Time-Dependent Simulation of Free-Electron Lasers

Time-dependent FEL simulations use a variety of techniques. Particle-in-cell codes have been used to simulate free-electron masers as stated in T. J. T. Kwan et al. (1977) and A. T. Lin et al. (1983); however, this is not feasible at short wavelengths. Most simulations use a slowly varying envelope approximation (SVEA). One such technique assumes that the envelope varies only in z combined with a field representation as an ensemble of discrete harmonics, which is equivalent to a time-dependent simulation, based in N. Piovella (1999) but is computationally prohibitive. A second technique uses an SVEA in both in z and t according to R. Bonifacio et al. (1989). The particles and fields are advanced in z using the same process as in steady-state simulations and then the time derivative describing slippage is applied. This is used in wiggler-averaged codes such as PROMETEO based in W. Fawley (1995) in 1-D and GINGER based in S. Reiche (1999) and GENESIS based in H. P. Freund (2000) in 3D. We describe the inclusion of this technique in the non-wiggler-averaged code MEDUSA, which is applied to amplifiers and oscillators