Universal classical mechanism of free-electron lasing without inversion.
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We present a universal mechanism which allows us to relinquish momentum population inversion in free-electron lasers, and extract strongly enhanced gain from a broad momentum distribution. This is achievable by appropriately designed magnetic fields between two sequential interaction regions, which cause, purely classically, absorption cancellation (by destructive interference) and emission enhancement (by constructive interference). 1995 The American Physical Society.