A Cherenkov radiator for FEL-synchronized VUV-pulses at a linac-based FEL

A possible way to carry out two-color IR+VUV pump-probe experiments at linac-based FELs is proposed. The idea is to supply an FEL facility with a gas cell filled with helium or hydrogen, so that the electron beam, upon passage through the undulator, could be used to generate ultraviolet Cherenkov radiation. Such a Cherenkov radiator is shown to provide a reasonably high spectral-angular density of photons in a wide range of wavelengths from visible (300-400 nm) down to the VUV (65-70 nm). When averaged over a macropulse, the intensity of such a source exceeds that of synchrotron radiation from a wiggler in a modern storage ring. The spatial quality of the source is inferior to that of synchrotron radiation, but the synchronization to the infrared FEL pulses, which is crucial for most applications, is obviously perfect.