Results of tests of a fast-repetitive nanosecond modulator with a peak output power of 3 GW are presented. It comprises of a type S-5N charging driver, a power compressor based on a pulsed forming line, and a gas spark gap. The modulator has been designed for the formation of high-current electron beams and high-power microwave generation in a pulsed-repetitive regime. Excitation of 10-GHz microwave pulses

ns long in a relativistic backward-wave oscillator with an elongated periodic slow-wave system has been studied. Optimization of the regime of interaction between electromagnetic fields and particles provided an average power of microwave radiation in the burst-repetitive mode (1 s;

Hz) of up to 2.5 kW at a focusing magnetic field (

T) below the cyclotron resonance region. The peak output power of the oscillator exceeded 2 GW.