Abstract
Gas-loaded free electron lasers using a dielectric gas of molecules with two energy levels are investigated theoreticly and numerically. In this case, the amplification of radiation arises in two wavenumber regions. One is the up-shifted ordinary FEL mode, and the other is in the vicinity of the resonance frequency corresponding to the energy difference between two levels of the gas. Furthermore, the latter modes are divided into two kinds, a slow beam mode and a fast beam mode. The fast beam mode grows faster than the slow beam mode in our parameter regime. To analyze the amplification and the saturation of such a gas-loaded FEL, we developed an FEL particle simulation code in the high-gain Compton regime. The results of the simulation code agree well with the results of the dispersion relation derived by the linear analysis, and saturation levels for three kinds of electromagnetic modes are also calculated.
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