Space Charge Effects in a Prebunched Beam Free Electron Maser (FEM)

Abstract

This chapter describes space charge effects in a prebunched beam free-electron maser (FEM). An electron beam prebunched at a frequency near to the synchronous free-electron laser frequency and passing through a magnetic undulator emits coherent synchrotron undulator radiation at the bunching frequency. In the collective regime, the spectral properties of the emitted super radiant power depend on the electron beam current density. For a low-density electron beam, the monochromatic super-radiant emission power is maximal at a bunching frequency equal to the electron-beam synchronism frequency, independent of the beam current. However, for a dense electron beam, the space charge effects become dominant and synchronism takes place with either the slow or the fast space-charge (Langmuire) wave, which propagate on the electron beam with different phase velocities. Consequently, radiated power maxima can occur at two well-separated frequencies.