CLASSICAL FREE-ELECTRON LASERS WITHOUT INVERSION

We present a classical approach to free-electron lasers without invers ion. The destructive interference of absorption in two sequential inte raction regions (wigglers) results in absorption cancellation and stro ngly enhanced gain. Appropriately designed magnetic fields between the two interaction regions separate emitting and absorbing electrons and adjust their oscillation phases in the two interaction regions accord ing to their velocities. In this way the adverse effects of wide (inho mogeneous) electron velocity spread are negated, and most of the inhom ogeneous distribution contributes to emission rather than absorption. This scheme does not require momentum bunching (population inversion) to ensure highly efficient operation.