The generation of a 2-GW microwave superradiance (SR) pulses has been demonstrated at 29-GHz using a single-mode relativistic backward-wave oscillator possessing the beam-to-wave power conversion factor no worse than 100%. A record-breaking radiation power density in the slow-wave structure (SWS) of ∼1.5 GW/cm2 required the use of high guiding magnetic field (7 T) decreasing the beam losses to the SWS in strong rf fields. Despite the field strength at the SWS wall of 2 MV/cm, a single-pass transmission mode of a short SR pulse in the SWS allows one to obtain extremely high power density in subnanosecond time scale due to time delay in the development of the breakdown phenomena.

We demonstrate both theoretically and experimentally the possibility of correlating the phase of a Cherenkov superradiance (SR) pulse to the sharp edge of a current pulse, when spontaneous emission of the electron bunch edge serves as the seed for SR processes. By division of the driving voltage pulse across several parallel channels equipped with independent cathodes we can synchronize several SR sources to arrange a two-dimensional array. In experiments carried out, coherent summation of radiation from four independent 8-mm wavelength band SR generators with peak power 600 MW resulted in the interference maximum of the directional diagram with an intensity that is equivalent to radiation from a single source with power 10 GW. Numerous scientific and technological applications stimulate interest in the generation of ultra-high power coherent radiation. Approaches that can be suggested to achieve this goal include the generation of radiation by a single source with an oversized electrodynamic system. In this case special methods (for example, 2D distributed feedback [1,2]) are required to produce spatially coherent radiation. Another method is the synchronization of a large number of moderate-power sources using a master oscillator [3][4][5]. DOIAt the same time for short-pulse sources, in particular, for sources based on Cherenkov superradiance (SR) of extended electron bunches moving in a slow wave structure (SWS) [6,7], there is an alternative opportunity, associated with the correlating the phase of a radiated pulse to the sharp edge of a current pulse. In fact, spontaneous emission of the bunch edge serves as the seed for SR processes. It gives rise to the stimulated emission including electron self-bunching and subsequent radiation of the short high-power electromagnetic pulse. If identical current pulses are sent simultaneously to several channels, identical SR pulses will be generated and the coherent summation of their amplitudes is possible. For two channel radiation sources such a possibility has been experimentally demonstrated in Ref. [8]. However the physical model describing the transformation of spontaneous Cherenkov radiation (i.e. the radiation from the unperturbed moving particles without the reverse effect of the field [9]) to stimulated radiation is still missing. The

Fulfillment of the condition that the voltage rise time across an air gap is comparable with the time of electron acceleration from a cathode to an anode allows a flow of runaway electrons (REs) to be formed with relativistic energies approaching that determined by the amplitude of the voltage pulse. In the experiment described here, an RE energy of 1.4 MeV was observed by applying a negative travelling voltage pulse of 860-kV with a maximum rise rate of 10 MV/ns and a rise time of 100-ps. The voltage pulse amplitude was doubled at the cathode of the 2-cm-long air gap due to the delay of conventional pulsed breakdown. The above-mentioned record-breaking voltage pulse of ∼120 ps duration with a peak power of 15 GW was produced by an all-solid-state pulsed power source utilising pulse compression/sharpening in a multistage gyromagnetic nonlinear transmission line.

In this work, experiments were made in which gyromagnetic nonlinear transmission line (NLTL) operates as a peak power amplifier of the input pulse. At such an operating regime, the duration of the input pulse is close to the period of generated oscillations, and the main part of the input pulse energy is transmitted only to the first peak of the oscillations. Power amplification is achieved due to the voltage amplitude of the first peak across the NLTL output exceeding the voltage amplitude of the input pulse. In the experiments, the input pulse with an amplitude of 500 kV and a half-height pulse duration of 7 ns is applied to the NLTL with a natural oscillation frequency of ∼300 MHz. At the output of the NLTL in 40 Ω coaxial transmission line, the pulse amplitude is increased to 740 kV and the pulse duration is reduced to ∼2 ns, which correspond to power amplification of the input pulse from ∼6 to ∼13 GW. As a source of input pulses, a solid-state semiconductor opening switch generator was used, which allowed carrying out experiments at pulse repetition frequency up to 1 kHz in the burst mode of operation.

Coherent summation of microwave beams has been demonstrated for two superradiance Ka-band backward wave oscillators producing over 700 MW of power. The explosive emission cathodes of the e-beam injectors were powered by stable splitted voltage pulses produced by an all-solid-state modulator. The voltage fronts were shortened to 300 ps in controlled delay shock-excited ferrite lines. The standard deviation of the phase difference between the microwave pulses was less than 2% of the oscillations period. The power flux density of the summarized radiation was the same as that of a single generator producing an output power of ∼3 GW.