Electromagnetically pumped free-electron laser with a guide magnetic field

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Abstract
The operation of a free-electron laser with an electromagnetic pump in the presence of a guide magnetic field is considered. It is shown that several different steady-state helical equilibria are allowed in a given configuration of the pump and guide fields. The study of the dynamics of the beam shows that the helical equilibrium is an exception rather than the rule. Thus only special initial conditions and careful tapering of the transition region at the entrance into the laser allow launching the beam on one of the stable steady states. The small signal gain in the laser is also derived. It is shown that the gain can be substantially enhanced when the beam approaches the orbital instability regime. Numerical examples demonstrate that the strength of the pump field in these conditions can be greatly reduced while still providing enhanced gain in the laser.