FREE-ELECTRON LASER WITHOUT INVERSION VIA TRANSVERSE-MOMENTUM DISCRIMINATION

A two-stage free-electron laser with a specially designed delay of ele ctrons in the drift region between the two wigglers allows us to obtai n essentially positive gain as a function of detuning, i.e., ''lasing without inversion'' (LWI). The key physical assumption is that ii is p ossible 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 absorbi ng electrons. We propose a scheme utilizing both transverse and longit udinal components of the electron velocity to achieve cancellation of absorption. Numerical simulations verifying the validity of the scheme are presented.