Two-color operation in high-gain free-electron lasers

Two-color operation in free-electron laser (FEL) amplifiers is studied usin g a 3D nonlinear polychromatic simulation. We assume the FEL is seeded at t wo closely spaced wavelengths within the gain band, and study the growth of the seeds and a discrete spectrum of beat waves that are outside the gain band. The beat waves grow parasitically due to electron bunching in the see ded waves with growth rates higher than the seeded waves. Injection of narr ow-band seeds ensures a discrete spectrum. An example is discussed correspo nding to an x-ray FEL; however, the physics is applicable to all spectral r anges.