Pulses, coherency, and spectra of the NovoFEL radiation

Free-electron lasers have specific time form of its radiation (usually continuous train of very shot pulses) and specific instabilities of the radiation. The fast instabilities change intra-pulse coherency of the radiation and produce large-scale side-band frequency modes. The slow instabilities change coherency between pulses and have an influence on fine structure of the spectra, which consist of many close longitudinal partly-synchronized modes. Spectral measurements of terahertz NovoFEL radiation by methods of Fourier spectroscopy, ultra-fast single-pulse spectroscopy, and new high-resolution spectroscopies based on ultra-long resonance waveguide vacuum Fabry-Perot interferometer and mixing of the NovoFEL radiation with ultra-monochromatic radiation of a gas laser are presented in the report.