A CHERENKOV RADIATOR FOR FEL-SYNCHRONIZED VUV-PULSES AT A LINAC-BASEDFEL

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 w ith a gas cell filled with helium or hydrogen, so that the electron be am, upon passage through the undulator, could be used to generate ultr aviolet Cherenkov radiation. Such a Cherenkov radiator is shown to pro vide a reasonably high spectral-angular density of photons in a wide r ange of wavelengths from visible (300-400 nm) down to the VUV (65-70 n m). When averaged over a macropulse, the intensity of such a source ex ceeds that of synchrotron radiation from a wiggler in a modern storage ring. The spatial quality of the source is inferior to that of synchr otron radiation, but the synchronization to the infrared FEL pulses, w hich is crucial for most applications, is obviously perfect.