1. In a free electron laser (FEL) system including an input particle beam, a method for varying the output wavelength of the FEL output beam by changing the energy of the FEL input particle beam, comprising:a. providing a primary accelerator cavity for supplying the input particle beam and a primary beam transport for transporting the input particle beam, wherein the input particle beam has energy with a nominal operating point and a primary accelerator bunch frequency;
b. providing a wiggler to periodically deflect the beam of particles inside the input particle beam;
c. inserting one or more secondary radiofrequency accelerator cavities between the primary beam transport and the wiggler;
d. setting the frequency of each secondary radiofrequency accelerator cavity to a harmonic or sub-harmonic of the primary accelerator bunch frequency; and
e. dithering the particle beam energy of the FEL input particle beam by operating the secondary radiofrequency accelerator cavities at the harmonic or sub-harmonic of the primary accelerator bunch frequency, said dithering varying the particle beam energy of the FEL input particle beam about the nominal operating point and varying the wavelength of the FEL output beam.