Linear gain of a free-electron laser with an electromagnetic wiggler and an axial-guide magnetic field.

Abstract

The small-signal gain for an electromagnetically pumped free-electron laser is calculated for an amplifier configuration which includes an axial-guide magnetic field. The large-amplitude electromagnetic wave acts like the magnetostatic wiggler in a conventional free-electron laser, and the expression for the gain is shown to reduce to the well-known result in the limit of a magnetostatic wiggler. Substantial enhancements in the gain are found when ${\ensuremath{\Omega}}_{0}$\ensuremath{\simeq}${\ensuremath{\gamma}}_{0}$(${\ensuremath{\omega}}_{w}$+${k}_{w}$${v}_{?}$), where ${\ensuremath{\Omega}}_{0}$ is the axial gyrofrequency, ${\ensuremath{\gamma}}_{0}$ is the relativistic factor for the electron beam, ${v}_{?}$ is the axial velocity of the electron beam, and (${\ensuremath{\omega}}_{w}$,${k}_{w}$) are the frequency and wave vector of the electromagnetic wiggler.