H.P. Freund and P.G. O'Shea
SAIC and University of Maryland
Two-color operation in free-electron laser (FEL) amplifiers is studied using a 3D nonlinear polychromatic simulation. We assume that the FEL is seeded at two closely spaced wavelengths within the gain band, and we study the growth of the seeds as well as a discrete spectrum of beat waves that are outside the gain band. We show that the beat waves grow parasitically with growth rates higher than the seeded waves. Beat wave growth is due to electron bunching in the seeded waves, and the injection of narrow-band seeds ensures a discrete spectrum. An example is discussed for parameters corresponding to an X-ray FEL; however, the physics is applicable to all spectral ranges.
