The present invention provides a relativistic magnetron including an anode with an entrant channel, the channel having an input end, an output end and a dimensional discontinuity between the ends. The channel is connected to the magnetron and has an anode defining an interaction space located between the dimensional discontinuity and output end. Also provided is a cathode, located upstream, a spaced distance away from the interaction space towards the input end, the cathode is adapted to send an electron beam into the interaction space where the electron beam forms a virtual cathode in the interaction space.