Time-dependent simulation of free-electron laser amplifiers and oscillators

Time-dependent free-electron laser simulations use a variety of techniques. Particle-in-cell codes have been used to simulate free-electron masers; however, this is not feasible at short wavelengths. Most simulations use a slowly varying envelope approximation in both in z and t, where 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. We describe the inclusion of this technique in the non--wiggler-averaged code MEDUSA, which is then applied to study temporal behavior in amplifiers and oscillators.