to lase on a selected harmonic

Lasing on the third harmonic of the fundamental FEL wavelength has recently been accomplished. Lasing on even higher harmonics appears to be possible and can offer advantages of various kinds. A serious problem is the necessity for suppressing lasing on unwanted harmonics and selecting only the desired one. We have devised a technique to select the desired harmonic that depends upon a modification to the design of the wiggler. We discuss the principles of this modification, the properties of modified wigglers, and give an example of a wiggler design that employs this technique.