OPERATION OF THE SAPPHIRE CERENKOV LASER

First operation of a sapphire-based Cerenkov free electron laser is reported. It is a tunable source of coherent millimeter and submillimeter 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. In addition to its role as a Cerenkov coupler, the sapphire forms a high Q resonator obviating external mirrors. The spontaneous Cerenkov emission is well-confined by this resonator and can build to watts of power. Measured tuning and threshold currents are presented and compared with the predictions of free electron laser theory.