This paper reports the first operation of a Sapphire-based Cerenkov Free Electron Laser (CFEL). The CFEL is a tunable source of coherent millimeter and sub-millimeter radiation. The phase velocity of light in Sapphire is about one tenth of that in free space so that Cerenkov radiation is obtained at modest electron beam energies; our electron gun supplies 10's of A/cm2 in the 20 to 100 keV range. The Cerenkov emission is confined by the resonator and can build to watts of power. The resonator geometry is critical both in terms of a passive cavity Q and because of its role in the electron/field interaction. Frequency, oscillation threshold current, and power have been measured.
E. E. Fisch, E. E. Fisch,
"Operation of the sapphire Cherenkov laser", Proc. SPIE 1576, 16th International Conference on Infrared and Millimeter Waves, 15766Q (1 October 1991); doi: 10.1117/12.2297955; https://doi.org/10.1117/12.2297955