Space-frequency model of ultra wide-band interactions in free-electron lasers

The principle of operation of intense radiation devices such as microwave tubes, free-electron lasers (FELs) and masers, is based on a distributed interaction between an electron beam and radiation. We developed a three-dimensional, space-frequency theory for the analysis and simulation of radiation excitation and propagation in electron devices and free-electron lasers operating in an ultra wide range of frequencies. The total electromagnetic field is presented in the frequency domain as an expansion in terms of cavity eigen-modes. The mutual interaction between the electron beam and the field is fully described by coupled equations, expressing the evolution of mode amplitudes and electron beam dynamics. The approach is applied in a numerical particle code WB3D, simulating wide band interactions in free-electron lasers operating in the linear and non-linear regimes. The code is used to study the statistical and spectral characteristics of multimode radiation generation in a free-electron laser, operating in various operational parameters. The theory is demonstrated also in the case of "grazing", resulting in a wide-band interaction between the electron beam and the radiation.