This paper presents the results of a series of experiments conducted by the High-Power Microwave Group of General Dynamics, Pomona Division, on the dielectric Cherenkov maser (DCM) oscillator. Here the device consists of a cylindrical met allic waveguide of inner radius 3.64 cm which is partially filled with a dielectric liner. Traveling through the lined waveguide is an annular relativistic electron beam. Liners of dielectric constant e = 10, 5, and 2.3 were investigated for liner thicknesses of 4 and 6 mm. The 6-mm-thick, ε = 10 liner generated an RF output of 200 MW for 20 ns at 3.8 Hz with electron-beam parameters of 700 kV, 12 kA, and pulse duration of 100 ns. The maximum measured power output for the other configurations was 80 MW at a frequency of 7 GHz (ε = 5), with several MW of power output from the polyethylene liner (ε = 2.3) at a frequency of ~9 GHz. © 1990 IEEE