Free-electron laser without inversion via transverse momentum discrimination

A two-stage free-electron laser with a specially designed delay of electrons in the drift region between the two wigglers allows us to obtain essentially positive gain as a function of detuning, i.e., "lasing without inversion" (LWI). The key physical assumption is that it is possible to tell which electrons emit and which absorb laser energy. We here show that straightforward attempts to realize LWI operation in a free-electron laser are frequently (but not inevitably) frustrated by the difficulty of telling the difference between emitting and absorbing electrons. We propose a scheme utilizing both transverse and longitudinal components of the electron velocity to achieve cancellation of absorption. Numerical simulations verifying the validity of the scheme are presented.