Generation of single and periodically repeated powerful ultrashort microwave pulses

Abstract

Generation of powerful ultrashort microwave pulses (USP) is of interest for a large number of practical applications, including radars with high resolution, plasma diagnostics, spectroscopy etc. In order to achieve this purpose many methods well-known in laser physics can be implemented in microwave electronics.In the last decade significant progress was achieved in generation of microwave USPs based on classical superradiance (SR) of extended electron bunches. Using of this effect a new type of pulse sources has been developed, which are capable of producing uniquely short (under 0.2-1 ns) electromagnetic pulses at super-high peak powers exceeding 1 GW in the millimeter waves (Kaband) and 3 GW in the centimeter waves (X-band) with a repetition frequency up to several kilohertz [1]. A natural continuation of this research is related with the promotion of SR sources in shorter wavelengths ranges, where the oversized waveguides should be used. In the first part of this paper the results of the experimental observation of SR emission with excitation of a surface wave in the oversized periodically corrugated slow wave structures are presented. The experiments were based on a theoretical consideration in the frame of the quasi-optical approach.A periodic train of powerful ultrashort microwave pulses with a repetition frequency of 10-100 MHz can be generated in electron oscillators with a saturable absorber installed in the feedback loop [2]. The mechanism of pulse formation in this case is similar to the passive mode-locking [3] in laser physics where such pulses are also treated as dissipative solitons. The second part of this paper is devoted to the theoretical analysis of solitons generation in a mode-locked two-section microwave oscillator consisting of Cherenkov TWT and a Kompfner absorber in the feedback loop. Generation of Sub-THz SR Pulses Based on Excitation of Surface Waves in Oversized WaveguidesCherenkov SR of electron bunch exciting the surface wave in an oversized corrugated cylindrical waveguide (Fig. 1a) can be considered within a quasi-optical approach [4]. In this case the radiation field near a shallow corrugation is presented as a sum of two counterpropagating TM polarized wave-beams: , (1) propagation and mutual coupling of which is described by two non-uniform parabolic equations. The synchronous interaction of electrons with a forward partial waves leads to development a self-bunching and formation of powerful SR pulse.Simulations show that the most optimal conditions for SR emission correspond to excitation of the backward surface wave near the Bragg frequency ( S -regime, Fig.1b). For parameters of an electron bunch formed by an accelerator RADAN (electron energy of 300 keV, a total current of 2 kA, a bunch duration of 500 ps) and a corrugated waveguide with the mean radius of 3.75 mm, corrugation period of 0.825 mm, and corrugation depth of 0.36 mm the operating frequency in the resonant point is of 0.14 THz ( 0 2 3.5 r O | ). In this case the power of generated SR pulse emitted in z directi...