Radiation stability and hyperfine mode structure of the terahertz NovoFEL

The hyperfine spectral structure of radiation−laser modes of the Novosibirsk terahertz free electron laser (NovoFEL) has been measured in two its main operating regimes: resonance unstable regime with maximum output power and stabilized regime with maximum spectral power. The monochromaticity of the mode structure is shown to be practically independent of the laser operating regime and reaches a value of 2·10−8. The mode composition weakly depends on the operating regime for a well aligned electron beam and consists of many longitudinal TEMq00modes (supermode). In the case of a poorly aligned electron beam, transverse modes of the optical cavity are also excited; its intensities strongly increase in the resonance regime.