Non-wiggler-averaged theory of short wavelength free-electron lasers

Short wavelength free-electron lasers are studied using a 3D nonlinear simulation using a superposition of Gauss-Hermite modes. The differential equations governing each mode are solved along with the 3D Lorentz force equations (no wiggler average is used) for an ensemble of electrons. This permits the self-consistent modeling of (1) beam injection, (2) emittance growth, (3) wiggler imperfections and (4) beatatron oscillations. Simulations are performed for a 1.4 Å FEL. Results indicate that beam emittance is the crucial limiting factor requiring Δγzγ0 ≤ 0.01%. The radiation is sufficiently guided that no severe degradation is found in the efficiency for moderate levels of wiggler fluctuations.