Collective effects in pulsed beam free-electron lasers operating in the Tera-Hertz regime

Intense radiation devices such as microwave tubes, free-electron lasers (FELs) and masers, utilize distributed interaction between an electron beam and the electromagnetic field. Our space-frequency theory is extended to consider collective effects emerging while ultra short electron pulses are propagating in the interaction region. The total electromagnetic field (radiation and space-charge waves) is presented in the frequency domain as an expansion in terms of transverse eigen-modes. The mutual interaction between the electron beam and the electromagnetic field is fully described by a set of coupled equations, expressing the evolution of mode amplitudes and electron beam dynamics. The model is used for the analysis and simulation of radiation excitation and propagation in pulsed beam free-electron lasers operating in millimeter wavelengths and in the Tera-Hertz frequencies. The approach is applied in a numerical particle code WB3D, simulating wide-band interaction of a free-electron laser operating in the linear and non-linear regimes. The model is utilized to study spontaneous and super-radiant emissions radiated by a an electron bunch at the sub-millimeter regime, taking into account three dimensional space-charge effects playing a role in such ultra short bunches.