Generating and frequency tuning of modulated high current electron beams and a specific efficient, high current, frequency-tunable device for generating intense radio frequency (RF), microwave electromagnetic fields in a rectangular waveguide. Current multiplication of a modulated seed electron beam is created by an energetic electron beam impacting a thin foil surface. The transmissive-electron-multiplier foils also mitigate both space charge expansion and improve beam propagation effects, by shorting of the radially directed electric field at the axial location of the foil(s). Foil thinness and intensity of the exit fields provide for a multiplication process occurring in a fraction of an RF period. Also included are both a self-excited microwave generator and an amplifier, using a temporally modulated laser to generate a modulated seed electron beam that is amplified. Methods to tune the oscillator are described that allow tunability over a full waveguide band.