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High power microwaves, up to Gigawatt levels in the centimeter regime, have been observed in reflex triode, foil and foilless diode systems. Generation efficiencies range from, 1% to 12%. The source of the microwaves is an oscillating virtual cathode - the nonlinear state which develops when the electron beam injection current exceeds the space-charge limiting current defined by the beam energy and wave guide geometry. This stable oscillation results in severe longitudinal charge bunching giving rise to large time dependent current variations. The experimental frequency dependence and broadband characteristics are explained by the scaling of the oscillator frequency with Â¿nb/Â¿, where nb is the beam density and Â¿ its relativistic factor, in conjunction with the Child-Langmuir rel ation. The optimal design for a narrow-band millimeter wave vircator is based on a foilless diode with a strong axial magnetic field. It will be tunable over an order of magnitude in frequency by varying the magnetic field strength.