[en] A research effort is underway with the goal of obtaining an energy of one kilojoule per pulse, with a 5 Hz pulse repetition frequency, from a high current relativistic klystron amplifier (RKA). The pulse length is one microsecond (1 GW peak power) with a microwave frequency of 1.3 GHz. This paper focuses on three areas: the theory, modeling, and experimental development of the microwave tube; the 1-μs-pulse-length field-emission diode electron gun developed for the RKA; and the BANSHEE pulsed power modulator that powers the RKA. This research builds on the pioneering work done by Friedman and Serlin on the RKA at the ∼100 ns pulse length. The one microsecond pulse length is almost an order of magnitude beyond what has been achieved with an RKA. Achieving a peak power approaching 1 GW for 1 μs requires a well behaved electron beam on that time scale, and an optimized extraction efficiency in the output cavity. The microwave tube design was guided by theory and particle-in-cell (PIC) code modeling that relate output cavity extraction efficiency to the amplitude of the beam harmonic current modulation and output cavity shunt impedance. The output cavity has adjustable tuning, loading, and output gap length to allow experimental optimization of the coupling between the modulated beam and the output cavity. An electron beam diode is used that delivers a peak current of 4 to 5 kA for a pulse duration exceeding 1 μs, at a beam kinetic energy above 600 keV. BANSHEE is the high voltage modulator designed for use as an electron beam driver for high power microwave tube development. The BANSHEE output pulse design parameters are 1 MV and 10 kA, with a 1 μs pulse width at a repetition rate of 3-5 Hz, driving a load of impedance of 100 Ω. BANSHEE is a thyratron-switched line-type modulator with a pulse transformer output stage. The modulator design is pushing the state of the art in thyratron technology and capacitor lifetime