Free-electron two-quantum stark radiation driven by the electric wiggler associated with density modulation in a hydrodynamic free-electron laser

Abstract

We note that in the free-electron laser (FEL) using a non-prebunched electron beam, the uncertainty in the axial position of an electron emitting laser radiation of wavelength λ is on the order of λ so that radiations from different electrons incoherently interfere each other. We also note that the net work done by the electrons is of zero frequency and, hence, cannot become laser radiation of intrinsically non-zero frequency. We confirm this with the fact that the net work done by the electrons per unit time in a hydrodynamic FEL is much less than the output laser power. We show that in the above FEL, the laser gain derived under the concept that the laser radiation is free-electron two-quantum Stark radiation driven by the electric wiggler associated with an axial space-charge wave produced by the laser wave and the transverse wiggling is in excellent agreement with the measured gain.