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).

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Experimental results of the observation of coherent stimulated radiation from subnanosecond electron bunches moving through a periodic waveguide and interacting with a backward propagating wave are presented. The subnanosecond microwave pulses in Ka and W bands were generated with repetition frequencies of up to 25 Hz. The mechanism of microwave pulse generation was associated with self-bunching, and the mutual influence of different parts of the electron pulse due to slippage of the wave with respect to the electrons; this can be interpreted as superradiance. The illumination of a panel of neon bulbs resulted in a finely structured pattern corresponding to the excitation of the TM01 mode. Observation of rf breakdown of ambient air, as well as direct measurements by hot-carrier germanium detectors, leads to an estimate of the absolute peak power as high as 60 MW for the 300-ps pulses at 38 GHz. These results are compared with numerical simulations. The initial observation of 75-GHz, 10-15-MW radiation pulses with a duration of less than 150 ps is also reported.

The first experiments on the observation of short pulsed superradiant (SR) emission with the excitation of a surface wave by a relativistic electron bunch moving in an oversized corrugated waveguide were performed. Subterahertz SR pulses with a central frequency of 0.14 THz, an ultrashort duration of 150 ps, and an extremely high peak power of 50-70 MW were generated. The experiments were based on a theoretical consideration including the quasioptical approach and direct particle-in-cell simulations.

To increase the output power of terahertz gyrotrons to several hundred kilowatts, we suggest using a planar geometry of interaction space with a sheet electron beam and transverse energy extraction. An advantage of this scheme in comparison with conventional cylindrical geometry is the possibility to ensure effective mode selection over the open transverse coordinate in combination with radiation outcoupling that leads to a substantial reduction of Ohmic losses. Similar to unstable resonators in optics for further growth of the radiation power it is beneficial to introduce waveguide tapering.

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

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A time-domain theory of frequency-locking gyrotron oscillators with low-Q resonators has been developed. The presented theory is based on the description of wave propagation by a parabolic equation taking into account the external signal by modification of boundary conditions. We show that the developed model can be effectively used for simulations of both single-and multi-mode operation regimes in gyrotrons driven by an external signal. For the case of low-Q resonators typical for powerful gyrotrons, the external signal can influence the axial field profile inside the interaction space significantly and, correspondingly, the value of the electron orbital efficiency. V C 2015 AIP Publishing LLC.