Electromagnetic emission produced by a propagating electron beam in a cylindrical drift chamber can be amplified by axially reflecting screens. Radiation appears at the first and second plasma harmonics with linewidths ∼0.1 &ngr;p. Amplification scales with &ngr;p2and lags electron‐beam voltage by several hundred nanoseconds, implying that electrostatic waves moving at the electron thermal speed must traverse the resonator before amplification begins. Rotating the reflectors beyond 30° lessens amplification, suggesting a broad reflection property.