The virtual cathode oscillator (vircator) can be one of the most promising high-power microwave sources due to its conceptual simplicity, high-output power capability, and potential tunability. Microwaves can be generated when the current of an intense relativistic electron beam exceeds the space-charge-limiting current of the structure. In this case, a virtual cathode comprising an electrostatic potential barrier is formed. Some of the electrons in the virtual cathode lose their kinetic energy and are then reflected back toward the anode.