Theoretical investigation of a short electron beam (extended bunch) interaction with a backward wave propagating in a slow wave structure demonstrates the possibility of producing ultrashort superradiance pulses with a peak power which exceeds the power of the driving beam (conversion factor K>1). It is shown that a nonuniform slow wave structure with optimized profile is beneficial in order to increase the conversion factor. The results of theoretical analysis are confirmed by the experiments. At X band using the SINUS-150 accelerator (4 ns, 330 kV, 2.6 kA) 0.6-0.8 ns superradiance pulses with a peak power of 1.2 GW and a conversion factor of 1.5 were obtained. Similar experiments at Ka-band based on the RADAN-303 accelerator (1 ns, 290 kV, 2.5 kA) demonstrated production of the superradiance (SR) pulse with duration 200 ps and peak power about 1 GW (conversion factor of 1.4).

This article presents results of theoretical and experimental studies on the production of ultrashort~a few RF cycles duration! microwave pulses of gigawatt peak powers based on superradiance from high-current electron beams. With the Cherenkov backward-wave-electron-beam interaction in a low-dispersion slow-wave structure, microwave pulses with a peak power greater than the peak power of the driving electron beam have been produced for the first time. In an experiment using the SINUS-150 compact high-current electron accelerator, with a 2.6-kA injected beam current and a 330-kV electron energy, microwave pulses of 1.2 GW peak power and ;0.5 ns duration~FWHM! were generated in the X-band. Production of superradiance pulses in a repetitive regime~3500 Hz! in the Ka-band has been demonstrated using a compact hybrid SOS-modulator. The effect of spatial accumulation of microwave energy in extended slow-wave structures with substantially nonuniform coupling has been demonstrated. In an experiment using the SINUS-200 compact accelerator, X-band pulses of ;3 GW peak power and 0.6-0.7 ns width~FWHM! were produced with a power conversion efficiency of 150-180% and an energy efficiency of ;15%.Calculations performed with various D J values have shown the following: With any value of j k . . 1, the optimum value of D J must be such~D J opt ; exp~Ϫj k !! that the region where the amplitude of the RF current saturates due to the non-188 A. A. Eltchaninov et al.

This paper presents a review of recent studies of superradiant (SR) regime of high-power microwave generation of relativistic backward-wave tubes. The fundamental possibility of producing microwave pulses with peak power substantially exceeding the driving electron beam power was demonstrated in theory and simulation. In experiments, beam-to-microwaves power conversion factors of 1.5 in the Ka band and up to 1.8 in the X band were achieved. High-efficiency Ka band generation was obtained at a low field (2 T) below the cyclotron resonance. With the use of a dc solenoid, SR pulses were generated in a batch mode at a pulse repetition frequency of 1000-3500 Hz.

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