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In the recent two decades, high-power microwave (HPM) generators producing short Ka-band and X-band pulses were intensively investigated (see Refs. [1–6] and citation therein). For generation of HPM pulses with duration of units of nanosecond and less influence of microwave breakdowns of electrodynamics systems, expansion of cathode and collector plasmas, and electron beam structure distortions becomes insignificant. Though the pulse duration is short, the radiating energy rises if coherent summation of radiation from several generators is provided. When the power in each channel is high, the wave beam could be formed where power flux density is limited only by breakdown strength of the media. Using approaches of the phased arrays, the directional pattern (DP) of summarized radiation could be steered. This is achievable from pulse to pulse, as well as during a single nanosecond pulse if summarized wave beams have a shifted frequencies. The related experiments with Kaband microwave pulses are presented below. Super-powerful microwave pulses containing about ten oscillations could be formed in the mode of Cherenkov superradiance (SR) of electron flux propagating in elongated slow-wave structure (SWS) [1]. As a result of a single-pass, high gain amplification of initial electromagnetic (EM) perturbation, the power of SR pulse may exceed the power of the driving beam [2]. This is also because of SR pulse is shorter than the electron beam (in space) and less than its propagation time throughout the entire SWS. If the interaction with following slow wave continuous, the beam energy transfer to SR occurs due to the wave-to-particles slippage [1]. For the case of counter wave (backward synchronous harmonic) SR pulse accumulates the energy from incoming “fresh” electrons [3]. Beam-to-wave power conversion factor K = (1–1.4) was obtained for relativistic superradiance Ka-band backward-wave oscillators (BWOs) [1–-3]. In the recent experiments [4], voltage pulses (Fig. 1; a, b) applied to explosive electron emission (EEE) cathode formed electron beams with power of ~ 2–3 GW. This was sufficient to excite a 29-GHz SR pulse (Fig. 1, c) with K~1. It was revealed that in the presence of high electric fields at the SWS wall ( 2  E MV/cm) arise breakdowns. However, rf power limitation was delayed enough to ensure SR pulse passage throughout the entire SWS. Electric field of ~1 MV/cm was achieved in quasi-stationary (QS) BWO having the SWS two times shorter than SR oscillator. This QS BWO generated 2-ns pulse with power of 0.6 GW. Figure 2 demonstrates, that both SR and QS oscillators have (from shot to shot) generation phases linked to the voltage pulse front (Fig. 1, a) at the point of maximum derivative by time. Obviously, the faster rise time of the beam current (that correlates with the voltage front steepness), the stronger a wideband, phase-imposing seed EM perturbation is formed, when the beam is injected into SWS.

Relativistic microwave oscillators with high power flux in a free space and interaction zone